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Old 01-22-2015, 03:27 AM   #1
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Default Circuit racers - list your diff and power.

Interested to know what diff ratios you guys run for circuit racing / track day cars....Cars where street use is not a consideration.

I have been running a 3.9 but wonder if a 4.3 might be better for me...I want better spool earlier in the corner.....am slowly working on the turbo, tune, manifolds etc but in the meantime I have some races to try and do well in.

1600 Turbo - 250 HP at wheels
6 speed and 3.9 (to date)
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Old 01-22-2015, 05:06 AM   #2
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200hp NA, 5sp and 4.875. A 6sp would not hurt at all and my tracks are small enough to not reach 7500 with that combo.

The higher gears you use (in a standard gearbox) the shorter the drops are, and you should spend as much time as possible on the 1:1 gear (for reduced loss for NA and for strength for FI).
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Old 01-22-2015, 06:17 AM   #3
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170whp and 4.1 OSG with a 6 speed.

We must run a factory Miata ratio under our rule set, and the 4.3 doesn't suit the tracks I run most often. So I run a 4.1.
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Old 01-22-2015, 08:19 AM   #4
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'96 1.8, 240-250whp, mostly track but not gutted, running a 4.10 and 5 speed, 7200 redline.
Rear end gear needed for tracks varies by track. At Sebring it could use a 4.44 to keep a higher gear in T7 and still not redline in the straight in 5th. At Homestead-Miami, Road Atlanta, and Roebling Road Raceway the 4.10 is about right to approach redline in 5th on straights. At Daytona a 3.63 would be nice because I hit the limiter far before the straights are over.

I think a 3.63 and 6-speed would help all of my issues and give me the best of both worlds. I just got the Supertech light double springs so I could sightly raise my redline a little but I know that's hard on the engine. I did it more for longevity.
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Old 01-22-2015, 01:37 PM   #5
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cool thanks for the info.

On a side but related question (six shooter reminds me)...after a few engine woes we are now temporarily using a NB 1600 low mileage engine (sub 30k) while my built motor is RE built....as the NB 1600 has solid lifters what do you guys suggest as a rev limit?

Im a bit worried the 4.3 might be slightly short for my main track....gonna be close but if I can rev the motor pretty hard/high on the end of the main straight it would be probably be okay....
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Old 01-22-2015, 04:28 PM   #6
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1. 6 / 280whp. 4. 1 stock. Considering 4. 3 if found cheap.
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Old 01-22-2015, 05:27 PM   #7
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Quote:
Originally Posted by mx5-kiwi View Post
On a side but related question (six shooter reminds me)...after a few engine woes we are now temporarily using a NB 1600 low mileage engine (sub 30k) while my built motor is RE built....as the NB 1600 has solid lifters what do you guys suggest as a rev limit?
The NB6 runs shim over bucket as OEM, so that will be your limiting factor. 7500 should be safe enough.
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Old 01-22-2015, 05:33 PM   #8
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250hp/330hp(12psi/25psi) '90 1.8 with a 3.63 R&P with a 6-speed. Can't imagine the car with anything other than the 3.63 at my power levels. Suits it perfectly.
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Old 01-22-2015, 07:13 PM   #9
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'95 1.8l, 230-260whp, 4.10 5spd. 7000rpm redline
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Old 01-23-2015, 03:02 AM   #10
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210whp, 5spd, 4.10, 7200 redline

Any more power and I would need the 3.90 at the track with the longest straight around here. At a couple of the other tracks a 4.30 would benefit me.

-Ryan
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Old 01-23-2015, 03:25 AM   #11
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I ran a 6-speed and 4.10s at 220-270whp, a 3.909 at 300-350whp, and I've also used a 3.636 at both 250whp and 320whp. All with a ~7400rpm limiter. The 3.909s were perfect at the higher power levels, and the 3.636 was doggy (IMO) with only 250whp but pretty good at 320whp. At WSIR and ACS in California, the 3.909s were not enough gear with 350whp.
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Old 01-23-2015, 07:25 PM   #12
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Quote:
Originally Posted by Savington View Post
I ran a 6-speed and 4.10s at 220-270whp, a 3.909 at 300-350whp, and I've also used a 3.636 at both 250whp and 320whp. All with a ~7400rpm limiter. The 3.909s were perfect at the higher power levels, and the 3.636 was doggy (IMO) with only 250whp but pretty good at 320whp. At WSIR and ACS in California, the 3.909s were not enough gear with 350whp.
Exactly my experience. You only ever use 3rd through 6th on any track with the 6 speed.

I however have dual use for my car and to make a usable 2nd for autocross I needed the 3.636 and an 8400 rpm rev limit to make it work well. At 320 hp on track in most cases the 3.909 is the gear to have though and only a few corners on different tracks the 3:636 is slightly better. I’m also very close to topping out 6th gear with a 3:636 at about 7800 rpm on one track I run. Pacific Raceway but as it tapers down off the end of the power curve nearing the 150mph mark it stops accelerating much anyway. Lap time seems to be much more closely related to the speed carried through the turn 2 sweeper past the end of the straight than the top speed at the end of the straight. I found that out after only downshifting to 5th gear for turn 2 I’m still thinking the trade off at the exit of turn two with the 3.909 would be better than a couple mph top speed at the end of the front straight with the 3.636 though.

Getting ready to try a Quaife wide transmission however. So I will be running it with a 3.909 for track and a 4.778 for autocross with that.

Last edited by bbundy; 01-23-2015 at 07:37 PM.
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Old 01-26-2015, 01:50 AM   #13
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5 speed with a 4.778, 131hp.

I have a 4.3 R&P that i will put up for sale in the morning thats just sitting and im sure someone here can put it to good use, because its currently taking up valuable shelf space in my garage.
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Old 01-26-2015, 03:50 AM   #14
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6spd with 4.1 230whp 7000rpm limit, stock msm turbo so spool is early and consistent I use 4-6 almost exclusively, rarely need 3rd gear unless it's the really tight stuff.
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Old 01-26-2015, 12:07 PM   #15
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232whp then 298whp, both 170tq, 4.10's & 6 speed.

I run mostly at Sonoma. I don't remember using 6th anywhere. I have 8k redline so that skews the numbers a bit.
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