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Discussion Starter #1
Looking for some comments regarding changing diff gears with a F/I engine. Current ratio is 3.5 in a 6MT with 375 rwhp. Have read several post of N/A cars with gear swaps and all seem to say 3.9 is the way to go.

Anybody out there with F/I and a gear swap? Would love to get some thoughts on your experience. Is 3.5 - 3.9 to big of a change? Had a ride in a 2010 370 the other day and it pulls like crazy and it is running 3.7 which is stock. Would this be a better setup with F/I?
 

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Discussion Starter #3
I'm not looking for more top end. Would like a little quicker acceloration but don't want to make 1st useless. Just wondering what some others have done and what their experience is.
 

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I could be wrong, but if you have an FI setup wouldn't you want to move to a taller final drive to take advantage of longer gears?

Scott.
Ideally yes, bc you have more power…but I know what the OP is talking about because I had this same decision but I had a 20" wheel and the additional power (also 375whp).
If it was my car I would keep your 3.5 FD, possibly go to a 3.7 if you've got a hard-on for switching out your FD. With a 3.9 you essentially negate 1st gear all-together and you now have a 5 gear car. My decision was to keep the 3.54 FD - It wasnt worth the hassle.

Engine Rev : Wheel @ FD=3.54
1g = 13.43
2g = 8.23
3g = 5.74
4g = 4.50
5g = 3.54
6g = 2.81

Engine Rev : Wheel Rev @ FD=3.9
1g = 14.80
2g = 9.06
3g = 6.33
4g = 4.96
5g = 3.90
6g = 3.10

For your reference-

Engine Rev : Wheel Rev @ FD=3.7
1g = 14.04
2g = 8.60
3g = 6.00
4g = 4.70
5g = 3.70
6g = 2.93
 

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Discussion Starter #5
Thanks 3point5 for your suggestion. I'm not sure what you mean when you say it wasn't worth the hassle. Are you referring to the time it took to calculate all the ratios? What gering did you have originally? Did you try all the different ratios? I know it seems like a lot of work to swap out the gears but I'm removing the pumpkin to do a suspension/differential bushing upgrade and was thinking since I'm there might as well do it. 3.7 seems like the way to go.
 

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I'm not sure what you mean when you say it wasn't worth the hassle.
the hassle of going from a 3.54(stock) to a 3.9 or 3.7 because I went from a 18" stock wheel to a 20" aftermarket wheel. It was gonna cost me about $1000 when it was all said & done - just not worth it.

the calculating took a few minutes…I have the stock 3.54 for a 6MT in the car now and for the immediate future.

You have a point of switching it out because you're gonna be up in those parts anyways and you can always switch it back if you don't like it. I don't have the ability to do the swap on my own but found a mechanic that would let me do it in his shop under his supervision so it'd be a learning experience.

I was going to swap out my rear axles with something from the drive shaft shop to support +/-650whp - I have a feeling my stock axles will be in torque-heaven if I put that much torque on them. But I'll cross that bridge when I get there.
 

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Discussion Starter #7
Still up in the air as to if I will change out the gears. But I do appreciate your comments. Took the car out for one last ride yesterday before winter weather catches me off guard. It felt pretty good, pulled hard but it always helps when the weather is cool outside.
 
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