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      01-08-2013, 07:37 AM   #45
pbonsalb
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Lower gearing (higher numerical ratio) in the diff means you will get less speed in each gear and hit redline at a lower mph in top gear. Raise the redline to 8600 and you will recapture a little of what is lost.

Go to www.diffsonline.com and select the rpm/mph spreadsheet under the technical tab. Plug in the E90M3 gear ratios, at least as many of them as you are interested in since it shows only 6, and the differential and tire sizes. Vary them to experiment and you can see rpm at various mph with various gears and tire sizes.
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      01-08-2013, 09:41 AM   #46
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Originally Posted by kaiv View Post
My times have been constantly improving thus far and without data acquisition it's difficult to say BUT I did notice one undeniable improvement at Auto Club Speedway: I can get the tach needle to hit 160 before slowing down into T1! The car would only do low to mid 150s before the bolt-ons. (to those unacquainted with the ACS Roval, it's a hp track)
Your car is awesome! Love the set up and hopefully I will see you at one of the SoCal track events. I've always wanted to do CalSpeedway and it looks like a really fun course. I'm going to Big Willow this Saturday (1-12) with SpeedVentures. I'm a newb and just beginning tracking.

By the way regarding your speed...I was just looking at my P-box data from my latest track event and couldn't believe how optimistic the stock speedometer is at that speed. On the back straight of Big Willow at the Dec 29 Shift-S3ctor event, temps were very cool and my speed-o needle was passing 160 just before the braking point on the back straight after turn 9. But my looking at my P-Box data I was only going 145. Comparatively, the speed-o is off only a few mph at 80 mph...but almost 15 mph at 160. Very strange. I'm running larger rear tires as well (275 Mich PSS).
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      01-08-2013, 10:10 AM   #47
pbonsalb
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Quote:
Originally Posted by Team Plutonium View Post
You'll also loose quite a bit of top speed.
Yes, assuming you need to go much faster than 180 mph.

A 6 Speed with stock 0.87 6th gear and stock 3.85 diff and stock 265/40/18 tires goes 193 mph at 8250 rpm. Changing to a 4.10 diff will reduce top speed at 8250 rpm to 181 mph. Even raising the rpm limiter to 8600 would not restore all of the top speed.

I did not do any of the math for a DCT or its differential options. They could be more or less. I know there are two popular alternative diff ratios.
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      01-08-2013, 09:10 PM   #48
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Quote:
Originally Posted by highyo View Post
what do you mean by top speed being affected? because of the different gearing? so now i can't go 155 anymore (not that i ever did)
with a speed delimiter, 155 would still be no problem, but 180 may be out of reach
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      01-08-2013, 09:28 PM   #49
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^Yeah what they said..

Quote:
Originally Posted by highyo View Post
what do you mean by top speed being affected? because of the different gearing? so now i can't go 155 anymore (not that i ever did)
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      02-22-2013, 08:51 PM   #50
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Quote:
Originally Posted by 4corners View Post
What about a 4.10 differential like the Dinan? It's cheaper than an exhaust.
That will probably be the first and only mod that I will do. Been down the sc path on my previous car so I want to stay NA. Seems to me most of the bolt on mods dont provide the bang for buck that a shorter ratio will do to acceleration. HP is for dyno queens, the real test of a mod is increased acceleration.
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