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Discussion Starter · #1 · (Edited)
I went to 21st Century Musclecars for a bullet-proof transmission rebuild of my MN-6. My factory tranny has been great for 41K miles and I was geting nervous about its lifespan. During the rebuild I replaced the main gear-set and added a LPE hardened, micropolished main shaft, etc. The gear-set retains the factory MN-6 1st thru 4th ratios but changes 5th from 0:74 to 0:80 and 6th from 0:50 to 0:70. For a street driven car this makes 6th gear a more usable ratio as you can see in my before and after comparison. I made notes before the changes and on my way home noted the RPM vs. speed differences from stock. I run stock 3:42 rear gears and the speeds were read off of my speedometer so they are approximate but close.

RPM 5th Stock 5th :80 Ratio 6th Stock 6th :70 Ratio
2000 63 MPH 57 MPH 94 MPH 67 MPH
2500 78 MPH 71 MPH - 83 MPH
3000 93 MPH 83 MPH - 98 MPH

I did not test stock 6th at 2500 or 3000 RPM as there was too much traffic but we all know it is up there. Also, the lower 6th gear at 6000 RPM gives approximately 196 MPH. So far I am very happy with the gearing and I will get the cobwebs out of the 6th gear slot in the shifter gate.
 

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Awesome info! I'm currently investigating changing my gear ratios so this is very timely.

Perhaps a naive question, but why do you have the MN6 vs the MN12 tranny? Don't you feel the gearing is really spread out...especially 1st?
 

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I think his car is an original FRC - those shipped with the MN6 on board.
 

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Rick,

Will you or someone else post the differences in the M6 and M12 ratios from 1st thru 4th? I think with the power I (and certainly you) am putting down the M6 would be the better trans. I believe the M6 has less aggressive gearing in 1st and 2nd, but the M12 has a large spread b/w 3rd and 4th which I think stinks.

I don't need torque multiplication in 1st and 2nd... but a closer spread from 3-4 would be nice, and of course what you did for 5th and 6th is definitely way more optimum than stock!

Good report! Glad you like it so far.
 

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Very cool.

I have my new trans going in here soon...just waiting on Page for the call to bring it in (doing the same, bolstering everything while we're at it).
 

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Discussion Starter · #7 ·
WA 2 FST said:
Rick,

I don't need torque multiplication in 1st and 2nd... but a closer spread from 3-4 would be nice, and of course what you did for 5th and 6th is definitely way more optimum than stock!

Good report! Glad you like it so far.
Wes, with a moded NA Vette in the 400 plus RWHP or FI car with higher HP you are correct, the last thing we need is lower gearing. Hell, a 4-wheel drive car could beat us to 3rd gear. IMHO, the MN-6 is much better than the MN-12 because of its higher gearing up to 4th gear. However, we are not pulling sticky slicks out of the hole. The closer spread you refer to between 3-4 is what I now have from 4-5 and 5-6. When do you do 200MPH around town in 6th gear? Why not take that gearing and put it down where it will do the most good excluding mileage? Mileage, who cares? Only GM meeting sticker mileage numbers. The gears for the MN-12 are, 1st:2.97, 2nd:2.07, 3rd:1.43, 4th 1:1. MN-6 are, 1st:2.66, 2nd:1:78, 3rd:1.30, 4th 1:1. The added cost to alter the ratios in the tranny with the new gears is the cost of the gears themself. It can only help and not hurt to lower 5th and 6th.

RC 45- You are correct, my car is a 99 FRC that came with the MN-6. It held up admirably well for almost 6 years with high torque but no drag strip launches. Not to rag on the MN-12 but the MN-6's have a better longivity record from what I have learned from the tranny builders. That is why I never went to a MN-12. I know this is not a popular thing to say on the ZO-6 VETE forum. I am just the messinger; others may disagree. ALL high HP moded street driven cars should benefit from this mod; unless MPG is important. :thumb:
 

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Good move Rick ;)
I changed 5/6th ratio's 10k miles ago and it is a wonderful mod. IMO the car should have come from the factory this way.
 

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Good post Rick... I may need to look at this mod for my car. Thanks for all the info.


Chris
 

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At $2.80 a gallon I enjoy getting 30+ mpg on the freeway with my M12. I agree that 5th should be taller, I put a lot of thought into this and in my opinion optimum gearing would be to have a "taller" first gear.

If first wound out to say 60 mph, 2nd- 90mph, 3rd 120 mph, 4th 150, 5th 180mph (.80) and keep 6th at .50 for fuel economy. 180 top end is plenty, in my opinion.

Kinda like gearing on a bike, with street tires the tall first gear would help with traction.

I run nitto 555 R2's, no wheelspin in first with 420 rwhp and stock M12 gearing.....
 
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