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Engine, Exhaust, Transmission Discuss the FR-S | 86 | BRZ engine, exhaust and drivetrain.


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Old 05-01-2013, 06:28 PM   #1
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Anyone w/ 3.727/3.909 Final Drive Post Boost?

Title says it all. Just wondering if anyone with forced induction has installed a taller (or is it shorter?) final drive gear. I ask because I would considering it when I get the Vortech kit. Not really aiming for high MPGs or anything, but a 3.909 would at least get u past 60mph in 2nd. At the same time, I fear the 3.727 would rob you of too much acceleration. Just looking for some thoughts & feedback.
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Old 05-01-2013, 07:10 PM   #2
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I was just thinking the same thing for my vortech kit yesterday, I would like to have a bit more play in 1st 2nd & 3rd. Interesting topic.
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Old 05-01-2013, 07:24 PM   #3
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or you could save money and raise the limiter past 8k that way you'll hit 60 in 2nd
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Old 05-01-2013, 09:09 PM   #4
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Turbo could go lower but a centifugal SC like the vortech would benefit from a bump up, 4.44 or 4.55
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Old 05-01-2013, 09:57 PM   #5
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Turbo could go lower but a centifugal SC like the vortech would benefit from a bump up, 4.44 or 4.55
Agreed. Generally speaking, turbos like a higher (lower numbers) gear. It helps build exhaust pressure to spin up the turbo. Centris like a lower (higher numbers) gear. It helps get the RPMs (and boost) up earlier. Positive displacement blowers aren't as fussy, but sure are a hoot to drive with a lower gear...
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Old 05-01-2013, 10:50 PM   #6
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or you could save money and raise the limiter past 8k that way you'll hit 60 in 2nd
Spinning engine rotating assembly possibly higher than they were meant to spin (no, an article quoting an STI employee is not good enough) to an unknown end is not "saving money" compared to a taller final drive.
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Old 05-02-2013, 04:22 AM   #7
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This discussion has been quite insightful, I'm glad we had it. After considering how turbo/centrifugal sc reacts to a final drive change, I'm thinking of just keeping it stock lol. I don't really wanna cruise at 4k on the freeway and yet pretty much been persuaded from going 3.727/3.909 so staying 4.1 I guess.
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Old 05-02-2013, 07:59 AM   #8
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Don't forget to consider any tire changes that will effect the "resultant" final drive ratio.

Stock tires (215/45-17) and stock rear gears (4.10) will result in 3000 rpm @ 70mph
Going to a 235/35-17 set up will will result in 3150 rpm @ 70 mph
Keeping the stock tires with 3.909 gears will result in 2870 rpm @ 70 mph
Going to a 235/35-17 & 3.909 set up will will result in 3000 rpm @ 70 mph, same as stock. You will have spent a lot a money to achieve the same result as doing nothing.

Last edited by Gary in NJ; 05-02-2013 at 09:02 AM.
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Old 05-02-2013, 08:09 AM   #9
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Don't forget to consider any tire changes that will effect the "resultant" final drive ratio.

Stock tires (215/45-17) and stock rear gears (4.10) will result in 3000 rpm @ 70mph

Keeping the stock tires with 3.909 gears will result in 3870 rpm @ 70 mph
I think your math is wrong. Going with 3.909 gears instead of the stock 4.10 gears will result in lower RPM at 70 compared to stock.
New gear ratio/old gear ratio x RPM
3.909/4.10 x 3000 = 2860 RPM not 3870.
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Old 05-02-2013, 08:59 AM   #10
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I don't have any trouble at all hitting 60mph in second!

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Old 05-02-2013, 09:02 AM   #11
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I think your math is wrong. Going with 3.909 gears instead of the stock 4.10 gears will result in lower RPM at 70 compared to stock.
New gear ratio/old gear ratio x RPM
3.909/4.10 x 3000 = 2860 RPM not 3870.
TYPO, should be 2870. Thanks for letting me know. I fixed it above.
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Old 05-02-2013, 08:26 PM   #12
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Thanks, I kept tire sizes in mind. I intend on going 225/40R18 on all 4 corners, which is just a smidge larger diameter than stock. Not really trying to build a dedicated track car, but just for street instead. Do you guys think 225s would be too skinny w/ RE-11 or AD08 or something along that tier? I'm sure they've dropped in price since the RE-11A & AD08R came out recently.

So using the formula above...
70mph w/ 225/40R18 & 4.10 final drive would result in ~2944rpm
70mph w/ 225/40R18 & 4.44 final drive would result in ~3188rpm
70mph w/ 225/40R18 & 3.909 final drive would result in ~2807rpm

Btw, has anyone figured out how to calibrate the speedometer to compensate for different tire sizes?
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Old 05-02-2013, 08:38 PM   #13
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Performance summer tires tend to run wide, so remember that. My 225/45 Pilot Super Sports aren't pinched on a 8" wide rim, but do poke a bit at the bead.

As far as the speedo is concerned, you need a module that intercepts the VSS signal. DCCD Pro/Spiider was working on this a few years ago in the Impreza world, I'd check with him. Really it's not a big deal as long as you stay within 3% of the original tire diameter.
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