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Building a high performance six

Started by _oldjohnno_ , Jan 26 2008 09:46 PM

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#1 _oldjohnno_

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Posted 26 January 2008 - 09:46 PM

There is a fairly detailed page describing how to build a quick six at Building the High Output Holden Six

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#2 _73LJWhiteSL_

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Posted 26 January 2008 - 11:11 PM

Very interesting. Thanks for the link.

Some good tidbits in there.

Steve

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#3 Heath

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Posted 26 January 2008 - 11:15 PM

Did you write that yourself? I don't have time to read it now but I have saved it and will definately read it over several times in the near future. Thanks!

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#4 _Yella SLuR_

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Posted 27 January 2008 - 08:36 AM

I thought the easiest way was to add a couple of cylinders and a V.

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#5 _Yella SLuR_

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Posted 27 January 2008 - 08:39 AM

Looks like a good comprehensive write up.

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#6 _70rey_

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Posted 27 January 2008 - 01:22 PM

mate wasnt a bad write up... and it is true u can get really decent horses out of a 202 motor.

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#7 Com_VC

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Posted 27 January 2008 - 01:36 PM

especially with the fitment of a turbo :spoton:

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#8 _mervex_

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Posted 29 January 2008 - 05:37 PM

I gotta agree with Com-VC. how much work ya gotta do to get THAT amount of power across a skinny rev range.How many of have been in or driven a well set up LJ or LX turbo 202 ,running 15 psi? The make really good torque between 2000-5000 rpm! Don't need to rev the tits off it!And you can drive it like a normal car if you wish! my mum used to borrow mine.
mervex

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#9 FastEHHolden

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Posted 29 January 2008 - 07:47 PM

That why I'm hoping to get a bit of work done on my car..its got a 234 Turbo (10 psi) and will be running LPG....should have gobs of torque..and combined with the TH350 it should be fantastic....the 4.11 full spool 9 inch is a bit of overkill though.

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#10 _waratah_

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Posted 30 January 2008 - 08:04 AM

i kinda think this thread should be pinned.

briallant article

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#11 _squirralien_

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Posted 20 July 2008 - 12:35 PM

has anyone got a copy of the article they can send me as the link doesn't seem to be working for me at the moment, I feel like I am missing the meaning of life lesson. I can send and email to who ever can email me a copy through PM
Thanks

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#12 _gstar_

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Posted 20 July 2008 - 01:31 PM

the best holden six performance build article was in a street machine a couple of yesrs ago, i think someone scanned it in and posted it up from memory but i cant seem to find it

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#13 Collo

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Posted 20 July 2008 - 01:43 PM

Link wont work for me either...

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#14 nzxu1

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Posted 20 July 2008 - 03:49 PM

:( Wont work for me either :(

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#15 _gtrtorana_

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Posted 20 July 2008 - 08:30 PM

Link wont work for me either...

Please make it work. Sounds like a good read

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#16 _tyre fryer_

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Posted 21 July 2008 - 02:06 AM

i think heath saved it to his computer if anyone wants to PM him.

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#17 _boynuz_

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Posted 21 July 2008 - 03:54 AM

Building The High Performance Holden Six
Introduction
This page is an attempt to provide some information and guidance for those wanting to build a higher output six cylinder Holden engine - specifically the red, blue or black straight sixes (sorry grey lovers). It's based on my own admittedly limited experience with these engines, plus what I've been able to glean from others. Reliable information for these old engines isn't easily available, and as usual anything you read in magazines should be taken with a big grain of salt - after all their primary responsibility is to their advertisers. The stuff you find on the web isn't necessarily any more credible, so use common sense and carefully consider anything you read before implementing it. Of course, that includes what's written here.

You'll notice that the information here isn't particularly detailed; it's more of an attempt to steer the reader in the right direction. You'll also notice that I tend to mix metric and imperial units - get over it, I'm an old fart.

Naturally, a page like this can never be complete, and I welcome suggestions and corrections. It's aimed at the builder with a limited budget, and is therefore focused on using the original Holden major components - eg. block/crank/head. Mainly we will discuss naturally aspirated engines, though we may touch briefly on issues related to blown engines.

Characteristics and Limitations.
The six is a reasonably light and compact engine compared to a V8 at least, and can provide reasonable performance in a lightish car like an early Holden or Torana. Having said that, the red motor wasn't a particularly advanced design even when it was first released and it's ultimately quite limited in output, mainly by the cylinder head design. As we will see later, the cylinder head is the key to making power with the six. On the plus side, they are a sturdy engine that can withstand big increases in HP and RPM utilising mainly factory components.

Naturally aspirated, don't expect to get much more than around 220hp for a streetable engine. This mightn't sound like a lot compared to todays injected V8s, but should be enough to get a light street car into the low 14s and possibly even high 13s without resorting to nitrous. Its possible to get 300+ hp from these engines using longer rods and extreme porting and cam profiles, but at these levels the powerband is painfully narrow. If you really do need more than say 240hp from a Holden 6, it may prove more practical and cheaper in the long run to use a Jzed/Duggan style head.

Choosing The Engine
If the engine is going into a registered street car, this decision may be already made for you. There are generally restrictions on displacement before engineers approval is required, and this usually means other modifications to brakes and steering for example will also be needed. Also keep in mind that later engines in registered cars usually need to have all the emission controls fitted and working.

Maximum horsepower is basically determined by the flow capacity of the engine components, so there won't be a big difference in output between say a 186 and a 202, though the smaller engine will produce its peak power at a higher speed. In general, you should choose the largest possible bore size (ie. 186/202) in order to minimise the chamber overhang problem that we'll touch on later. The 202 will probably prove to be a bit quicker and more pleasant to drive in a street car, though the 186's slightly better rod ratio will extend its top end a little. There is one other advantage to the 202, and that is the availability of a relatively cheap performance piston/ring package in the ACL Race Series parts. As far as horsepower goes, there will be very little difference between a 186 and a 202.

Don't overlook the EFI 202 from the VK Commodore for something to transplant into a daily driver - these are as cheap as dirt and are very close in output to the XU1 - but more civilised.

Stroker motors are fairly popular, and can provide about 235cu in in the most popular configuration. Usually they are based on the 221 Ford crank. The original Ford flywheel flange is cut off and the journal turned down. Then the rear journal and flange is cut off a Holden crank and a hole bored into the journal so it can be pressed onto the turned-down Ford rear journal and welded on. The other journals are then ground to suit the Holden rod and main bearings. Provided it's done properly it's quite durable. Slightly modified stock length rods can be used with Ford 250 pistons. A fair bit of work has to be done to make room for the longer crank throw; notches have to be ground into the sides of the crankcase, and the sump needs a bit of hammer work as well. Also the cylinder bottoms need to be relieved a little and the camshaft needs to have some flats ground into it for big-end clearance. Is it worth the extra expense and effort for about 15% more capacity? For a street engine I guess the extra cubes would make for a very nice torquey engine, though I doubt that peak horsepower would be any more than for a 202. For a very high output engine I think I'd stick with a 3" or 3.25" stroke, if only for the better rod ratio. Still, the 235 could be a very strong and responsive street engine.

Which Block?
Again, if it's for a registered car you may not have much choice. A track-only car can make use of a late blue or black block, complete with counterweighted crank and better rods. But if you want to avoid having to use emission controls you'll probably be stuck with an older (HJ or earlier) red block. These can be fitted with the counterweighted cranks with a bit of work, and it's also fairly easy to adapt the later 12 port heads to these blocks. There is nothing special about the old HP blocks, and while the XU1 blocks allegedly are beefier good luck in finding a block or the money to buy one.

Keep in mind that some of these blocks are over 35 years old, and most have been rebored a couple of times. They are also likely to have a fair bit of corrosion in the water jackets, so you might have to check out quite a few before you find a good one. Pull out the water pump and knock out some of the welch plugs so you can get a good look in there. Finding a good 202 or 186 will be the hardest, many of these will already be bored 0.040" oversize and this is as far as these cylinders should be taken if you plan to make much power. I know lots of them have been bored 60 thou over, and 60 might be ok for a standard street car engine but any more than 40 on a high output engine is risky. Even if the engine doesn't fail outright the lack of bore stiffness will cost you power and cooling can be a problem.

If you are building a 3" stroke motor (186/192) keep an eye out for 179 blocks, or even better, the less ancient 173. These engines are cored the same as the 186/202 blocks. This means they can be bored out to 192 specs, but you could start off with the std 186 bore size and have stiffer cylinder walls plus room for a couple of rebores. A blue 173 block with its lightweight non-counterweighted crank and heavy duty rods would make the basis for a good (and cheap!) drag race engine.

Block Prep
Nothing special usually required here, unless of course you plan on using a different type of crank. The age of these blocks dictate that you should give any block a good clean and check it out thoroughly for cracks or any other damage before you invest any time or money in it. Line boring isn't usually required, though cutting material from the deck might be be needed to get the squish/quench clearance down - more on this later. Keep an eye out for cracking around the head bolt holes which is quite common, though minor cracks here don't seem to cause any problems. These holes also tend to strip threads so consider fitting inserts and/or studs while you're at it. Bore finish is quite important, and should match the rings requirements. It's generally accepted that an automatic machine can give a better, more consistent result than hand honing.

The Crank
There are quite a few different types of cranks for the little Holden. The 202 engine uses a 3.25" stroke while everything else from the 138 red up to the 186 is 3" stroke. There are also variations in material. All the 3" cranks made before the introduction of the HK in mid '67 are steel, plus the 186 X2, the 186S and the 186 XU1s used steel as well. All 173s and 202s are cast. Strength and durability doesn't seem to be an issue for either type, and remember Brocky won Bathurst in a cast cranked 202 Torana so I wouldn't be too concerned about the lack of steel 202 cranks.

The later 12 port 202 (but not the 173) engines had fully counterweighted cranks that make life quite a bit easier for the mains. For a street motor or any engine that is subject to sustained high speeds I'd go for the counterweighted crank, though for some forms of racing the lighter non-counterweighted crank might have an advantage. The 202 cranks have larger main journals than the others, and there are also variations in rear main seal dimensions so if you are planning to use a 202 crank in an earlier block (perhaps to make an engine that's bigger than the numbers on the block indicate) you will have some machining to do. The main tunnels can be line bored or alternatively the cranks can have their main journals turned down to fit the older blocks, apparently with no ill effects. Bearing clearances should be about .002", which might be considered a little bit on the tight side for hi-performance engines. Similarly, you want to keep the rod side clearance fairly close to the stock figures in order to prevent throwing too much oil around.

The red 202 cranks had smaller diameter oil holes, and while these cranks were fine for normal use they were prone bearing problems at high speeds. If you're running a 202 that will be subject to prolonged high speed work it would pay to either use a later crank or drill the oilways out slightly - though I doubt that this is something that can be done properly with a Black & Decker and a long series drill bit.. There is no need to crossdrill the journals so I wouldn't bother. It's normal practice to slightly chamfer any sharp edges or corners on the oil holes but don't get carried away and flare them too much - it just reduces the bearing area.

Some people like to run knife-edged cranks, where the outer circumference of the counterweights are bevelled back to an edge. Sometimes the leading edges of the counterweights are bevelled too. The idea is to reduce the windage and drag on the crank, and it also reduces the rotating mass. While this sounds cool I'm not sure it's worth it on a dollar-per-horsepower basis. I know that the oil wrap-around effect on the crank can cause drag and sap power at high revs, but unless you plan on going the whole hog with a special sump design and scrapers and so on I suspect the gains from running a knife edged crank on its own would be minimal. The reduced weight would certainly help acceleration, but if that is a major concern you could simply run an early non-counterweighted crank for a lot less money.

You could probably get away with using a new standard balancer on a 3" stroke engine or a mild 202, but for high RPM work you'll have to use a competition style balancer, especially on a 3.25" crank. If you decide to use a stock balancer consider fitting some sort of retaining ring or flange to the front to stop the rim from walking off the hub. There are a few different types of heavy duty balancers around and while they aren't cheap they can be good insurance. A stock balancer may come apart at high speeds, with possibly disastrous results. Depending on what combination of balancer and timing cover you are using, the timing marks may not actually indicate TDC so remember to check it and re-mark it if necessary.

Rods
Use the heavy duty rods from either the 4 cyl Starfire engine or the later blue/black engines - they are the same rod. Always replace the rod bolts when stripping and reassembling the engine. For a mild street engine, particularly a 3" stroke that won't be revved much past 5000rpm the stock red motor rods should be fine.

Balancing
Some people get anal about this, balancing their components to within a gram or two. On a straight six with a 120deg crank (like our Holden sixes) there is little point to this as these engines are inherently balanced anyway. Provided the piston and rod weights are reasonably matched, I wouldn't bother with balancing at all. If it makes you feel any better though, go ahead and do it.

Oiling and Oil Pumps
The stock holden system is simple and relatively trouble free, though some of the early 202s I think had smaller oil holes in the crank and could pick up the bearings under prolonged high speeds and loads. The Holden, like nearly all engines, has a bit of a problem supplying an appropriate amount of oil over a wide range of speeds. Increases in the crank speed do lead to a slight increase in oil requirements due to the increased throw-off, but its nowhere near the increase in oil flow provided by the pump as revs increase. The net result is less-than-ideal flow and pressure at low speeds but too much at high speeds. The standard size oil pump should be sufficient for nearly any high performance engine, and the only application I can think of where a high volume pump might be useful is an engine that runs at unusually low speeds and high loads; a turbo engine perhaps. It's surprising how much power is absorbed by an oil pump, and if you've ever primed a Chev with a power drill and dummy distributor you'll have experienced it first hand. Whenever there is an excess capacity the unused oil blows over the relief valve, and the energy that goes into this work is converted into heat. In other words it makes the oil hotter, and this is another reason to avoid the high volume pumps.

It's worth remembering these motors are pretty long in the tooth so you'd want to check out any used pump carefully before using it again. Check the clearance between the tips of the gear teeth and the pump body and reject any pump with more than a few thou clearance. Also check the end clearance and keep it down to 2 to 3 thou. Backlash between the gears isn't really critical but check for badly worn or scored gears, or worn shafts and bushings. If there is any doubt about a used pumps condition it's best to replace it.

Naturally all the oilways in the block and crank will have to be thoroughly cleaned, and if you avoid excessively loose bearing clearances you should have no trouble maintaining enough oil pressure. Two thou should be enough to ensure a reasonable flow across the bearings, but not so loose as to drop the pressure too much at lower speeds. Check the alignment of the oil holes in the block with the holes in the bearings and if necessary elongate the holes in the block slightly to match. Don't shim the relief valve spring too much; a well built engine should live happily with 50psi at full load and speed.

Choose your oil carefully and be aware that many modern petrol engine oils will be unsuitable for a high-output Holden straight six. Unfortunately you can't find out much about an oil just by reading the container - even the SAE viscosity numbers cover such a wide range to be almost useless. An example of this is an oil marketed as say SAE30 that is at the high end of the 30 range. This oil may actually be more viscous than another oil at the low end of the 40 range that's marketed as an SAE40. It pays to check the makers Technical Data Sheets where you will find accurate specs on the viscosity at various temperatures as well as other info. While we're on the subject, viscosities really have little relation to an oils lubrication abilities so there is no point in running a thick oil, the increased drag just robs power. If you can't maintain adequate pressure with a 15w-40 oil there are problems.

For those running a flat tappet cam - and this will be nearly everyone - look for an oil that contains zinc dithiophosphate (ZnDTP). Almost none of the modern petrol engine oils and only some modern diesel oils have it, the reason being modern roller cammed engines don't need it and also because it tends to foul catalytic convertors and oxygen sensors. You mightn't find the ZnDTP level mentioned in the tech data sheets but it will probably be in the Material Safety Data Sheets. More than likely you will end up using an oil designed primarily for diesel engines and these generally work very well. Just don't put it in a very high mileage engine that hasn't previously been using diesel oil. The high detergent levels will quickly loosen up the accumulated crud in the engine with unpleasant results. Avoid synthetics unless you really know what you're doing.

Pistons
Not a lot to choose from here, at least not when compared with whats available for the Chev motors for example. And if you plan to build anything besides a 202 the choices are quite limited.

Don't expect to be able to buy forged pistons off the shelf; if you need forged items you'll probably need to get them custom made or else adapt pistons made for something else. Cast high-silicon-content pistons are generally adequate for naturally aspirated engines of the specific outputs we are talking about here, though at the top end of this range the safety margin is getting a bit thin. Still, we've probably all seen some sixes that have copped some pretty extreme treatment and still held together. The main thing of course is to avoid detonation.

More than likely you'll end up using the cast ACL Race series pistons in a 202, and these seem to hold up well. There are flat top versions as well as one with a small dish - the flat top is the one to use for even if you plan on reducing the compression, and we'll get into this further when we talk about combustion chambers. These piston and ring packages were designed to be used in higher than normal output applications, and should be fine in nearly any normally aspirated engine. The early 202s had a habit of breaking off the skirt at high revs, though this is not a problem with the ACLs.

If you're building something other than a 202, the only pistons that will be easily available will be the stock replacement type. These will stand up to much higher pressures and speeds than they were originally designed for, but still you need to be realistic in your expectations. They were never intended for very high compression ratios, and it mightn't be possible to get them with a flat top. The stock type piston/ring package wasn't meant to do very high revs, and they will need at least another 2 or 3 thou clearance over stock for high performance work. Be extra careful to avoid detonation with these because they can be hammered to death very quickly. They'd probably be OK up to about 200hp but if you are going after every horsepower you can get it might be best to do whatever it takes to get some forgings or at least some Hypereutectic type castings.

Something to think about if you are considering using non-standard pistons: there are some Ford 250 pistons that are reasonably easy to adapt. These have a lower compression height and are the pistons used in the stroker engines. You could use these with a longer-than-stock rod to pick up some useful power gains. It's a bit outside the scope of the basic modifications that we are discussing here, but if you have to run special pistons anyway I'd certainly give it some thought.

Whatever pistons you use, the rings are generally the moly faced type, so you'll need to have the bores finished to suit (400 grit or finer) and preferably with a honing plate fitted. High performance ring packs are generally thinner and lower in tension, and there is a measurable decrease in friction with these. With stock pistons though, the ring choices will be limited.

Cylinder Heads
Ok, we're starting to get into the juicy stuff here - the head is the key to making power with the little Holdens. There are basically two different head designs used on the six, the 9 port as used on the red motors and the 12 port used on the blue and blacks. We'll look at the 9 port first.

Nine Port Heads
While engines from other manufacturers of the same era had head porting that was at least adequate or even too big (eg Cleveland or square port BB Chev), the 9 port Holden head was barely able to feed stock 149 motors. The bigger, later motors were equally asthmatic despite having bigger valves. Where other engines responded well to intake, exhaust or especially cam upgrades, the old Holdens never really woke up until the head was modified. The intake ports in particular were abysmal, but on the positive side even the most godawful butchery of the ports nearly always produced an increase in power. Perfectune recognized an opportunity to provide an exchange head with improved porting and bigger valves, and sold squillions of their YellaTerra heads. The mods were basic and mainly carried out on automatic machines, keeping the prices low. Power and fuel economy could be substantially improved with nothing more than a head upgrade. There are still a lot of these YT heads around and on a mild performance engine they do a reasonable job, with the so called "Bathurst" style heads capable of making 200 odd hp.

Lets look more closely at the 9 port heads. The most obvious feature is the siamese inlet ports, with the six cylinders grouped into three pairs and each pair sharing an inlet port. The valves are arranged like this: EI IE EI IE EI IE. Cylinders 1 and 2 share an inlet port, as do 3,4 and 5,6. Traditionally siamese ports have been considered unsuitable for high performance engines and in many cases (eg. BMC 4's) there is good reason for this. However, in the case of the Holden motor port sharing can hardly be blamed for the heads poor performance. If we look at the centre two cylinders (3&4) for example, we see they are 360deg apart in the firing order, and even with the longest duration cam there is never a time that both cylinders have their intake valves open at the same time. So obviously there is no chance for one cylinder to rob it's neighbour. The end pairs of cylinders are slightly different, and there is a short period during each cycle where one intake is closing while the other is starting to open. But this period is so short (and occurs at a time when there is so little flow) that any inter-cylinder influence will be negligible. It's not the fact that the ports are siamesed that hurts the flow, it's the basic design of the port along with that head bolt that passes through it.

The valves are all inline and only slightly canted and this, combined with the fact that the ports are quite low, makes for a sharp, almost right angled bend in the valve pocket area. Add to this a cast iron pillar that runs up the centre of the port near the gasket face and things are looking even worse. This pillar is where the head retaining bolt passes through, and is quite thick, almost a third of the port width. Over the years there have been several approaches to solving the head bolt problem, the most common being to cut the thick pillar out, replacing it with a thinwall steel tube. This is what YellaTerra did, and it's quite effective in increasing flow. Some people have cut the pillar out and installed a socket head cap screw in the floor of the port to clamp the head down, then screwed a flush fitting plug into the hole in the port roof. I doubt that there is much difference in flow either way, but the conventional steel tube approach is the most convenient.

Fortunately there is a lot of meat in the port walls to work with, and it's easy to get big increases in flow and power output. If you're serious about making power, you should leave the port work to someone with the experience and equipment to get good results, and these people can get a 9 port head to flow enough to make over 310hp. In fact, in terms of sheer bulk flow you'll probably get more from a 9 port head than a 12 port, though of course bulk flow is only part of the story.

The standard valve sizes are too small, and you should aim to use at least XU1 or VH/VK size valves. The centres are fairly widely spaced, so there is plenty of room for bigger valves and seats. The downside to this is that the valves tend to be badly shrouded at the sides of the chamber, and it's pointless to try to widen the chamber because the side walls already overhang the cylinder walls. And anyway, there just isn't enough material between the adjacent chambers to lay the walls back much and still have sufficient thickness in between to support the head gasket. Of course the shrouding becomes worse as the valve size is increased, partially negating the benefits of using those big valves. Not only is the gas flow restricted by the chamber wall, it has to negotiate the ledge at the top of the cylinder bore. If you lay a head gasket on a cylinder head you will see that the openings aren't perfectly round, and match the shape of the chamber. Now lay the gasket on the block deck, and you can visualise the step or ledge under the chamber. Obviously the smaller the cylinder bore, the bigger the ledge, and it's a good reason to use the biggest available bore size. It's not uncommon to see these ledges on each side of the top of the bore chamfered or radiused back with a grinder to match the chamber, but if you decide to do this I'd be careful not to go too deep. A chamfer here may or may not help flow, but it will definitely expose a part of the piston above the top ring to a lot of heat so I'd be wary of going more than about 3mm deep.

We'll talk about combustion chambers more after we look at the 12 port heads as they are pretty similar with both types of head. If youre doing the head work yourself, all I can suggest is that you resist the temptation to make the ports huge and concentrate on slightly raising the roof of the ports, tapering them back from the port face to the valve bowl, so in effect the angle under the valve is less severe. Of course, you need to be able to match your intake manifold. Larger valve seats will have to be blended in and the bowl area can be opened up. Don't grind the port floor, except to clean up any dags. There is no need for significant widening on any reasonable street engine. The biggest gains will come from fitting oversized valves and from reducing the width of the head bolt boss. It isn't strictly necessary to cut it out and fit a steel tube, just narrow it and streamline it.

The earlier engines had intake valves of about 1.49" in diameter, and these are hopelessly undersized for nearly any application. Later red 202s and 173s had 1.625" valves, but these are still a bit small for anything but the smallest or mildest of engines. For a high output application you really need an intake valve of around 1.7" to 1.75" diameter. There's probably not much point going beyond this because the shrouding just becomes too tight. The YellaTerra heads generally used valves 3/16" oversize.

The exhaust ports flow quite well by comparison, and again there is plenty of meat to work with. There is a thick wall dividing the centre four exhaust ports, and these also have a head bolt passing through them. Unfortunately on some heads the wall doesn't quite extend all the way to the gasket face so these ports are at least partly interconnected, and I assume this would reduce the benefits of using tube headers or extractors. As with the intake, resist the temptation to go overboard with the grinder. Work out what size primary pipe size you will be using on the exhaust and match the port to this (keep it slightly smaller actually), trying to keep the cross sectional area fairly constant. Ideally there will be a step up of about 1 to 1.5mm all around the port into the exhaust manifold flange. You will probably find you will remove little if any wall material apart from a cleanup and some streamlining of the guide boss. The 1.275" exhaust valves of the earlier engines will be much too small, but you could get away with using stock 1.48" blue/black valve in even fairly highly tuned engines.

We can summarise the 9 port heads like this:
The standard head is extremely restrictive and won't make much power no matter what other engine components you have
Siamese ports might be less than ideal, but on the Holden 6 the firing order makes flow robbing from port to port a non-issue.
An expert head porter can achieve massive flow increases, up to 300 odd hp, and even an amateur can get good results with care.
Oversize valves are necessary, but it's no use going overboard because of the chamber shrouding.
There are many different types of manifold available for the 9 port, more than for the 12 port.

Twelve Port Heads
The introduction of the blue engines brought a completely new head design, and it was a massive improvement over the old red motors. Obviously, there is now an individual port for each cylinder, and the exhaust dividers now extend to the port face so extractors will work properly. The valves are bigger, and there is improved cooling with some additional water holes. When fitting a 12 port head to a red block use a gasket as a template to drill matching holes in the block deck. The new intake ports are much higher and narrower so the air flow now has a less severe angle to negotiate. The intakes will probably look very familiar; they resemble a slightly smaller small block Chevy port.

Flow wise they are comparable to a Bathurst style or YellaTerra 9 port head, but should be capable of a much fatter torque curve than that of the 9 port head. The reason for this is that the individual ports allow the use of a true individual runner intake manifold such as is used with the EFI setup or six throat IR carb setup. The peak horsepower thus obtained probably won't be much different to the 9 port but the extra midrange will certainly boost performance. There is potential for substantial flow improvement with these heads as well, and if you want to make the most of them it's probably best to leave the port work to an experienced specialist. Do-it-yourselfers could probably gain a bit by giving the ports a general cleanup and slightly narrowing the intake port dividing wall around the head bolt.

The exhaust port is also improved, and exits the head pointing slightly downwards. This might sound a bit odd, but I guess the idea behind it is to ease the transition into the exhaust manifold. The carb versions have bigger chambers and also have lumps in the roof of the exhaust ports where the air injection nozzles screw in. These can be plugged and ground back. Be careful not to enlarge the exhaust port as it will already be about the right size for a 1.5" primary tube, however it mightn't hurt to slightly streamline the somewhat chunky valve guide boss. Neither port has the wall thickness found in the 9 port head, so go easy with the grinder.

These heads have a reputation for being a bit prone to cracking, and the quality of the castings certainly doesn't look that flash. I think that the cracks might have more to do with the cars they were installed in than a problem with the head though. The old Commodores ended up having the radiator top tank lower than the cylinder head, so it was easy to get a pocket of air or steam trapped in the head, especially if the cooling system wasn't bled properly. The Nissan engines also suffered cracked heads in the same type of car, but were trouble free in other cars. The bottom line is this: get the head crack tested before you invest time or money in them, and if you have one of the old Commodores bleed the cooling system thoroughly.

The combustion chambers are very similar to the 9 port heads, and have the same problems with valve shrouding and the same step at the top of the cylinder bore. If you plan on using factory 12 port heads I suggest taking a look at the VK EFI version; not only does it have better flow characteristics than the blue heads it also doesn't have the air injection humps intruding into the exhaust ports. The downside is the big open chambers that do little for compression or squish. The blue 173s have a much better chamber, and these heads could be ported to give the same flow as the EFI head to give the best of both worlds.

As far as I know, YellaTerra can still provide 12 port heads. These look quite good, with oversized valves and improved porting. They are also said to be cast from better material. Keep in mind that headwork doesn't come cheaply, so it's quite possible that a complete new head will be cheaper than rebuilding and modifying a used one. The biggest single disadvantage to using the 12 port would have to be the scarcity of good intake manifolding. The factory EFI works quite well, and even the factory 2 barrel manifold would be acceptable for a mild daily driver, but what these heads really like is a good, true IR (individual runner) type manifold with three sidedraft two-barrel carbs. A good triple Weber or Dellorto setup on these motors will be very quick, and should make good power over a wide rev range. Select your manifolds carefully; some are very badly designed will give disapointing results. In particular approach anything that is claimed to fit both 9 and 12 port heads with suspicion.

Combustion Chambers
There are useful power gains to be had by getting the chamber into shape. Specifically, at TDC we want to reduce the clearance between the piston and the squish/quench pad to the bare minimum. This is the flat area of the head alongside the chamber bowl, and it's primary role is to induce turbulence in the chamber as the piston approaches TDC. It does this by "squishing" the mixture out of the confined space into the bowl area, but there is a downside to this as well. Because the mixture in this area is in such close proximity to the relatively cool metal of the head and piston, it doesn't burn along with the rest of the cylinder charge when the piston is at or near TDC. It either doesn't burn at all, burns only partially or only burns when the piston is significantly past TDC and it is too late to derive any usable work from it.

How much power can be gained? Lets look at an example engine, a 202 with 11:1 compression that makes about 220bhp. Lets assume the quench area covers about a third of the head area, and we have a piston to head clearance of 0.070". In this example the volume of gas in the quench area at TDC works out to about 12cc, or roughly 22% of the total chamber volume. In other words, about a fifth of the mixture in the chamber won't be producing any useful power. Lets say we reduce the clearance to about 0.35", so that we now have only about 11% of our available mixture in the quench area. In theory at least, we have just picked up over 20hp. In the real world of course the increase mightn't be so dramatic, but at any rate there are good gains to be made for very little effort.

Remember that the piston crown is part of the chamber, and any dish in the crown will have to be taken into account. Wherever possible, it will pay to use a flat top piston, or at least try to limit the dished area to that part of the crown that matches the bowl area of the head. How tight should the piston to quench pad clearance be? Basically you want to run as close as possible without actually having the piston smack the head at high revs. And that depends on how much piston rock there is at TDC, bearing clearances, thermal expansion of the piston and so on. You should be safe at 0.040", but I'd be nervous about anything less than 0.30". You can juggle the clearance by altering piston height, deck height or gasket thickness. Obviously, if there is any sign of piston/head contact you'd want to get a thicker gasket in there pretty quickly.

Compression Ratio
There is no magic number here, the ratio needs to be selected with regard to the type of fuel used and also the cam timing. I'm not convinced that the petrol currently sold is as bad as some people make out, though there does seem to be some variation from batch to batch. Cam timing, and in particular the intake closing event has a huge effect on cylinder pressure and therefore compression ratio selection. At slower speeds, a delayed intake closing (such as is used with hi-po cams) will regurgitate lots of cylinder pressure back into the intake, making it possible - and desireable - to use a much higher ratio. As speeds increase and the engine gets "on the cam" the momentum of the intake charge causes this reversion to decrease or stop altogether, leading to much greater cylinder filling and pressure, and this in turn to a jump in torque and horsepower. At these higher speeds there is also a big increase in the amount of turbulence in the chamber, and this is why we can get away with the higher pressures at high speeds without detonation. It's also the reason that no more spark advance is required over a certain speed.

Keep in mind that the Holden chambers are pretty ordinary so don't get too greedy. As a very very rough rule of thumb, I'd suggest limiting CR on an engine with relatively short cam timing (similar to a stock cam) to around 9.5 to 1. Higher revs and longer cam timing will allow CRs of up to say 11:1 or 11.5:1 to be used with decent petrol, but if you are going to run high compression you'd better make sure you're right on top of the cooling, ignition timing and mixture distribution, otherwise it's easy to rattle the engine badly.

Builders of blown engines often have to reduce the ratio, and to be honest I'm less than impressed with some of the methods used. Things like pistons with deep circular dishes and decompression plates certainly reduce the pressure, but they kill any squish that may have existed in the chamber. This is just throwing horsepower away. If I was building a blower motor I'd be trying to keep the clearance between the piston and the flat face of the head to a minimum to promote turbulence (squish), just like in a normally aspirated engine. Then to reduce the CR, Id try to make the chamber deeper, possibly cutting a bowl in the piston that matches the head chamber but without touching the squish pad area. This approach should make significantly more power than simply spacing the head away from the deck.

For a naturally aspirated high-performance engine, you'll probably be using a small chamber head to get the desired ratio. Measure the chamber volume carefully to verify the ratio, the usual method being to cover the chamber with a piece of perspex with a small hole in it, through which you can pour light oil from an accurately graduated container or burette. You may need to machine material off the head to increase compression, and obviously if this is taken to extremes the stiffness of the head may be reduced to the point where it becomes difficult to keep the head gasket in. In these cases a custom domed piston may be the only solution.

Here is the formula for calculating CR (the volumes for the cylinder and chamber can be in cc or cu.in so long as you use the same units for both. One cubic inch equals about 16.39cc):
CR=(D + V + DC + G + CC) / (V +DC + G + CC)
where:
CR = Compression Ratio
D = Displacement of one cylinder
V = Piston Volume (will be a negative value with a domed piston)
DC = Deck Clearance Volume
G = Gasket Volume
CC = Combustion Chamber Volume

The formula to work out gasket or deck volume is:
V=0.7854 x d x d x g
Where:
V = volume in cc
d= Bore diameter in cm
g = gasket thickness (or deck height)

You can use the above formula with a slight modification to work out how much a chambers volume will be reduced by if it's machined by a certain amount:
V = 0.7854 x d x d x tm x ca
Where:
V = volume reduction in cc
d = bore diameter in cm
tm = the amount machined off the head in cm (10 thou is about 0.025cm)
ca = is the proportion of bore covered by the chamber bowl eg. if the chamber area is about 60% of bore area use 0.6 for ca

Induction - Carbs, Manifolds and Injection
What do we want from an induction system? How about this?

A sufficient volume of air and fuel to allow the engine to develop its maximum potential power
Fuel mixed thoroughly with the air, and in particle sizes uniformly small so they burn quickly and completely
Uniform mixture distribution from cylinder to cylinder
Satisfying number one is easy; it's just a matter of making everything big enough . Number two is a bit harder, and seems to be particularly challenging for certain popular American carburetors. Number three depends mainly on manifolding, with some types it takes care of itself, others will take some work to get right. So how do we know when we have got it right? There are two key indicators, firstly the engine will make good power. And secondly it will have good fuel consumption relative to the power produced. I know this is supposed to be about high performance engines but it really is important to monitor fuel consumption as well; it's a sure sign of how efficiently the engine is running as a whole. Every now and again you might hear some young bloke tell you how he has built an engine so incredibly powerful he can barely back it out the driveway without refilling the tank. The correct response of course is to smile and nod, and to think to yourself: "You f*ckwit. If you ever get that thing running properly it will make twice as much power and use a fraction of the fuel." Even a triple carbed, long-overlap cammed engine should give reasonably good mileage on the freeway, very similar to a stock engine with similar gearing if everything is set up right. So if your engine turns out to be thirsty in normal use, rest assured there is more power to be had by getting it to run efficiently. A good illustration of whats possible is the modern fuel injected engine; these are making much more power and torque than the engines of say 30 years ago, but at the same time using much less fuel and putting out less emissions. Burning every drop of fuel as completely as possible isn't just good for mileage, it's good for horsepower levels too.

Types of Induction Systems
We'll come back to injection systems later... in the meantime we'll check out the two basic carburetor/manifold layouts: common plenum systems and individual runner (IR) systems. We'll look at the IR systems first..

IR Manifolds
With these systems, each cylinder has it's own individual manifold runner and carburetor/injector butterfly. If you've had anything to do with multi-cylinder motorcycle engines you'll be familiar with these setups, they are like individual single cylinder engines on a common crankshaft. There are obvious advantages with regard to maintaining uniform mixture distribution, and its also easy to make all the runners the same length. Often it's also possible to make the runners relatively straight as well, and this helps reduce fuel separation.

One peculiarity of these systems is the "quick-gulp" intake characteristic - because each carb supplies only one cylinder there will be a period of roughly 270deg where the flow into the cylinder occurs followed by roughly 450deg of little or no flow before the next inlet stroke starts. And because the carbs are flowing only part of the time we need to use a total throat area of roughly three times that of a conventional system. This explains why these types of engines require carbs that at first glance appear to be way too big.

Examples of IR manifolding include the triple SU and Weber setups. Even though the 9 port heads have siamese ports the crank layout and firing order dictate that essentially only one valve in each port is open at a time, so in effect they run as an IR manifold. The 12 port motors are particularly well suited to IR manifolds as it's possible to use longish runners with a fairly constant cross section all the way from the carb to the valve. This can result in a useful boost to midrange power.

The triple Weber manifolds for the 9 port motors are a bit of a weird one - basically we have two carb throats and runners joining into a common port at the head face. In effect it's like two carb throats feeding each single cylinder. I know some people have had good results from these, but it hardly looks the most efficient way to do it, and I'd be careful to keep the venturi size down if I was using one of these manifolds. Old timers may remember one of the HDT Toranas at Bathurst, where the rules specified that the number of carb throats couldn't exceed the number originally fitted. This particular car had two huge DCOE Webers fitted - one throat had been blanked off while the other three fed the three siamese ports of the 9 port head. To me this is a more logical way of feeding a 9 port than using triple two-barrel Webers.

To summarise, IR setups generally give very good results, but are more expensive and require a bit more work to synchronise the butterflies and tune properly.

Common Plenum Manifolds
This is where all the manifold runners connect to a common chamber that is fed by a carburetor or carburetors. The stock single carb layout is an example. Provided the carb and the runners are big enough bulk flow shouldn't be a problem, but maintaining good distribution can be a challenge. In the old days it was common to fit dual or triple Stromberg downdrafts to the old Holdens, and I suspect the improvements these brought were as much a result of better distribution as the increase in flow. They have the advantage of simplicity, and because the carburetor isn't subject to the violent pulsing that can occur in an IR setup they can be a bit easier to jet for clean running over a wide rev range. On the downside it's almost impossible to make the manifold runners the same length, and curves in the runners are unavoidable. Fuel drop-out is a problem, and this is the reason most factory manifolds are heated. The heat does wonders for smoothness and mileage, but it reduces the power output.

In summary they are relatively cheap and simple, but with limitations regarding runner lengths and distribution.

Downdraft Common Plenum Setups
For the best performance, I tend to think one of the sidedraft setups are the way to go, though I guess if you were trying to keep an engine looking reasonably stock or you just want to keep it simple a downdraft setup might fit the bill. For mild daily drivers the Weber from the XE Falcon works pretty well with a 12 port head and stock manifold. Even the stock Varajet flows about 380cfm, and can make good power.

For higher outputs, some people have had good results with a small 4 barrel carb on a 9 port motor, but whatever you do steer clear of the 350cfm (or 500cfm) two-barrel Holleys. A Holden 6 with a well sorted Holley 350 will start and idle well, run cleanly and produce a moderate amount of power. But it will also use more fuel than the other small two-barrels, without any power advantage. They obviously have a pretty severe problem with mixture quality and/or distribution. One peculiarity they have compared to the other 2 barrels is the butterflies open together, and they swing open on an axis that's at right angles to the engine. Maybe this contributes to the poor distribution. I know there are a squillion blokes out there with Holley equipped sixes, and they all swear that their engines run just fine. Which they do of course, but I'd bet good money that they would make significantly more power and use less fuel with a better carb.

Although downdrafts mightn't be the ultimate in performance, a decent two-barrel on a stock 12 port manifold is a cheap and simple solution for a mild street engine. A four barrel might provide sufficient bulk flow for good peak power but it will be difficult to match the mixture quality and distribution of a set of triples.

CV/CD Carbs
By CV carbs I mean the constant velocity/constant depression carbs such as the SUs or Zenith/Strombergs as fitted to the XU1s and any number of pommy cars. And I may as well say right now that there is probably no performance advantage to be had by using one brand over the other. There may be a minor advantage to the SUs in that their piston design eliminates any potential diaphragm problems. I've used CV carbs on a variety of engines over the years, and they have never failed to give excellent results.

It's practically impossible to overcarb an engine with CVs, and they seem to tolerate being fitted to overcammed street engines very well. Inch and three quarter CV carbs are almost the defacto standard on a hot six, and they will certainly perform well on a very wide range of engines from near stock to quite high outputs. On a very well breathing 186/202 it may pay to go for 2" units to achieve the engines full potential, and unlike fixed venturi carbs there will be no penalty from using the bigger carbs as the throats only open as far as necessary.

Some people are wary of using these carbs because of their undeserved reputation for being hard to tune and/or keep in tune. The reality is that as long as the linkages are in good condition and well set up they require very little attention at all. There must be no play at all in the bushes or rod ends, and the main shaft must be set parallel to the throttle spindles. Also it pays to run individual return springs directly on each butterfly to minimise the effect of any play in the links. Once the carbs are synchronised and the mixture set they will run for a very long time without any further fiddling. While there is a huge variety of metering needles available it's very likely that the factory fitted needles will be very close to ideal, and it seems that the standard needle can cope with a wide variety of engine types.

Summary: CV carbs will give excellent results on nearly any engine.

Injection Systems
While there are mechanical competition-only injection systems, they are beyond the scope of this article so we'll focus instead on the factory VK setup.

Commodore EFI Systems
The EFI from the VK can perform very well, despite being very simple - almost to the point of being crude. The system has few inputs, and has no feedback devices such as oxygen sensors etc. The injectors are wired in two groups of three, but all fire simultaneously at 120deg intervals. The manifold design provides good midrange torque and can also flow enough for 200+hp. A bit of judicious portwork at the transition area leading up to the gasket face should pay off with even more flow and power. The relative crudity of the fuel control makes the fuel economy from these units less than spectacular and in fact better economy can be had from the Varajet equipped engines. Improvements to the controls make these units perform even better, with gains in both power and economy. There are limitations with the stock ECU with regard to compatability with longer duration cams. Aftermarket, tunable ECUs are readily available though, as are bigger injectors and throttle bodies etc. A popular budget mod is fitting the Delco ECU from the Camira. I won't go into much detail here on EFI's but there are plenty of people out there who have used these successfully, and there are some good how-to's on the web.

The strong point of the EFI system is the manifold, and while it's not practical to use these manifolds with carburetors (at least not without horrendous distribution) it is possible to use them with LPG. For an LPG-only engine the compression ratio can be raised to make up for the slight drop in output that LPG normally brings. There won't be any distribution or fuel drop-out problems and the long-runner manifold provides a good top end with a nice fat midrange...

For 12 port projects, a VK EFI may well be the best choice.

Air Filtration
To ensure your engine lasts for any length of time you will need effective air filtration, and that means paper filters. Not fly screen or foam, or oiled cotton gauze - paper. When using IR type manifolds the quick-gulp flow characteristics that require relatively high capacity carbs will also dictate high capacity filters. Small individual filters on each carb are unlikely to be big enough. It's probably better to duct all the intakes together into a common airbox and use a single BIG paper element. Also keep in mind that at low speeds a long duration cam might spit back a bit of fuel, so try to keep the filter far enough away from the carb inlet that the element doesnt get wet. Realistically I know that space limitations make all this difficult, but if you can set up a good filter system you'll maximise both horsepower and engine life.

Fuel Pumps
Obviously an adequate fuel supply is essential, but that doesn't necessarily mean swapping out the stock pump. These were used successfully at Bathurst on fairly high output engines so they can't be all that feeble. If you think you'll need more flow a small block Chev pump is a direct replacement. I tend to favour mechanical pumps for reasons of quietness and reliability, but an electric pump is fine too so long as the pressure and flow ratings are suitable. SU's in particular don't like any more than a couple of psi fuel pressure and will dribble and puke if overpressurized, so use the appropriate regulator. And don't forget to use a good quality filter.

Camshafts and Valve Gear
Basically we are going to have to choose beween hydraulic or solid flat-tappet cams - I'm going to conveniently ignore rollers as being outside the scope of these basic notes. Common wisdom has it that any street driven vehicle should run a hydraulic cam, but there are some very good reasons to go with solids. Everyone knows solids aren't susceptible to the lifter pump-up issues that hydraulics can sometimes suffer from. But there is also a substantial performance advantage that goes with solid lifter profiles. Hydraulic lifter cam profiles need to have a short but gradual ramp just before the valve acceleration starts in earnest. The same thing happens on the closing side. The net effect is that, for a given duration at 0.050", a hydraulic cam will hold the valves off the seat for longer than a solid lifter cam will. These ramps are so gradual that no useful flow occurs, but it's enough to bleed off cylinder pressure at lower speeds. By contrast a solid lifter cam will leave the valve firmly on its seat before slapping the tappet rudely. In other words, for a given peak horsepower, a solid lifter cam will give a stronger low end and midrange than a hydraulic cam. Or looked at in yet another way, for a given low or midrange torque, a solid will give a better top end.

Hydraulic lifter cams are quiet and maintenance free, and for a mild daily driver they are the logical choice. But if performance is the priority, definitely go for the solids. They have a reputation with some for needing constant adjustment, but in reality once everything is bedded in they should rarely need attention. Millions of engines are fitted with solid lifters as standard and these often go for years between adjustments. Granted, the stresses on a high revving Holden 6 valvetrain might warrant more frequent setting, but a few times a year should be enough. Besides, adjustment is much easier and quicker to do on the 6 than say a V8, so it doesn't take long at all.

When comparing cam profiles between different manufacturers check the duration at 0.050" only, the advertised duration figures are too "rubbery" to be meaningful.

Some very very rough rules of thumb for cam selection:

Longer duration = more top end, less bottom end
More lift for a given duration = more area under the torque curve
Closer lobe centres/more overlap = more bottom end and less on top (but more overlap also makes for rougher light throttle running and idling)
Wider lobe centres = better top end at the expense of low down torque, better light throttle running and idling
Slightly retarded cam timing = more top end, less bottom
Slightly advanced = more bottom end, less on top

So what cam to use? Here is another very rough guide:
Mild daily driver, smooth enough for granny - 200 to 210deg @ 0.050", 0.39" to 0.41" lift (hyd)
Slightly warmer, rough idle but still fairly civilised - 215 to 225deg @ 0.050", 0.4" to 0.44" lift
Hot barely streetable cam - 230 to 240deg @ 0.050", 0.44" to 0.48" lift
Bigger again, maybe just barely streetable for occasional use- 240 to 250deg @ 0.050", 0.490" to 0.5" lift
Animal, dont even think about trying to use it on the street -250+deg @ 0.050", 0.520" to 0.570" or more lift

When you get the cam, make sure you get all the related components from the same manufacturer so that you can be sure the springs, retainers and lifters etc. are compatible. You may need to make the spring seats bigger/deeper for big springs and high lifts. If you aren't sure how much material is in the spring seat area do a practice run on an old head first or cut an old head up to see how much meat is in there.

Drive the cam with an alloy or steel gear, but for chrissakes don't use those damn straight cut gears - their only virtue is their cheapness and the constant whining is irritating. Don't try to be clever with cam timing; at least to start with set it exactly where the cam grinder intended it to be run; you can experiment with minor variations later. If you need to reposition it more than a few degrees for best performance you've got the wrong cam... Carry out all the usual precautions ie. check for coil bind, retainer to guide clearance, valve to piston clearance, rocker slot, etc etc. The press in rocker studs are good enough for a mild engine, but use screw in studs for setups with higher lifts and heavier springs.

The factory rocker setups are fine with any reasonably streetable cam, though you'll need some form of adjustment for solids. Roller rockers are nice to have with bigger cams, say over 0.48" lift, and the shaft mounted jobs eliminate stuffing about with guideplates. If you run ball-mounted rockers (or stud mounted rollers) on a head that originally had aluminium rocker bridges you will need guideplatesand hardened pushrods. Standard rockers are 1.5 ratio, you may find rollers in other ratios or adapt rockers from other engines (eg Cleveland). Be aware that higher ratios increase the loading on the entire valvetrain so I'd avoid using them unless I really needed the extra lift. If you decide to try the cheap Chinese rollers carry a couple of spares in your toolbox. Take the time to get the rocker geometry right; with so many things like deck height, cylinder head dimensions, valves etc being altered it's almost inevitable that a non-standard pushrod length will be required. At the mid-lift point, an imaginary line drawn from the valve tip contact point through the centre of the rocker pivot should be perpendicular to the valve stem.

Follow the manufacturers cam break in procedure closely and make sure you use an oil with sufficient zinc additive to protect highly loaded flat tappets, eg. Rimula Super. Most modern petrol engine oils aren't suitable, and using them will lead to cam and lifter failure.

Keep in mind that many of these cams will provide insufficient vacuum for brake boosters and so on - you may need to provide a separate vacuum source such as an electric pump or an alternator from a Japanese diesel with a built-in vacuum pump.

One more rule-of-thumb: if you can't choose between two different cams always go for the milder one. An overcammed street motor will be slower than it should be, and more painful to live with.

Exhaust
The exhaust has a profound effect on performance, but to get the maximum returns can take a lot of work. There are two basic requirements: firstly we want a system that offers free flow with a minimum of backpressure, and secondly we want tubes dimensioned to provide the best possible scavenging and power output. Satisfying the first requirement is fairly easy, but realistically you aren't likely to fully achieve the second one with any standard off-the-shelf extractors. They just won't have the right proportions. If you really want or need to get the absolute maximum from your engine you're going to have to make your own. But for any street driven car I wouldn't even bother with trying to tune the exhaust; I'd settle for a set of well built extractors with appropriate pipe diameters and wouldn't worry about getting the lengths right. You'll get good performance from these without having to spend countless hours cutting, bending and welding tubes.

There are factory dual outlet cast manifolds for both 9 and 12 port motors, and these can give surprisingly good performance from mild to moderate motors, often as good as what you'll get from extractors. Not only do they perform well, they are quiet and stay leak free. With hotter engines tube extractors are probably what you'll be using. For a very hot 202 or 192 the 1.5" primary pipe will be about right, though maybe still a fraction big. For milder or smaller motors they are are too big so try to find something with say 1.375" or 1.437" tubes. It may be difficult to match smaller primaries to the exhaust ports though.

If you need true tuned extractors for a competition oriented engine, you'll probably have to make them yourself. I wish I could tell you the exact dimensions to use but unfortunately every engine is different so it's something you will have to find out experimentally. And if you make further changes to the engine later on then you'll have to go through the whole exhaust tuning routine again. I can tell you that the best size primary pipe for a very highly tuned 192/202 will be no bigger than 1.5". Secondary pipes should be just a shade bigger than primaries. Collector size will be 2.25" to 2.5" and combined primary/secondary length will probably be somewhere around 30" to 36". Yeah, I know these are very vague figures but it's all I can offer at the moment. I hope to put some tables up later showing some common combinations and their corresponding pipe sizes. You might be thinking these diameters seem to be a bit on the small side but remember we're talking about a relatively small engine. If you use too-big pipes the engine will be a dog, and both torque and horsepower will be well down.

Ideally all the primary tubes should be exactly the same length, and most off the shelf extractors will have quite a lot of length variation. Having said that, I'd rather have extractors where at least some of the tubes were roughly the right length than a set where all the tubes were the same incorrect length. The straight six has much more room to fit exhaust manifolding compared to a V8, but even so you may come across primary pipes that dive down at a sharp angle right from the port face. Wherever possible leave a few inches of straightish pipe at the flange before sweeping the pipe down. Check the port match at the flange and head face, some extractors are way off and will need a bit of work with the die grinder.

Exhaust pipe size is important for muffled street engines, the smaller ones will probably like a single 2" pipe best, moderate 173/192s a 2.25", and very hot 192/202s a 2.5" pipe. Don't use twin systems, they will kill performance. Baffled mufflers often flow as well as or better than the straight-through absorbtion types, even though they are more difficult to accomodate neatly. A rough rule of thumb is that big, bulky turbo style mufflers perform better from both a flow and noise perspective than the small canister types. OEM muffle

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#18 _squirralien_

Guests

Posted 21 July 2008 - 10:13 AM

Thank you very much I will print that out and bind it now and maybe add some pictures to make a nice book out of it, I am sure I will be reading this a few times.
I really appreciate this and I think this on its own in a thread should be pinned in this section, without all our other messages, or maybe some of the messages with the useful information in them,,, Can a moderator go through and do that, make a thread just with the useful info and pin it.
Once again, thank you very much and I am sure that goes for a number of people on the forum.