With a blower or turbo, the numbers that the G produces aren't that great. I mean, the numbers in terms of power are pretty good, but getting all that power to the ground and up to speed isn't really efficient. Even with a vortech (vortech being on the lower end of FI), the M3 has 100 less horsepower at the crank, but pulls of similar times. Anyone think of changing the rear diff. or maybe even upgrading the lsd?

 

There are threads about the 350EVO gear swap on the boards, where a noticable different is seen, however the top end loss is pretty significant. My 5AT revs high at 80 already, I'd hate to see it with higher gears.

I will get the Quaife diff replaced as I dont have the lsd installed.

 

i think the 3.9 gearing will be too much for FI cars. With shorter gears u have have faster accelerations and less traction down low. I think for autos that are going FI, the 3.5 should be perfect.

 

A number of folks with F/I are going to a 3.3 FD. The taller gearing results in better grip down low, and more time between shifts. You'll get better gas mileage cruising on the highway, too.

I upgraded from my OEM open differential to the Quaife ATB differential, and I'm glad I did. Quaife is the creme-de-la-creme as far as differentials are concerned. No wear-and-tear, and a life-time warranty... even under racing conditions. It's one of the most costly after-market diffs out there, but it's well worth it (about $1500).

In my humble opinion, there is no better place to secure a Quaife than Forged Internals. They have a web-site. Check it out. Make sure you talk to Sharif. He's top-notch. Good luck.

 

Maybe for stick the 3.3 is good but for autos traction isnt much of an issue down low with a superchager. So Dan K needs to clarify if he has a stick or auto.

I always assumed that if u accelerate faster u can hit boost faster so why do ppl want taller gears unless u are making some serious hp (>450)? Then I would start worrying about traction.

 

From what I have read here and elsewhere, the primary benefit of taller gears is being able to stay in each gear longer. I think in the 1/4, the taller FD gear allows you to avoid a shift near the finish line. One less shift equals a lower ET in the 1/4...

 

thats a myth. Taller gears are only good for Hi speed racing bc it takes longer to reach redline but for quarter miles shorter gear will be better. U accelerate faster and able to build boost faster and not drop down in rpms as much compared to taller gears.

Why do u think race cars have like 3.9 -4.XX final drives bc they want to shift faster and get a faster mph.

 

All things being equal you will have better times with shorter gears. The only problem is that as guys like me are finding out even with drag radials traction is limited with the gear ratio of the 6mt. If you put a 3.9FD into a Turbo Z I would bet you first gear would be almost unusable. You would build boost faster in a Supercharger application but in a turbo app since 1st would be so short their would barely be enough load to build full boost. Most race cars have a higher final drive so that their top gear is usable on that particular track.

 

The performance benefit for 5AT owners with a 3.9 final drive is incredible. Well worth the $500 or so. However, you lose cruise control and MPG will drop. If only we could change the 5th gear ratio to offset the 3.9 then all would be well!

I've been considering a torque converter as an alternative to help with low end torque but figuring out which one to get so that it will give you what you want is complicated. It's also about $500 installed.

 

can you email me details? randys_g@yahoo.com

 

I'll be glad to provide more details. Just specify which details. Thanks.