Best OEM final drive for 5 speed T25/2554 DD/AutoX/HPDE - Miata Turbo Forum - Boost cars, acquire cats.

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Old 03-31-2015, 08:57 PM   #1
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Default Best OEM final drive for 5 speed T25/2554 DD/AutoX/HPDE

So, my car is daily driven, and will be used for 4 Autox a year or so, and HPDE like twice a year. 5-speed, and guestimated final numbers should be a round ~225ft-lb, 235hp.

I'm considering swapping my 4.30 to a 3.90 to have a little longer in each gear. I have an offer to trade my diff flat out, though mine has 72k on it, and the mileage on the 3.90 is unknown. Otherwise, his asking price is $500 without the carrier. Looking to hear from people on both sides, as most people I've asked their opinions are running a 4.10.

My fear is, as far as enjoying the turbo DD, I'll lose some of that *back of your seat* feeling at this power level. I expected fuel economy gains on the highway, but in reality, I will only be going from [email protected] to 3709rpm. So, not really all that much of a change.

AutoX I think will be benefited by having a higher top speed in 2nd gear.

I can't speak for HPDE, as my first event is in May.

So, Tl;Dr: What is your ideal final drive for a T25/2554 setup with a 5 speed, that will be daily driven, with bimonthly autocross time, and semiannual HPDE?
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Old 03-31-2015, 09:07 PM   #2
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I'm about to pull the trigger on a 3.9 r&p for like $90. Its no 3.63 but I think it will help with dding.
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Old 03-31-2015, 09:20 PM   #3
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I'd switch the 4.30 for a 3.90 if it was straight across that's for sure.
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Old 03-31-2015, 09:23 PM   #4
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True. That's a steal.
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Old 03-31-2015, 09:33 PM   #5
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Quote:
Originally Posted by aidandj View Post
True. That's a steal.
Is it? From what I'd heard with the 4.30 becoming Spec Miata legal they were going up in price.
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Old 03-31-2015, 09:34 PM   #6
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maybe.
but the 3.90 is better.
so who cares lol
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Old 03-31-2015, 09:36 PM   #7
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I liked the 4.10 with the 5spd in my similarly/slightly more powered 95. Doubt a 3.9 would ruin the car at all.
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Old 03-31-2015, 09:38 PM   #8
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thing is, when you turn the boost up, and you will turn the boost up, and break the 5sp, and you will break the 5sp, and get a 6sp, you'll be so much happier with the 3.9
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Old 03-31-2015, 09:54 PM   #9
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For autocross 5speed +turbo power plus DD and a few hpde not too ridiculously high RPM limit you want the 3.909. with the rev limit turned up above ~8000 rpm the 410 would be faster and better use of the power band.
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Old 03-31-2015, 09:56 PM   #10
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Quote:
Originally Posted by 18psi View Post
thing is, when you turn the boost up, and you will turn the boost up, and break the 5sp, and you will break the 5sp, and get a 6sp, you'll be so much happier with the 3.9
See the problem is, as I know you personally have made evident, stock rods and 5-speed are good friends. lol.
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