Building a 71 Supreme and going to do a 4 speed conversion. Here's what I am planning on finishing up with:

Engine:
'70 350 block - oversize yet unknown
W-31 pistons
#7 heads with big valves installed (engine currently has #6's)
406768 442 M/T cam
OE cast intake and exhaust manifolds
Carb and distributor built to W-31 specs

Trans:
M-21

Rear End
2.56 Limited Slip - the original rear end to the car and it is in great shape.

This car is going to be for fun - not any serious racing. I am also wanting it to be stock appearing, using the parts I have or will be gathering. Will this combination be OK with the 2.56's ?

 

Hell no to be blunt. It will be nearly un-driveable to take off from a stop. The worst 1st gear ratio trans (2.2) and a very high rear (2.56) designed solely for an automatic makes for a bad choice even with a stock cam it would be very pesky taking off from a stop sign. Just to be driveable at all you would need to bump up to a 3.08-3.23 gear at the very least if you kept that tranny and then it will still be a little tough. To keep the 2.56 gears the only way to really work is go with the good old formerly Doug Nash (Richmond 5 speed) the has a 3.28:1 1 st gear and a 1:1 5th (no OD) and not use the M21 http://www.summitracing.com/parts/rmg-7022726a

 

Yes, with that combo you can expect to burn up a few clutches!

 

I wouldn't, I think those gears will take all the fun out of your fun build.

 

Must agree with the gang, your gas mileage will be great, but it won't get out of it's own way.......

 

+1. An ill wind blows...

- Eric

 

I have been considering gears and trans combinations and like the idea of a Super T10 with 3.somthing first gear and 3.08 rear gears.....

 

It would be good for running on the salt flats. Street not so much.

 

Yeah there is a light duty ST10 BW with lower gearing from a late 2nd gen F body 305 application that might work if you found one

 

Thanks all. Will begin the hunt for a 3.08-3.23 posi carrier and 3.23 gears. At least it's not a 68-70 rear end!

 

You'll be glad you did. You still might wanna swap that M-21 for an M-20 with the 2.52 low, though. I had a Camaro with a 3.55 Posi and an M-21. Powered by a Corvette 327/350 hp L-79, it still took a little clutch slippage to get it rolling off the line with any authority.

 

You can keep your 2 series posi unit and just a nice set of 80's-2000's GM 1500 series truck 3.08-3.42 gears and use a ring gear spacer.

 

The tallest gear used with a M21 from factory is around 3.55-3.73 on a Chevy normally besides 3.90-4.33 etc on a Pontiac 400 although my old 70 TA RAIII 400 was factory M21 3.55

 

Years ago I had a 1968 Cutlass 350 with a M21 four speed. When I bought it there was a 3.08 under it and I felt like it leaped across the intersection... I couldn't ease off a dead stop. Money was tight at the time and I came up with a Chevy 3.31 that I swapped in. That was much more comfortable to drive. Then a car I sold and wish I had back was a 1972 442 350 4bbl and 4 speed with a 3.42 posi. That car was a kick in the pants to drive. From that experience I'd encourage you to consider the 3.42 ratio.


John

 

Let's be clear. The M21 and M22 are racing transmissions designed to keep the engine in a very narrow RPM band on the track. They SUCK on the street. Sell the trans and buy a T10 with the 2.88 first gear. You'll be MUCH happier. Better yet, get a five speed.

 

Or....ditch the transmission. You can keep the rear end & install a Richmond 5 speed which uses a 3.07 1st gear & a 1:1 5th gear. You will love it.

 

Working on changing to a M-20. Looks like the tallest rear gear offered from the factory with a M-20 was 2.78 in '71. Guess I'm kind of Old Timey because I want to put a Muncie in it.

 

I think you meant 2.73 as 2.78 was in 1970 and only in Type O. Seriously the 5 speed can be had cheap as many are pulling for 6 speeds. They work well with power of mild SBO. The M-20 is nothing to get excited about but if you do get one get a 1971-1973 version.

 

Good for you! Don't go restomod! It's a slippery slope -- first a 5-speed and before you know it, you're rockin' 22s with O-ring tires, a purple digital dash, videoscreens in your headrests, and a chrome steering wheel.

 

Just keep the M21 and do some 3.73's !

 

NOT. Please put down the bong & step away from the key board.


He is converting an automatic equipped car over to a 3rd pedal car for fun; why not maximize the fun factor and make it driveable? His plan to run a close ratio 4 speed with a 2.56 diff was a path to failure. Not sure how you go from installing an extra gear to what you implied would happen.

 

This is easier and cheaper choice combined with a 3.08 you can drive easy off the start and cruise very well but catch enough rev's to help it get into the cam cruise range on the highway.

 

Ahhhhhh, the good old days..........

 

If I had the bong, it might go all the way including the mural of 2Pac on the hood.

 

Sorry -- should have put one of those little smiley dudes on that post to show that it was a joke.

 

Muncie = Cool.
For what it's worth, even the racers in Pure Stock find they like the M20 better than the M21 close-ratio. And by "like" I don't just mean it's easier to drive, it also performs better.

 

The PSMCDR guys can use M22W's heavy duty Muncies built with M20 gear spacing:-)

 

I converted my M-21 to a M-20 because I have a 3.23 rear gear. Just had to replace the cluster gear and the impute shaft. Anxious to see how it drives!