Why not 4.10's on a MT-82 Boss/CJ NA Combo

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SmokeGreyHatch

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After doing more math on 1/4 mile speed/tire size/gear ratios, a 4.10 seems perfect for a car on a 28" slick trapping less than 125 mph. Having a CJ intake, I have zero drop off in power compared to the boss which peaks around 7k and carries it to 8k (but there is a drop of 10-15 rwhp). Mine keeps climbing even at 8k. I want a gear that puts me into that powerband the quickest.

The MT-82 is 1.32:1 in 4th. According to the Richmond gear calculator:

Gear & Transimission Calculators - Richmond Gear

Trapping 125 mph on a 28 inch slick (growing 0.5 inches down track) would put me through the traps at 7976 rpm. At full weight I know my car won't trap that but should ET better with a 4.10 because I'm in the usable powerband more often. Even at 120-122 mph which is what I'd expect it to trap, it'd turn 7657-7784 rpm through the traps. I've turned this car 8k datalogging and I know it's much harder on it, but we're not talking road course hard. It's a brief period. The 28 inch slick may even grow more than 0.5 inches which would just result in less rpm through the traps.

Is it the fuel mileage aspect? At 70 mph it's still only turning 2411 rpm in 6th gear.

Thoughts from anyone running 3.73's-4.10's in a boss/cj NA car? Should I just throw in the 3.90's I already have staring me in the face?

Thanks,

Matt
 
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Blazer707@TBR

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You will end up trapping more then you think.

Ill be going 3.90's also, its a good all around gear for the M6's, 28" slicks and high RPM shifting.

-Thomas
 
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SmokeGreyHatch

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Blazer707 said:
You will end up trapping more then you think.

Ill be going 3.90's also, its a good all around gear for the M6's, 28" slicks and high RPM shifting.

-Thomas
Click to expand...

sean04 said:
I think your going to end up trapping higher than you think.
Click to expand...

Thanks guys, that would be nice. I'll just see what the dyno says and go from there based on my weight, but I'll probably just stick with the 3.90's for the wiggle room.
 
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SCalla1384

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Lol. Nah I think 4.30/4.56's would be over kill. I think you'll be trapping alittle higher then you expect. I'd go with 3.90's. 4.10's would be a good idea, but I think leaving yourself alittle bit of room is better.

Seems like these cars make peak power for a solid few hundred rpm. Not just peaking. So if your making peak power from 7000rpm til 7800rpm, I don't see the need to go with 4.10's. the transmission gearing pretty much keeps you in the fat spot of the powerband anyways.
 

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