219 replies to this topic

[LC-GTR-1969]

tell us what you really think oldjohnno!  hahahaha

 

i have an open mind, so I will be comparing the two in as objective manner as possible before I make my summary.  

 

I do know that the my engine starts and idles great with the webers. 1500rpm to 3000rpm is the weak point, and above 3k the engine is quite crisp. Not surprising at all considering its a fixed venturi setup running chokes which are too big for street use (in reality).

 

Once I have the SUs tuned to the point that my webers are, I will write up a comparative summary. 

[jd lj]

In regards to my post quoted by old Johno the comparison between the webers and su's wasn't actually a test as the owner was just using what he had available at the time. I agree it's not a fair comparison of the two set ups if they're not comprehensively tuned.

[LC-GTR-1969]

In regards to my post quoted by old Johno the comparison between the webers and su's wasn't actually a test as the owner was just using what he had available at the time. I agree it's not a fair comparison of the two set ups if they're not comprehensively tuned.

Fair enough, but I have heard and observed, that webers can be quite good for smoothing out over-cammed engines. Thats not to say that SUs wont do similar, however. I cannot be sure as I havent tested the SUs yet.

 

I am going to try UP needles, what jet should I use? .125?

[LC-GTR-1969]

Any IR setup will tolerate more duration than a shared plenum intake... it's the isolated runners, not the Webers...

 

You are very likely correct and I tend to agree, but nonetheless, the end result is the same from where I am sitting... An over cammed/ long duration cammed engine will idle relatively well with webers when compared to many other induction setups. Whether it be the manifold attached to the webers or the webers themselves, to me, whilst interesting for scientific conversation, is somewhat irrelevant to the average punter who just wants their cranky engine to idle well and run crisp. If you can get webers for the right price, go for it I say... If there are other individual runner setups that will do a similar job of smoothing things out, great, go with whatever floats your boat or whatever one can find I say...

 

Ultimately, I am not a one eyed weber man, hence why I have invested some cash to have a go at 2 inch SUs for some comparison. In saying that, just dismissing webers as being a rubbish carb is crazy, because they are not a rubbish carb. Complex, yes. Easy to tune, no, Perform, Yes, when tuned well and used in the right application, they are fantastic. But that is not saying that a very well tuned SU or Strommy would not perform equally as well, I just think its horses for courses. I find some modern quad cam engines overly complex and sometimes over engineered for my liking, and I hate working on them, but that does not mean they do not have advantages. Maybe a bad example, but just a point I am trying to make

 

I have experimented with a 1310 austin engine in a sprite, swapping from twin 1 1/2 SUs to a single 45DCOE weber. My observations with that setup, was that the SUs had more snap and response below 3500rpm. Above this, the SUs were relatively flat. The weber, was the opposite- somewhat flat below 3000rpm and above this, the response was awesome. The 5-6000rpm range the weber was superior. In a straight line, it is hard to say which would be quicker though, as the SUs would snap out from a start better, then fade off, and the opposite for the weber. I suspect over 200 meters they would be very similar. Both setups were tuned well. Weber had smallish 32mm chokes.

 

At the end of the day, with that siamesed port engine, they were both strong in different ways, but they certainly made the engines behave somewhat differently. I have always liked 2 stroke bikes and carts, and webered engines seem to behave somewhat 2 stroke like in my experience, from which my take on it is that the fixed choked setup has a sweet spot where the airspeed is ideal, which creates a harmony zone for the engine, just like when a 2 stroke is on the pipe. In saying that, this may give many the shytes... Just an observation...

 

By the way old johnno, I also love Mikuni carbs, if only i could get 6 and a suitable manifold! 

Edited by LC-GTR-1969, 11 February 2016 - 07:49 PM.

[LC-GTR-1969]

You only need three. Six throats on a 9 port makes no sense whatsoever. They'd be trivially easy to adapt to SU manifolds.

maybe I will try some after I have tried the 2 inch SUs!

 

by the way, it was 1 1/4 inch SUs on the 1310 engine, not 1 1/2 inch. The mini we have is running 1 1/2 and I got mixed up...

[N/A-PWR]

He He Oldjohnno,

 

another non-comparison, which I didn't believe at the time,

 

was the two LC Torana's on a 1/4 mile drag race,

 

where  the 192 triple weber won by a couple of lengths, vs 208 500 holley both on NOS.

 

So two 45mm throats gave better top end than two 32mm throats? That's hardly surprising, nor is it a valid comparison. 

 

not sure which way to think about the unknowns, hmm.

good luck with the comparisons 69

[warrenm]

Fair enough, but I have heard and observed, that webers can be quite good for smoothing out over-cammed engines. Thats not to say that SUs wont do similar, however. I cannot be sure as I havent tested the SUs yet.

 

I am going to try UP needles, what jet should I use? .125?

UP needles should be a good place to start, Yes .125" jet

[LC-GTR-1969]

So two 45mm throats gave better top end than two 32mm throats? That's hardly surprising, nor is it a valid comparison. I don't mean to sound like a smart arse but it seems that most comparisons involving Webers also involve some other carb that isn't similarly sized. And why do so many people say things like "I love my Webers, the car runs so much better now (except for that massive flat spot between xxx and xxxxrpms...). And it sounds so cooooool.." Why after spending a squillion dollars and hours on tuning does it still not match a Holley tuned by a brown dog with a box of jets and a set of number drills?

 

Fuk Webers.

 

Fair point, and to an extent I agree, and which is why I clarified my answer- i was trying to remember back a few years...

 

But remember that the weber is choked down to 32mm, so the weber is not quite seen by the engine as a 45mm throat. Albeit, the larger the throat to choke/ venturi ratio will actually provide a greater venturi effect to my understanding, but nonetheless flow in the weber was limited to a 32mm choke area.

 

My current setup has 2 45mm throats with 40mm chokes feeding the 9 port, so if the SUs are better, or worse, it is also not a great scientific comparison, as some could say my current weber setup is overcarbed, and lacking signal, which may explain why the SUs may perform better, and on the opposite, if the webers perform better, some may say its as they had more total flow area... 

 

Hence, its hard to get a really clear comparison, nonetheless, with most NC guys running webers, and most drag guys running 2 inch SUs, to my observation they both seem to have their advantages.

 

Dont get me wrong, I do love the elegant simplicity of SUs, but cannot discount webers as a performance carb as they are good when working right.

UP needles should be a good place to start, Yes .125" jet

 

thanks heaps, I will buy a set and whack them in as a place to start!

Edited by LC-GTR-1969, 12 February 2016 - 05:56 AM.

[warrenm]

Set the jets .060" down from the bridge as a starting point.

A bit of light reading.

http://www.terryhunt.../picsb/pics.htm

[SA EH]

Has anyone had a 2" su on a flow bench?

 

I mean "actually" instead of just googling flow rates.

[Ice]

Im with OJ a mate spent 7K on a red 6 with Webber's it wouldn't pull the skin off a custard
took them off and swapped them over for a 500 2 barrel Holley with bigger jets it went like a scalded cat
go figure

[LC-GTR-1969]

Set the jets .060" down from the bridge as a starting point.

A bit of light reading.

http://www.terryhunt.../picsb/pics.htm

 

Great reading mate, thank you very much for sharing.

 

I pulled apart the carbs last night and to be honest 2 out of three look really good, all things considered. One carb seems to have binding a little in the throttle spindle, so I am suspecting galling in the spindle bushes. But otherwise, they cleaned up quite well.

 

Very happy and excited to recondition them and whack them on the tory... Now I just need some time. Sick of being contantly busy, I haven't even taken the car out for well over a month as no time, let alone start repairing things...

Edited by LC-GTR-1969, 13 February 2016 - 06:21 AM.

[jd lj]

OJ, since I have no experience with mikuni's what are the benefits of these carbs over others, from my limited knowledge of them don't they use similar principles to dcoe's. I'm assuming that possibly they have a more adequate auxiliary ventruis and a different progression circuit?

As many of us know the main problem with dcoe's is the mid range flat spot where the progression circuit starts to taper of before the main circuit has come into effect enough to fill this hole in the mixture curve. Extra progression holes are found on some models to help with this problem but can also cause others.

Dcoe's are well suited for circuit racing because the engines would rarely see rpm's low enough to worry about the mid range flat spot, being at higher revs and only really using the main circuit then these carbs simply do everything required from them and can be calibrated the to the large air flow required.

Fortunately there's a very clever guy out there who has after a lot of effort designed a more efficient auxiliary ventruis (only for 40dcoe's at this stage). These new auxiliary ventruis have a much better signal strength and the main circuit can come into action around 1000rpm earlier and eliminate the flat spot. The main circuit can easily handle the increased size of its dynamic range and the progression circuit can now be reduced to match the now larger main circuit.
That's enough information on that for now.

It was said in another thread recently that a red motor prefers a larger droplet size in its fuel atomisation over a finely atomised mixture. Why is this so? From my understanding a finer atomisation will burn at a leaner ratio than a coarse mixture and a coarse mixture also requires more heat to burn than a finely atomised mixture. How does this relate to what I mentioned earlier about the red motor preferring a coarser atomisation, anyone?

From my own tests dcoe's produce a quite large droplet size from the auxiliary ventruis which as stated above won't burn as readily as a finer one, has anyone tested a su carb to see what sort of atomisation is produced by them?

James

[LC-GTR-1969]

It was said in another thread recently that a red motor prefers a larger droplet size in its fuel atomisation over a finely atomised mixture. Why is this so? From my understanding a finer atomisation will burn at a leaner ratio than a coarse mixture and a coarse mixture also requires more heat to burn than a finely atomised mixture. How does this relate to what I mentioned earlier about the red motor preferring a coarser atomisation, anyone?

From my own tests dcoe's produce a quite large droplet size from the auxiliary ventruis which as stated above won't burn as readily as a finer one, has anyone tested a su carb to see what sort of atomisation is produced by them?

James

Do you think this could offer some explanation as to why my engine produced peak HP with what would normally be considered an overly rich AFR? 11.7-11.8 was the range where it was happiest above 5000rpm. This was confirmed by comparing track mph and jetting. any leaner and whilst the midrange cleans up a little, trap MPH speed drops.

[jd lj]

If you look at the hp vs afr graphs you see that peak hp is produced around 12.5AFR, as the mixture leans out the power drops a lot quicker than it does as compared to the AFR being richer. Obviously though once the AFR gets too low that's still not good either.

When you tested the lap times with different jetting was this on the same day and the same conditions. Air, tyre and road temperatures plus other factors would all make a difference.

If I was jetting the carbs from scratch with 40mm chokes my starting point would be something like 50f8 idle jets, 45pump jets with a blank discharge valve and medium pump rod springs, F2 emulsion tubes, 160mains and around a 200 air corrector and the floats set at 8mm on brass floats. This fuel level should be close to 2mm below the passageway leading to the auxiliaries / 25mm down from the top of the tower that houses the emulsion tubes etc. From your stated AFR with the 165mains I'd expect that using a 160mains would put the AFR very close to the 12.5AFR.

[N/A-PWR]

Had to google these one's Jono:-

 

Reference - Post #11

 

  On page 81 of David Vizard's Tuning the Aseries engine,

 

  he flow test a ton of different carbs that could and have been used on our engines.

 

 This test list max flow for a given single carb, so mutiply by two for twin carb application. 

 

 HS4 133 CFM 

 HS6 210 CFM 

 HIF44 240 CFM 

 Weber 28/36 26/27 venrturis 225 CFM 

 Weber 32/34 195 CFM 

 Weber 40DCOE 36 mm venturis 175 CFM per barrel 

 Weber 40DCOE 38 mm venturis 242 CFM per barrel 

 

Reference - Mikuni

 

 Mikuni HSR42: 213 CFM

 Mikuni HSR45: 237 CFM

 Mikuni HSR48: 270 CFM

 

Reference - What size suits

 

 Q: How do I choose the right size carburetor for my engine?

 

 Edelbrock responds: A simple formula can assist youwith this.

 

 Multiply your cubic-inch displacement by the maximum rpm limit,

 

 and then divide by 3,456 to represent the volumetric efficiency.

 

The result is the amount of cfm the engine requires at the maximum rpm limit.

 

 Example:

 350 ci x 6,000 rpm = 2,100,000

 2,100,000 / 3,456 = 608cfm

 A 600-cfm carb would would be sufficient.

 

Has anyone had a 2" su on a flow bench?

 

I mean "actually" instead of just googling flow rates.

 

Be very interesting what you come up with Jono.