For the Big Block Buick guys 400, 430, 455 do you prefer a M-21 or M-20. It sounds like rearend Gear may swing your decision. I have the m-21 rebuilt and ready to go but there is a M-20 near by that needs gone through. Rear gear choices 364 and 323. I am leaning one way but will listen to arguments.
Buicks have lots of torque. Even with my 350, the M21 seemed too close. I kept skipping gears. 1st to 3rd gear more than anything. I can imagine it would be worse with a 455 torqued up. In my 350 car, I swapped in an APE M23Z. It has a 2.98:1 first gear. With 3.08 rear gears, it really makes it more enjoyable to drive. No more skipping gears and the RPM drop is better between gears. If you have 3.23 gears, the M21 will be sluggish in 1st. It might feel like you are needing to slip the clutch more. With 3.08 gears, it felt like I was taking off in second. With the M20, it will be an easier launch. With 3.64 gears it will likely be better but I'd still opt for the M20 over the M21, given the choices.
Seems Buick missed the mark going w/the M-21 in every application (esp. '71-2 350 w/ std. 3.08). I assume because of the torque they chose to, but the other divisions reserved that trans. for the higher ratios. It certainly seemed to work well w/the BBB/3.64. & even my '70 350 w/3.23 (then 3.42, 3.73). Never driven a BBB/M21/3.42 or 3.23. Depends what you're plans are for the car. I think in this case any of the combos will be ok. Bit of a PIA but I think the only way to know for sure is to decide on the rear gear & try both trans. If it were me, I'd probably go 3.64/M-21 to start.
You could use the M20 with either rear gear ratio. The M21 will perform better with higher numerical gears so if planning on using this trans, I'd use the 3.64 only.
GM missed the mark with the Muncie ratios, period! It's nice that AGE has finally offered decent ratios.. I would go with the M20 gears.
I have extensively over the past 33 years used both the M-20 and 21 in my 455 car with 2.56, 3.31, 3.42, 3.73 & 4.11 rear gears. The M-20 is more street friendly due to the lower 1st gear. If you use the 3.23 you will like the M-20 better going stop light to stop light. If the engine has been modified with a larger camshaft and no corresponding compression increase it might not respond well when moving from a dead stop with the 3.23/M-21 combo. Keep in mind that the M-20 is the weakest of the Muncies. Torque will damage the M-20, I have stripped the teeth from 2nd gear more than once. If you use the M-20 REFRAIN from 3000 rpm power shifts. On the track there is no problem since you are shifting at high rpm where the torque is down. At this time my 455 car is 10:1 with small valve heads, a GM Stage 1 cam, M-21 and 3.42 gears. It is a dream to drive in any situation.
I am leaning toward the 3.64 and M-21 just because I believe that's the way GM built it. I am not getting rid of the m-20 or 3.23 so I will have options if I want to try the other down the road. The m-20 was in a 71 Olds cutlass with 3.42 gears. If I want to do any significant highway miles, I thought the 3.23 would help with rpm and drivability.
That's why I said "depends upon your plans with the car." 3.23 certainly would help on the highway. 3.42 might be the happy medium.
