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Hola. I was thinking of holding off on building my 400 till the summer when i have more time. I was thinking of swapping my turbo 350 for a 4 speed. What's the difference between an M20,21, and 22, and which would be best. ( the car was originally a manual so it still has the clutch and all that.)
 

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Those numbers refer to different gear ratios, for one thing. Most of the Muncie M21's & M22's had a 2.20:1 first gear ratio. Those require more rear end gear--such as 3.73 or more. You want one with a higher number 1st gear, to give you more take off gearing when using less rear end gear. Most M20's had a 2.52:1 first gear. You can Google up lots of Muncie info.

http://www.5speeds.com/muncie2.htm

Up thru early '74, GM used the Muncie 4-speed. Sometime during '74 they switched over to a Borg Warner Super T-10 4-speed. So, your '75 would have had the ST-10. Looks like most of the ST-10's used in Pontiacs(designated code M21) had a 2.43:1 first gear ratio, according to this site.

http://www.pontiacpower.org/BW.htm
 

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a 2.43 ST-10 would work well with something like a 3.31 for a street driven car.I found the 3.55 a little too low for much highway driving.A 3.08 would require more slipping of the cluth leaving a stop.Tom
 
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