Quote:
Originally Posted by kmbkk
So I’m strongly considering this swap. I have a boosted (turbo) manual FRS, tuned on E85. I found an M85 LSD locally for a decent price, with 180k miles on it. My car is a DD, not tracked. I’m trying to decide between the 3.7 and 3.9 LSDs. Any big concern going with the M85?
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180k miles is quite a bit.
How much power are you making?
What is your expected reasoning for wanting a 3.7 or 3.9?
Is there a speed you normally pull to on hwy pulls? Or would like to be able to pull to? You want the top of your 1-1 gear (5th) to be right at or slightly over whatever speed you want to go to as fast as possible to that speed, but the less aggressive (numerically lower) will be slower at any given speed below the more aggressive (numerically higher) ratio top of 1-1 gear (5th).
assuming stock 24.6 tire diameter.
A 4.1 will be fastest to 130mph
A 3.9 will be fastest to 137mph but slower than 4.1 to 130mph
A 3.7 will be fastest to 144mph but slower than 3.9 to 137mph and 4.1 to 130mph.
This is also assuming starting speed of 0mph and not factoring any traction advantages or disadvantages one has over the other, just purely potential mechanical torque. Also each will have specific speeds in which they are faster. Like a 3.7 in 3rd will be faster than a 4.1 once it’s shifted to 4th but then lose the advantage once the 3.7 also shifts to 4th etc. so it can get pretty technical and complicated if you want to pick the exact best one for your needs.
Personally to simplify it i just go off what i want my 1-1 gear to hit so i can do pulls to that speed as fast as possible. That’s why i chose a 4.3 with my 25” diam tire i can hit 126mph in 1-1 gear for fun 40/60mph - 120/125mph pulls as fast as possible.
You could technically calculate all of this off your 4th gear and then sort of treat 5 as an overdrive in a sense.
Which would be
4.1 - 107mph
3.9 - 113mph
3.7 - 119mph with a taller tire could be really good for 120mph pulls.