I need to replace my incorrect M21 in my stock '69 Z. For strictly a street car is the M20 or M21 a better choice. I need opinions. Thanks,
Gary

With 3.73s the M20 is better.

Huh? No, the m20 is a wide ratio tranny and combines well with higher gear ratios.

The m21 is a close ratio tranny and combines better with low gear ratios.

You want a m21 w/ 3.73's.

Dave

I know that the M20's lower 1st gear gets the car moving easier, but what other advantages are there? The M21 close gear ratios can keep the engine running in the power band. What other advantages are there? Thanks.

M20... without a doubt!
Peter

http://www.interlog.com/~muncie/designop.html

You need to tell everyone your rearend. Then, take Scoob's advice depending on whether your gears are low or high.

There was a 67 M21 in my 69 Z when I first bought it with 4.10's in the rear and 27" tall tires. The car shook like heck when the clutch was released. I read some post's about using the M20 vs M21. So I found an M20 that was closer to my Z's build date and istalled it. Really made a big, big difference in how the car launched. The shake was completely gone and the car moves alot quicker when I release the clutch. Since then I have replaced the Weld wheels and 27" tires that were on it with rally wheels and 26" tires, removed the Comp. Eng. traction bars with J-bolts and my Z is much smoother on take off.

My car has 3.73's.
Copo Pete: Please elaborate.

If you replace your M21 with a M20, you will think you have added about 50 horse to the motor. The difference is unbelieveable. I run a M20 with 4:10's in both my Chevelle and Nova. The Chevelle picked up 2 tenths in the 1/4 mile with no other change than the tranny. You will NOT be sorry! The car will be much more enjoyable to drive. Especially in the city with lots of stop lights!
Peter

Thats what I was looking for. I know the close ratio is "more cool" to drive, but what is more pratical for the street. What was the % of '69 Z's that got M20's?

The point Scooby is making is that you may find yourself having to shift out of 1st gear too soon with the M20 and rearend at 3.73 and numerically higher. You may not like that for everyday driving (think "creeper" gear).

Then again, if you're on the strip often like COPO says, you'll like the pickup you'll get in ET. It's all about how you'll use your car most.

I scanned the net for a short while and didn't see any production numbers on Z's by transmission type. Someone on this site probably knows.

Originally posted by ckaram:
The point Scooby is making is that you may find yourself having to shift out of 1st gear too soon with the M20 and rearend at 3.73 and numerically higher. You may not like that for everyday driving (think "creeper" gear).

Then again, if you're on the strip often like COPO says, you'll like the pickup you'll get in ET. It's all about how you'll use your car most.

I scanned the net for a short while and didn't see any production numbers on Z's by transmission type. Someone on this site probably knows. Nope, compare my 95 Z28 6 speed with 2.62 first gear and 4.10 rear gears, not like a creeper gear at all. 1st gear is like a monster out of the hole. It's my everyday driver and I've put 182,000 miles on it since 95.
Go with the M20, you'll love the 2.52 first gear.
Ron

Chevy wouldn't have made the volume of Z's with the M20 if there wasn't some value to it drive-wise...

To each his own graemlins/beers.gif

I too am going to have to go with the M-20.

I have a Nova with 27 inch tires and 4:10 gears. I had a M-21 for a while and hated it......getting started from a light required a nice balance between slip or kill it.....an M-20 with its lower first gear you can almost just let off the clutch without touching the gas and the car will go.

The M-21 and M-22 was originally made for road racing...hence the very close ratios. They also worked great in big blocks that had alot of torque to get moving.

For your application I would definatly go with the M-20, you will enjoy it a ton better.

Hope this helps.

An M20 with 3.73's is what I run. It's a rocket. I switched back and forth from the 20 to the 21 and I like the throttle response out of the hole and general driveability much better with the 20. Second gear advantage 1.88 on the 20 vs. 1.64 on the 21. Third gear 1.46 on the 20 vs.1.28 on the 21. The only disadvantages I can think of is the jump from third to forth is wider on the 20 but you have the weight moving by then so it's not that noticeable and the other disadvantage is the price of tires these days!
Just my opinion!
Jim

Great info guys! Just what I was looking for. Thanks.