Clint,
What will you be using your car for? Mostly a street car, drag or road racing?
Here's the ratio difference:
M-20: 1st 2.52, 2nd 1.88, 3rd 1.46, 4th direct drive
M-21: 1st 2.20, 2nd 1.64, 3rd 1.28, 4th direct drive
If you go with 3.42 rear end gears, I'd use the M-20. M-21 would be better suited for lower gears like the 3.73's you're considering. When I bought my Camaro in 89' it had a M-21 with 3.08 rear gears and was a dog off the line.

 

The tooth count on the close-ratio M-21 makes it slightly stronger, but the wide ratio M-20 provides more torque multiplication.

M-21's were typically installed on cars with 3.73 rear gear or higher (numerically). If you run the 3.42's go with the M-20.

In fact, I would go with the M-20 with the 3.73's. The engines we build today have a much greater useable RPM range, and the need for close ratio stuff isn't as necessary.

If you're looking for a very sturdy Muncie or Muncie parts, including the SuperCase, here's a good place to call:

www.autogear.net

FWIW

 

Thanks for the fast replies and the detailed explanation on the differences. From both of your replies it sounds like the M20 is the way to go.

Thanks,

Clint

 

I give another vote for the M20 for your setup.

 

M20 on the street do to the 1st gear ratio.

 

What HP are your plans for the BB402?

 

Jody,

It should be a mild build but I'm hoping to get around 375 - 400 hp out of it. I just had the block bored .020" over and using Keith Black HP 160 pistons, heads are cast iron #3964290 and using a Comp Cam XE268H cam kit, Edelbrock Performer RPM, 750 Holley, and Headers. Any ideas where I might be with this combo?

Thanks everyone for your help,

Clint

 

Sorry Clint, didnt mean to hijack your thread. Maybe this will help with your decission before you spend the money. Maybe your car will be better in 1st being a BB having the extra torque and a smaller cam?

Jody,
Can i just change out 1st gear in my current m-20, i just took out a Saginaw and the m-20 was totally rebuilt already have $1,100.00 in it

 

Jody,
Can i just change out 1st gear in my current m-20, i just took out a Saginaw and the m-20 was totally rebuilt already have $1,100.00 in it

Sorry Don,

The M22Z must be used as a complete gearset when doing a conversion.

 

I've got an M-20 with a 3.73 ratio...works very well--not planning to make any changes!