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      12-08-2012, 12:43 PM   #15
Finnegan
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Drives: Z4M/. Z3M, E36/46 M3
Join Date: Oct 2008
Location: Teaching the dog to slalom

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Quote:
Originally Posted by Gtfour View Post
I had a 3.91 with 2-way Carbonetics LSD installed on my car this summer.

The shorter gear ratio compare to OEM 3.62 was a nice upgrade but the LSD was too agressive for me.

I recently installed a 4.10 pinion/ring in my OEM diff with the M-Variable LSD. I love it! It's so rev-happy compare to the 3.91. Even with my ESS VT-445, the 4.10 is a great match. I can't wait to try it next summer on the track.

I still don't understand why people choose a 3.91 over a 4.10???

Especially when NA
Hmmm....Should I have gone with the 4.10?

If I'm doing this correctly, and assuming stock sized tires:

3.62 -> 3.91 = 8%
3.62 -> 3.91 = 13.25%

3.62 -> 3.91 8% with 265/35 tires (2.81%) = 10.81%
3.62 -> 3.91 8% with 265/35 tires (2.81%) = 16%

I went with 3.91 because FI is a future enhancement I plan at some point; and my rear tires are 265/35, which already gives me close to 3% more wheel torque.

But my math assumptions could be off--but FI is in the future. Like Caddy, I figured off speeds (current and factoring in future improvements in times) and didn't like the shift points req'd for my local track with the 4.1 (ran numbers with and without tire factors).

I went the R&P direction because there's a good local shop (off road / diffs only) that will do the work for under 200. They did a great job on my friend's E46 M3. R&P was 545 shipped (Bobby Rahal BMW). Those costs, plus additional parts, fluid, REM and polish, and install are a better deal than an entire diff swap. Plus I know my diff's history...

Last edited by Finnegan; 12-09-2012 at 04:36 PM..
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