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Discussion Starter #1
I am trying to figure out what rear ratio I may have in my C3. It is the original rear that came with a 327 300 hp M21 tranny. Is there a typical set up that it would have come with considering it seems to be the basic model.

Reason for asking..........
I have replaced the 327 with a 572 and need to make some adjustments to the rear end as the new engine doesnt rev like that 327. It gets to 60 at the drop of a hat but then it sounds like its doing 150 at 75 mph.
 

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I am trying to figure out what rear ratio I may have in my C3. It is the original rear that came with a 327 300 hp M21 tranny. Is there a typical set up that it would have come with considering it seems to be the basic model.

Reason for asking..........
I have replaced the 327 with a 572 and need to make some adjustments to the rear end as the new engine doesnt rev like that 327. It gets to 60 at the drop of a hat but then it sounds like its doing 150 at 75 mph.
Acording to my book, in '68 the 327 300hp 4 spd came with (2) possible ratios: 3.36 and 3.08 and the auto came with only the 3.08

Hope this helps
 

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jack up the car put a mark on the driveshaft. rotate the tire one revolution while counting the number of times the drive shaft rotates. easy ! dont be surprised if it rotates more than 3.3 times. back in the day 4:10 or even 4:56 was the norm for killer street cars.

good luck

paul
 

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Discussion Starter #7
You guys are a great help I appreciate the responses.

Maybe I should just ask. What transmission / rear end set up would give me the best match between launch speed and top end. I would like to be somewhere in the middle.


Engine is a 572 620hp 650 torque with 20in rims on rear. The hidth is about 28 - 29 inches from ground to top of tire.

Current tranny is M21
 

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An M21 is the close ratio trans and usually not used the base motors. Not sure if it could have been ordered with one, maybe. Anyway, the close ratio 1 st gear is a 220 and not the best match to a 308 or 336. With the torque of a BB it will work but SB's wouldn't have run all that well off the line with them. Usually the 21's and 22's came with 370 or 411 diff's. The M20 is a wide ratio and what came with the base motors when a 4 spd was ordered. Usually 336 was matched to them but some bought the "highway" gear which is a 308.

With all that said, you can toss it out since you're going with a huge engine. If you "use" all your power your diff and M21 will be reduced to trash in short order. 650 hp is well beyond a stock 10 bolt I wouldn't ever recommend a Super 10 bolt although I've built a few for guys in the 550-600 range. All depends on hook up and launching.
Muncies and ST-10s are rated about 300 ft/lb of torque if I recall. Most of the high power cars I build diffs for, these are over 600hp up to 1000hp, use 12 bolt conversions and built TH400's.

Here are some pictures of some abused 10 bolts and 4 spds.












Here is a ST-10













 

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What transmission / rear end set up would give me the best match between launch speed and top end. I would like to be somewhere in the middle.


Engine is a 572 620hp 650 torque with 20in rims on rear. The hidth is about 28 - 29 inches from ground to top of tire.

Current tranny is M21
A Tremec T-56 Six Speed (or derivative thereof)!!!:partyon:
 

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Discussion Starter #10
Thanks for all the help. Turns out it was not reaching wide open throttle. I made some adjustment to the pedal connection and it came up of the floor a lot. Made all the difference in the world.
 
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