I am converting my '70 big block from automatic back to 4-speed, which is what the original car was. The car has the original 3.08 HD,positraction rear end. My local tranny vendor tells me that with this rear end I should put in a wide ratio, M20 for better out-of-the-hole performance. A couple other people tell me that an M21 would work fine and that an M21 close ratio is what was probably original to the car. Please let me know what you think. This is obviously a big and expensive decision. Thanks.
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Re: Correct differential and tranmission combinati
Dave -
The '70 powertrain chart in the Spec Guide shows the 3.08 axle on a BB as standard with M20, with a 3.36 available as the "Perfomance" option; the 3.36 axle was standard with the M21, with the 3.08 available as an "Economy" option. Obviously, starting torque isn't an issue with an M21 and a 3.08, but clutch-slipping wear might be. -
Re: Correct differential and tranmission combinati
Dave----
If I were going to use one transmission or the other and didn't already own either one, I'd recommend going with the M-20. That's the best transmission for street applications, anyway.
In your case, it's not absolutely essential with the 3.08:1 since you're dealing with an LS-5. However, since you have to buy one or the other and the price of either is going to be about the same, I'd definitely go with the M-20. Absolutely. Positively.In Appreciation of John HinckleyComment
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Re: Correct differential and tranmission combinati
Dave
I just converted my 69 m-21 with a 4:11 to a 3:36. It works fine, but just a little high in 1st gear. If I were you and I was going to keep the 3:08 I would definately go with the M-20. I think that would also work a little better with mine, but I am done for now.Good luck.
TomComment
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I have one
Have a 1970 454 with 3.08 and M20 with A/C - a nice cruising gear, doesn't require excessive clutch slippage.
Good luck,
MarkComment
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