Scrrem

I am still going through the #'s on my 69 BB coupe. It has a #'s matching M21 and a 3.08 rear axle. I had someone tell me they only put the M21 in cars with 3.36 or higher gears. The rear has the proper BB end cap straps, but that doesn't mean it wasn't replaced. Is this an uncommon combination?
Rich

Revi

Not sure how common it was, but it was an available option.

Page 90, https://www.gmheritagecenter.com/doc...t-Corvette.pdf

Easy Mike

The 3.36 may have been standard for the M21, but the buyer could specify optional gear ratios, so a 3.08 M21 could have been possible. What is the code on your differential?

Gordonm

If that is truely the combo you have it has to be a dog off the line. Your overall gear ratio in 1st gear would be 6.77. You should be at least in the 9s or 10s. You can probably do about 80 mph in 1st gear. While not very desireable for performance I guess someone could special order it. Most 2nd gear over all ratios are in the high 6s or low 7s so its like you would be starting off in 2nd gear. Lots of clutch slipping to get rolling.
IMO I think the M21 is a terrible street transmission. The close ratio was designed for road racing to keep the motors in peak HP The M20 wide box was a much more street friendly transmission. Fire suit on for this

JohnnyQuik

Well Hagerty thinks the M21 adds $1800 to the value of my '70 big block, so I guess I'll keep it. With 500 lb-ft of torque on tap, the first gear ratio doesn't matter so much.
https://www.hagerty.com/apps/valuati...vette?id=47218

Z06LMB

East Bay 68

When I had the m-21 rebuilt for my 68 L68 (427) my guy recommended the m-20 2.52:1 first gear, he said the regular m21 first gear could be a pain to use around town with my 3:36 rear diff. So that's what I did, made sense to me, we'll see for sure at some point .

Scrrem

2at41769w

Scrrem

Yeah, I assume the previous owner envisioned the issue you described with the performance and installed 3.73 gears which help the launch off the line while retaining the #'s matching M21.

62corvette

Yeah--they also say +$2500 for power steering. Fat chance.

car junkie

Lot's of off the wall numbers there. 119K for a number 1 car! Maybe the restoration cost but not market value. Value guides for the most part are worthless. Maybe this is a gimmick to over insure cars for higher premiums.


Value Adjustments

+$1,800 for close ratio 4-spd. -8% for auto trans. +$200 for pwr wndws. +$2,000 for a/c. +$1,850 for hard top. +$900 for custom interior (leather). +$2,500 for pwr strg. +$1,500 for tilt/tele strng whl

Easy Mike

AT is the code for 3.08:1.

rkeller411

m-21 needs 3.73 or 4.11 to truly enjoy the launch of the car. Orherwise m-20 is better at lower gears. I had previous owners change out factory 3.73 to 3.08 in BB 396/375 I put it back and it was so much better.