what is typically more desired the m20 or m21?

[Brandon T.]

I know all the differences but seems some perfer the m21 to keep the rpm's up on road courses and some perfer the m20 for more grunt but overall which is more desired?

[Terry D.]

Re: what is typically more desired the m20 or m21?

Brandon
Depends on the kind of driving you do the most.
Terry

[Brandon T.]

Re: what is typically more desired the m20 or m21?

So, when I find a car with an m20 I guess I shouldn't let that turn me off from it? I have never driven an m20 our 68 is a m21. I just always think of the m21 as a plus and maybe I shouldn't.

[Doug F.]

Re: what is typically more desired the m20 or m21?

my 63 had a close ratio with 336 gears and even with the 340 hp it was a pig off the line, i put in a wide ratio and it is now a pleasure to drive, the bottom line is less than a $1000 will take care of almost any trans issue, even if it's a 3 speed and maybe even an auto, on a $50,000 car, that ain't too much

[Michael J.]

Re: what is typically more desired the m20 or m21?

To me the choice is all about what was in the car originally.

[William C.]

Re: what is typically more desired the m20 or m21?

Back in the day.. I had a friend ask me how to order his '69 Z-28. I told him to spec the 4.10 rear with a wide ratio trans. The car was a ROCKET! went thru the hands of three of my buddies and about 50K mikes before I lost track of it, and it still was one of the quickest Z-28s around.

[Patrick N.]

Re: what is typically more desired the m20 or m21?

Brandon what rear does your M21 have? I have an M20 + 3.08 in my L68 so it is nice to drive on the highway. (My tank sticker shows my car originally had a 3.36 rear). When I looked into this, the general understanding I got was if you have a 3.55 or higher rear you should match that with the M21 but will high-rev on the highway. On the other side, 3.55 and lower the M20 is a good match. People do seem to like the lower M20 first gear 2.52 over the M21 2.20 off a launch but that is matter of opinion i guess. I don't think shifting from 1-2 and 2-3 is different between the two. 3-4 on the other hand goes to the M21.

[Joe R.]

Re: what is typically more desired the m20 or m21?

Patrick,

I've heard that the 3:36 is the best ratio for street / strip with a big block. I assume that's with the M21, since it was the only transmission available from GM on the SHP engines.

Joe

[Tim T.]

Re: what is typically more desired the m20 or m21?

In the early 60's the M21 was developed for road racing as it kept the same change in RPM from gear to gear when down shifting (heel & toe) and thus was easy for the drivers to maintain the RPM's without much thought.
In that era, there were many less miles of Interstates, thus much less high speeding driving and premium "real" gas was 25 cents a gallon.
Today is a different world. Yes, if you are mostly road racing, use an M21 with approperate rear end for the road course you are racing on.
If you are driving on the current roads ( as most of are) and racing little (or none) then you must decide what is most approperate for you.

I currently have a 63 SHP 340 HP with a M20 and a 3.36 posi and couldn't be happier. Good off the line performance and good Rpms (3200) when crusing on the interstates. Mine was originally an M21 with 4:11 rear in its day and was road raced. And yes, out of habit, I do heel and toe and find it east to match RPMs.

Hope this helps,

Tim

[Michael M.]

Re: what is typically more desired the m20 or m21?

It also depends on what transmission you like. The M20 (wide-box) transmission is good for three series rear ratios (3.36:1, 3.55:1, 3.70:1) because the first, second, and third gear ratios in the tranmission are lower. So it feels like your rear end ratio is a little lower than you have. I prefer the M21 (close-box) transmission. The M21 goes good with 3.70:1 and four series gear ratios.

[John D.]

Re: what is typically more desired the m20 or m21?

My 67 L79 has M20 and 3:36 and I think its about the best all around street combo...

[Brandon T.]

Re: what is typically more desired the m20 or m21?

Someone asked whats in my car our 68 is an l36 427 with the m21 trans and 3.08 rear end.

[Philip A.]

Re: what is typically more desired the m20 or m21?

My understanding is that prior to 1966 you did not have an option choice between M20 or M21. If the car had a SHP motor then it got a M20 WITH CLOSE RATIO GEARS; non-SHP motor cars got M20 with wide ratio gears.

[Patrick N.]

Re: what is typically more desired the m20 or m21?

Patrick,

I assume that's with the M21, since it was the only transmission available from GM on the SHP engines.

Joe

Hi Joe, Do you know if all 3.36's came with M21? My tank sticker is a postage stamp but the one piece that is clear is the 3.36 rear. Never sure if the car came with M20 or M21 originally but an M20 was what was in there when I rebuilt it. At some point I could see myself putting the 3.36 back in my car as that is what it came with and could easily swap out the Cluster gear and Input shaft to make it M21- someday though

Best,
Pat

[Erik S.]

Re: what is typically more desired the m20 or m21?

my 1968 L71 still has the original 4.11 and M21. With my current restoration I will convert to 3.36 (keeping the original 4.11) but will also swap to M20 for at the same time for better driving experience purposes