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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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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or you could save money and raise the limiter past 8k that way you'll hit 60 in 2nd
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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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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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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. |
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New gear ratio/old gear ratio x RPM 3.909/4.10 x 3000 = 2860 RPM not 3870. |
I don't have any trouble at all hitting 60mph in second! :D
Scott |
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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? |
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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