[Formulajones]

Ha.....Mick above mimic'd about identical to what I just said about the M20 gear ratios.

[thirtypointer]

Originally Posted by STEELCITYFIREBIRD

What CFM carbs?
Are they calibrated to be run as a dual quad setup?
Cam?

Could/should be a monster..when tuned well, most DQ setups suffer from being way to rich just about everywhere. Unless something is way out of bounds hard to kill the torque of a big cube Pontiac @ 3K.

Edlebrocks 1404s (500 cfm)
Edelbrock intake
Cam: Xtreme Energy XE274H 274/286, 230/236, .488/.491, 110 Hyd

They have been tuned. Should be scary to drive, right!

[thirtypointer]

Originally Posted by tom s

What 2-4 intake?If a E intake,get a Offy.Remember a apair of 500s are not 1000!I would not run anything less than a a pair of 750s on your engine.I have been running 2-4s on pontiac since 1962.Both my 421 HO and my 421 SD have 2-4s.The HO has a a pair of 625s for 100% street driving,the SD has a a pair of 750s.The 428 RA V im building will have a pair of 750s.Tom

Edlebrocks 1404s (500 cfm)
Edelbrock intake

Are those carbs way too small in your opinion?

[tom s]

For sure too small for the CI and really not a good 2-4 intake.If you want more power a QJ and a good modified stock intake would be better.If you like the eye candy and performance a Offy intake and a pair of 3705 max wedge AFBs properly jetted for a in line intake would be my choice.Progressive linkage for the street and solid at the track.Tom

[69hardtop]

My setup is mild 400/M20/3.55 rear/26 tall tire and I’m pretty happy with overall driving manners.....but I usually don’t push it much over 65 mph

[TAQuest]

Originally Posted by tom s

A M21 with a 3.36 will be a dog and a clutch killer at the strip IMO,Tom

Not a dog or a clutch killer on the street. Don't take it to the strip but why would it be any different?
Burns the tires in 1st, 2nd, and 3rd with factory traction bars. Blew away a turbo diesel last night and that was going from a light up a bridge.
It's near perfect up until I need another gear, 5th. However, that keeps me out of trouble with speeding tickets.
25" tires though. And a very happy engine.
Since the question was asked with 3:55 gears in mind, that should make up for the 25" tires.

[thirtypointer]

Originally Posted by lust4speed

I ran dual 650 AFB carbs non-progressive with 3.36's and the M20 with the Old Faithful cam in the 428. Car always left silky smooth with no effort.

I just solved a problem with the current combo which is a 462, Quadrajet, and a little larger cam that complained a lot on take off. I would have to feather the clutch on normal take offs while playing with the throttle. Problem ended up that I had leaned out the carb's low speed circuit too far and the engine didn't like it. I had decreased the primary jet size two sizes by following my AF meter. After ignoring the meter and going back up three sizes the drivability is now back to where it should be.

Talking about wide ratio vs close, back in '67 when I ordered the car I settled on the 3.36 rear and wide ratio for two reasons. The first is I had to commute fairly large distances, and the second was looking at drive ratios. Ever notice that the first three gears with a wide ratio and 3.36 rear are almost the same as a 3.90 rear and close ratio trans? Yeah, healthy drop from 3rd to 4th, but I remember that being about 88 mph with the original tires. Most street races are over way before that, and torque does a pretty good job of pulling through the gap anyway.

On the two fours - gas mileage was excellent but when I sold them and put on a 800cfm Quadrajet and Torker I manifold the car went a tenth and a half faster (and lost one MPG). The AFB's were silky smooth and never gave a problem, and I suspect the horsepower limitation was the Offy manifold. Still did okay on Harry K's chassis dyno producing 413 RWH at 5,160.

Anyway getting back to the point, if your carb is off it will definitely screw up low speed drivability.

Thanks. I am thinking maybe carbs are the issue. I am taking off the way you described in your 462 paragraph. I was thinking the tranny and gears were my issue!


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[TAQuest]

Originally Posted by thirtypointer

Thanks. I am thinking maybe carbs are the issue. I am taking off the way you described in your 462 paragraph. I was thinking the tranny and gears were my issue!


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I had a timing chain set issue and it made it so it struggled off the line. Had to slip the clutch.
Fixed that and the car is happy with the ratios.
I think a lot of folks that have issues with the m21 ratio are really having issues with their engines and passing the buck to the ratios.
A happy engine gives you more flexibility with trans and gear ratios.
When you get it right you need those ratios or you just sit and spin.

[unruhjonny]

I had a close ratio Muncie four speed with a 3.55:1 rear in my car for many years - from what I have come to understand, it wasn't ideal, but it was fine for me.

I also had compression on my side, which helps torque numbers...

Pontiac did build some cars with a close ratio Muncie and 3.42:1 - so that's honestly the lowest (numeric) gear ratio I would use with a close ratio transmission - but even then I'd suggest only doing it with a higher torque engine...

[lust4speed]

Originally Posted by tom s

... and really not a good 2-4 intake...Tom

Lets pursue this a little further. I always thought that the Offy manifold was limited by the runner design of the manifold -- basically carbs sit really low and runners have to go uphill and make the sweep into the head. The Edelbrock "looks" like a better design since carbs sit higher and ports look to have about the same angle as a normal 4-barrel manifold.

I sold my Offy dual quad manifold and carburetors a few years ago and recently picked up an Edelbrock dual quad manifold with almost new 625 cfm Edelbrock carbs in exchange for doing some engine work. Thought it was going to be a step forward, but wouldn't be the first time I've made a move backwards based on what looks like it would work better.

Thinking about putting them on my son's 65 2+2 421 engine. He doesn't race it anymore and pure street duty now, but I'm interested to hear whether the Offy would have been a better choice.

[Chris65LeMans]

When I first bought my car, the M21 was paired with 3.31s in the rear. After I worked on the tune a bit, it ran great and - with the lousy tires on it - spun the tires in all 4 gears.

During restoration, I switched to 3.73s and good rubber - that REALLY woke it up.

[geeteeohguy]

An M20 with it's stock gears will have zero problems taking off from a stop, even with a 3.08 rear gear, which was optional. I suspect you have a swapped in M21 right now, or else you have engine/tune problems. Have owned/driven many GTO's with M20's with rear ratios from 2.56 to 3.90, and the only one I had to feather the clutch on was the car with the 2.56 swapped in. The original M-20 in my '65 (the one with the small countershaft) has a lower 2.56:1 first gear than the later '66-up 2.52 geared boxes.

[tom s]

Mick,look at the placement of carbs on a factory 421 SD intake,then look at the placement on the E intake.The factory spend maybe millions on dyno work as did chevy on their dual quad intakes.Chevy found theirs were better with the carbs close together like the E.The Offy is like the factory pontiac 2-4s.Also the E AFBs although they race well at WOT just dont seam to drive as well as other AFBs without a lot of tuning work.As said I have been doing the 2-4 stuff on pontiac for over 55 years and have tried virtually all AFBs.The factory SD pontiacs are by far the best.Buick 60s and the 348 chevys are very good for 625ish replacements.For 750s the factory SDs are the best,the single 4 409s are good and my choice is 3705 mopars with jet changes as they come jetting for a Xram intake and have staggered jetting.Have not tired the new E AFBs I think they are the AVS?Tom

[i82much]

Originally Posted by lust4speed

I just solved a problem with the current combo which is a 462, Quadrajet, and a little larger cam that complained a lot on take off. I would have to feather the clutch on normal take offs while playing with the throttle. Problem ended up that I had leaned out the carb's low speed circuit too far and the engine didn't like it. I had decreased the primary jet size two sizes by following my AF meter. After ignoring the meter and going back up three sizes the drivability is now back to where it should be.
.

I had a similar issue from going to slow on the idle feed restrictor on a Holley. I think there is a *very* good chance that this is a carb tuning issue. Light throttle, low RPM, the clutch pulls the engine down and not much signal to the carb. Need to richen up the idle transition circuit.

[thirtypointer]

Update: I just switched my setup to a single carb and intake. Cliff’s Q-Jet and a Performer RPM. This really worked well. It really woke up with lots of low end power and responsiveness. No track yet and only a few miles into it, but thinking the tranny wasn’t the core issue as many of you suggested. Thanks for all the help!


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[Formulas]

The transfer from idle to main jet is a very important area for drivability and ET. and light throttle pinging
And the hardest area of most carbs to modify

[Formulajones]

Originally Posted by thirtypointer

Update: I just switched my setup to a single carb and intake. Cliff’s Q-Jet and a Performer RPM. This really worked well. It really woke up with lots of low end power and responsiveness. No track yet and only a few miles into it, but thinking the tranny wasn’t the core issue as many of you suggested. Thanks for all the help!


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You're still on the M-20 with 3.55's correct? That's what you started the thread with. There is nothing wrong with that setup.

It was the idea you had of going with an M-21 that would have made what you were complaining about worse for you.

[thirtypointer]

Originally Posted by Formulajones

You're still on the M-20 with 3.55's correct? That's what you started the thread with. There is nothing wrong with that setup.



It was the idea you had of going with an M-21 that would have made what you were complaining about worse for you.

Yes still have the M20 and 3.55s. It was the duel carbs not tuned correctly.


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[TAQuest]

Originally Posted by thirtypointer

Yes still have the M20 and 3.55s. It was the duel carbs not tuned correctly.


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M21 would work fine also.

[unruhjonny]

Originally Posted by TAQuest

M21 would work fine also.

Agreed.

Originally Posted by unruhjonny

I had a close ratio Muncie four speed with a 3.55:1 rear in my car for many years...

Just saying.