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Old 12-10-2013, 11:51 AM   #21
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3.9 + 6 spd is way short for my 265 whp.
Spend too much time shifting and too little time accelerating in-gear.
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Old 12-10-2013, 08:09 PM   #22
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Originally Posted by supersuckers View Post
Yeah, thought of that. So that leaves the 3.6 and 3.9. The 3.9 doesn't appear much different than the 4.1, and I've seen some state that you really want 300HP for the 3.6. So I'm starting to think the 3.9 + 6 speed may be the sweet spot for track gearing on a 250WHP car? I don't run the car above 7k, so that would get me to 145mph.
If it's a track day/HPDE car, it really doesn't matter. Either the 3.6 or 3.9 will drive fine. One will be faster than the other depending on the track config, but you're not talking huge differences in time here and if you're not *racing* then it just doesn't matter.

(If you are racing then it does, and you should look at your datalogs and crunch some numbers to figure out which rear end is better for which track -- it won't be the same for all tracks).

For SSM autox, the 3.9 is slower because it has too many 2->3->2 shifts on an SCCA-compliant course.

The simplest thing to do is buy a 6-speed and 3.9:1 Torsen. Put them in, see what you think. If you like the 3.9, sell your 4.1 Torsen and move on. If you don't, buy the 3.6 or 3.3 gears, swap them into the 4.1 housing, then reinstall the original diff (was 4.1, now whatever the new gears are) and sell the 3.9 for the same price you paid for it.

(my car has a 6-speed + 3.636 because the primary requirement is autox. I did the above, except that I decided to keep the 3.9 as a spare instead of selling it.)

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