132 replies to this topic

[mick_in_oz]

I wonder if the high manifold vac reading with RPM is due to the vac secondary carb, or more to the point the secondary diaphragm spring being a little on the heavy side for the engine?

 

It's been years since I have run a 780 vac sec carb, and then it was on a standard stroke 308, and I never put a vac gauge on it while racing it, only to tune the around town stuff.

 

My thought would be to change to a much lighter vac diaphragm spring, and see what the result is. Simple easy place to start.

 

AND LEAVE THE BALL BEARING IN!!!!!!!!!!!!

[axistr]

UC308Hatch, Ron they machined down the original piston. The slave cylinder bore is 15/16. My pedal is extremely light even lighter then my SLR with same master set up with concentric slave. You could go for a 7/8 slave bore which will give you I higher friction point. My friction point is around 4" up from the floor.  No worries on the last comment that's what the forum is all about, plenty of idea's to help each other out, we cant all be experts in every field and its great to have such a vast knowledge from some very talented members on here.

 

 

Thanks Mick, The Holley 780 Street avenger carby's come with three different vac secondary springs. I installed the lightest spring, I like the secondary's to cut in early on high torque engines. I think my problem may be multiplied due to ignition timing change with the increased vacuum. I've been doing a lot of reading and most agree that the Edelbrock performer R.P.M manifolds are the go for engines up to 400ci. But some experts say that when duel plain manifolds are used you need a bigger C.F.M carby over a single plain manifold. They state that the duel plane division in the manifolds designed to pulsing is contained to two butterflies and not across the four, great for torque but reducing it's capacity to flow at higher R.P.M. This is something I didn't even think of when I did the initial sums on the carby size choice. 

 

Doing a bit more research on E.F.I. I noticed Holley have reduced the price of their multipoint HP injection kits by $600 US. Down to just over $2,500.  Holley have had the cheque book out lately and purchased the M.S.D group which included Mallory, F.A.S.T, Mr Gasket and a few others. Holley are now bringing out anew range of throttle body fuel injection kit including the integrated E.C.M with timing control for under $1000 US. Might have to sit back and see what's happening in the E.F.I industry as there may well be some big bargains soon.      

[LS1LX]

That sort of engine you should really run a double pumper. I don't see any benefits with a hot motor running a vac sec carb, its still going to juzzle the juice.

[axistr]

Yep your properly right LS1LX, but I am running tall driveline gears. 9" with 3.0-1 ratio and the gear box has a .65 overdrive.

If I had lower gearing I would have used a double pumper. I am not that concerned about the fuel economy, I just didn't want it to bog down badly in the lower rev range which is a common problem with big 4 squirt jobbies. When the vacuum drops they wont allow the fuel to siphon in unless they pull the rev quickly. Just  another reason for looking into E.F.I and there added benefits.  

[LS1LX]

3.0's are you running on the salt lakes? Add some 3.7s and this machine will feel like you put a 100hp shot of NOS in the bum.
Personally I wouldn't waste the time or money on those low grade aftermarket efi setups. I honestly think you will be disappointed at the end of all the hard work for the small result gained if any.

[76lxhatch]

^ That doesn't add up, your rear tyres would have to be huge (like 70 series on a 17" wheel!). I get around 1800rpm with a 3.45:1 diff, 0.64:1 overdrive and 275/40R17 tyres

[Bigfella237]

Yeah something wrong there... those tyres, ratios & revs say your diff is somewhere around a 3.08 or 3.36

 

A 4.11 with your tyres should be doing almost 2,200 @ 100km/h (unless your speedo is a long way out)?

 

Edited by Bigfella237, 06 December 2015 - 04:15 PM.

[UC308Hatch]

Is that program one you wrote, or is it in here somewhere? 🏻🏻

[Bigfella237]

It came from the right-side of my brain!

 

Yeah wrote it myself in Java, I got sick of doing all the maths by hand all the time.

 

The "Wheels" tab calculates front & rear track, hub-to-hub measurements, rim width, offset & backspace...

 

 

...the "Tyres" tab does all the usual stuff plus gives min and max rim widths for that tyre size...

 

 

...the "Speedo" tab does much the same as the oldholden website's online calculator...

 

 

...and the "Notes" tab is just a scratchpad that saves whatever you paste in there.

 

Before you ask, no don't want to release it, I'm always tinkering around changing stuff so it's kind of a work-in-progress and I don't want to get into the whole tech support / releasing updates / version history thing, sorry.

[UC308Hatch]

Awesome stuff Andrew, 🏻🏻 I'm impressed 🏻

[ls2lxhatch]

Maybe your tacho is under reading.

 

Gearcalc is one of many free calculators.
http://locost7.info/gearcalc.php

 

According to Gearcalc a 245/45/17 (RKM 488) with a 0.65 overdrive and a 4.11 diff will produce 100 km/h at 2173 rpm.

 

 

The formula to calculate Km/h for a given rpm, gear ratio and diff ratio is

 

Km/h = (RPM * 60) / Gear Ratio / Diff Ratio / RKM

Km/h = (2173 * 60) / 0.65 / 4.11 / 488

Km/h = 100.00

 

Km/h = (1800 * 60) / 0.65 / 4.11 / 488

Km/h = 82.84

 

The small variation between calculators is typically in the calculation of the number of revolutions per kilometer (RKM) based on the rim diameter, tyre width and profile. Some tyre manufacturers (Toyo for one) specify then revolutions per kilometer (RKM) which eliminates tyre circumference calculation discrepancies.

 

I have written a similar program. My program and Gearcalc calculate the RKM for a 245/45/17 at 488. The RKM in my program and I suspect in Gearcalc is rounded to the nearest whole number.  Bigfella237's program calculates the RKM at 487. The difference in RKM between Bigfella237's program and Gearcalc could be that Bigfell237's program rounded down and GearCalc rounded up.

 

The small difference in RKM is responsible for the small difference in rpm calculated by GearCalc and Bigfell237's program.

Edited by ls2lxhatch, 07 December 2015 - 04:34 AM.

[ls2lxhatch]

Toyo specify a RKM of 502 for the Proxes4 245/45/17 which would produce 100 Km/h at 2236 rpm.

 

http://toyotires.com...pdf/proxes4.pdf

[Bigfella237]

 ~ The difference in RKM between Bigfella237's program and Gearcalc could be that Bigfell237's program rounded down and GearCalc rounded up. ~

 

Yep the variation is in the rounding, I've looked at this before and decided the difference wasn't worth worrying about, the gap between your program and mine is 5 engine rpm (2168 vs 2173) and you'll never pick that up with any tacho I've ever seen.

 

As you say, these are all ballpark figures anyway... different tyre brands, amount of wear, inflation pressures, load ratings, etc. all affect the numbers slightly so 5 rpm is hardly worth a mention, but anyway...

 

Grant, if the tyre size and diff ratio is correct as stated then something else is out, either the speedo and/or tacho is reading incorrectly or your gear ratio is much taller than you thought, which is what I'm tipping is the problem?

 

Not sure which trans you have but if we re-run the calcs with an overdrive gear ratio of 0.50:1 as fitted to the T56 Tremec ("MM6" trans code) then I come up with 1668 rpm @ 100 km/h, just about spot on what you're seeing!

 

 

REALLY SORRY FOR THE THREAD-JACK FOLKS, it kinda got outta control.

[axistr]

No worries Grant, It was one of those things, when I restored the hatch the diff already in the car was a nine inch 3.0-1 and I slipped in a true track L.S.D. Funds didn't allow for a ratio change at the time, but the top loader was buggered so the extra money when into the Tremic TKO box. I would have preferred a set of 3.7 gears. But on the brighter side it has plenty of torque to push it and I managed to drive it to toranafest and back to Windsor on fifty litres of premium unleaded.

 

I am going to hijack my one thread and add a couple of photos, My wife bought me a new Sony digital camera a while ago so I buggered around in the shed last night in the shed. Its amazing all the photos where taken in pitch black, I run a light over the cars with the shutter wide open and the results come up better than expected. Its a real cool trick I picked up for another photographer.

 

 

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[76lxhatch]

The red looks really good in the photo of the two together

[Bigfella237]

Those photos are unreal and both cars are a credit to you, I'm tempted to say you're a lucky man but really luck didn't have anything to do with it!

 

Just on fuel economy vs gearing, as I'm sure you know, it's not purely a case of taller gears equal better mileage either, my Landcruiser is reasonably thirsty on 35" tyres but if I step down to 33's it's remarkably better (goes better too).

 

With a TKO I think you'd be surprised what a difference a lower gear set would make, and a low final drive ratio is a LOT of fun in traffic too!

[LXCHEV]

Loving the photos, awesome! What kind of light do you use to do that, and far back do you stand etc?

[axistr]

Thanks Bigfeller, 

 

I think the words your after are "the harder I work the luckier I get"

 

The red always look good in any light and never seams to change colour much. The yellow seams to change in colour depending the angle and light conditions, its actually chrome yellow.

 

Lxchev I just used a small cordless Narva LED light. Its how you use it that makes the difference.

 

[axistr]

Finally got a EFI kit for the hatch, Holley multipoint HP kit, I can run the distributor I have but will need an ignition box to do so to control my timing by the Holley E.C.M. Much easier is just to run a chev small cap H.E.I electronic distributor from a TPI based (87-93) engine. The Holley E F I kit comes complete with the harness and plug to suit this distributor. The only problem is I think I am going to have to move the engine further forward by around 15mm. Sound easy if you say it quick but 15mm is going to be very tight around the extractors.

 

Has anyone got a small cap H.E.I distributor from a TPI engine that can take a few measurements across the cap so I know how much I actually need to move the engine forward.

 

My Mallory distributor measures 86 10mm below the HT lead connections and I currently have only 2 mm clearance to the firewall drip rail.    

[76lxhatch]

If you can use a Chevy HEI then couldn't you just run a standard GM ignition module and a reluctor pickup dizzy that has the advance locked out? The modules are cheap and that would sure beat having to move the engine forward...!

[axistr]

The Chev T.P.I distributor uses two plugs and six wires to complete the ignition system wiring. The T.P.I dizzy also directly outputs to the coil not the E.C.M but ECM still controlling the timing. I think I would be flat out getting this ignition module in either of my distributors, Mallory are very small in diameter. The Holley system allows for hall sensors, magnetic pick up, trigger ignition boxes and direct fire coil ignitions systems, but will not control the one coil systems directly. (ECM controlling ignition timing) They all so state you can't run a C.D distributor only C.D boxes or the ECM will be damaged.

 

Sorry but I don't know how to post up here Holley link for the common ignition wiring diagrams. 

 

 

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