Swap in 3.9s or keep 4.30s for Autocross? - Miata Turbo Forum - Boost cars, acquire cats.

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Old 02-16-2009, 02:58 AM   #1
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Default Swap in 3.9s or keep 4.30s for Autocross?

I have a standard 5 spd and the stock 4.30s in my '99 right now. I have a nice set of 3.9s and the speedo gear along with a torsen 1 ready to drop in to replace the 4.30s with the open diff.
Are the 3.9s going to suck for autocross or am I going to love it. We have relatively large courses and I frequently bounce it off of the rev limiter in 2nd with the current setup.

So who's run both sets and can tell me if I'm going to love it or hate it. I'm assuming that the car is around 240 hp right now for those that take that into consideration and I reach peak boost around 3100-3200 rpm in 4th gear.
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Old 02-16-2009, 03:02 AM   #2
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You can always change the "gearing" in the tires. When I ran CSP I used 225-50-13 Hoosiers and they drastically shortened the gearing. So, with the 3.9 you gain some streetability, an LSD, and possibly some highway MPG, and by sticking with the old diff you get nothing but a shorter gear that can be done with a tire swap.
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Old 02-16-2009, 03:07 AM   #3
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That's a good idea/option. I'm not looking to be super competitive anyway. I'm always gone for most of the season and never have any sort of chance to make enough races to be in it for points. I'll be running my 225/45 RT615s this season both on the street and at the races.
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Old 02-16-2009, 08:49 AM   #4
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Definitely go with 3.9.
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Old 02-16-2009, 09:24 AM   #5
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I think you'll love it. I can't stand an open diff, especially at autocross. You're making enough power that the longer gears won't really slow you down. On the other hand, if you think you'd prefer a 4.10 torsen you shouldn't have a problem getting someone to trade. I know I'd do it in a second.
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Old 02-16-2009, 09:44 AM   #6
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There are a lot of variables that go into the decision. But for street tires, I would say the 3.9 is the best bet. The 4.30 open is going to turn all your torque to smoke from the inside rear tire as you exit every corner. For 225 race rubber the 3.90 is probably still okay. You are right on the edge for 275s. It would depend a lot on how often you encounter tight corners and whether or not it dropped you out of your turbo's power band on a regular basis.
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Old 02-16-2009, 01:59 PM   #7
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Dibs on your 4.3 if you decide to sell it?
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Old 02-16-2009, 02:04 PM   #8
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i have 3.9 and i like it fwiw. Better even as stock. I too had the problem with running out of 2nd gear and thats why i jumped on a cheap gearset. Its better now.
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Old 02-16-2009, 02:16 PM   #9
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Cool. Thanks guys I'll swap in the 3.9s and keep my 4.30s around just in case I don't like it.
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Old 02-16-2009, 03:23 PM   #10
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Swap in the 3.9's and let me know how you like them. I think I'm gonna end up doing the same thing with a RX-7 LSD one day if I don't swap in a ford 7.5" first.
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Old 02-17-2009, 09:50 PM   #11
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I've run 2 auto-x's with my 5-speed + 4.10's...I say keep the 4.30.
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