Gearing

[Low Level E30]

I have a choice between a 3.91 LSD and a 4.10 LSD for my 325i --- what do you guys think? Which would YOU rather have?

The car is for fun, not a DD, but pretty regularly driven --- some street, some highway, some road course.

I realize the obvious benefits - less RPM, slower acceleration, higher theoretical top speed (although I'd never reach it), better gas mileage, etc w/ the 3.91, as compared to the 4.10.

Have any of you used either or both sets of gears on a road course? Is there any noticeable difference between the two?

Any input?

Thanks!!

[rwh11385]

clutch type 4.10 vs. viscous?? 3.91

clutch type hands down. and 4.10 because it is fun. fuck, my daily-driving 325is has a 4.10 and 3% undersized tires. 13% shorter gearing is fun

[Low Level E30]

This will be a clutch-type 3.91 from a Touring, not a viscous IX diff.

Isn't 1st gear way too short?

What's the latter part of the gearbox (3rd to 5th) like out on a road course, with the 4.10?

Also, what if you throw a lightweight flywheel into the mix? Still want a 4.10?

Thanks for the assist!

[rwh11385]

oh okay. then that levels the playing field at least.


nah, i like first gear. highway is a little bit silly, and so has been my gas mileage, but oh well. i'll live.

the car fucking flies out of turns. (best way to put it) you feel like you're pulling quicly, then slamming on the brakes, turning, and repeating it.

and i think a ltw flywheel would be good with the combo. if my clutch ever kicks the can, i'm gonna pick one up

[Low Level E30]

I should've mentioned this at the beginning --- the car is also stripped (no A/C, tar, carpet, etc), running race shells and a rollbar, pushed by a 3.73. A lightweight flywheel will get me through 1st and 2nd a li'l quicker --- so would you guess a 3.91 to be the perfect compromise, or is a 4.10 still a better choice?

I think you're gonna say "4.10" :D

[Low Level E30]

What is your 5th gear RPM at 75mph w/ the 4.10?

[Digitalwave]

My car turns 4k RPM at 80mph in 5th with a 4.10LSD.

I honestly think for a road coarse and DD car, a 3.91 would be a better choice. For an autocross car, a 4.10 would be nice. I usually start my car from a stop in 2nd gear with the 4.10 during DD use.

[Jarvis]

It's really not that hard to get used to a 4.10 vs. the stock 3.73. Since you don't really care about highway revs/gas mileage and are upgrading no matter what, I vote for the 4.10.

[DanGillan]

i've been rocking the 4.10 for a year now and i wouldnt change a thing.

[rwh11385]

My car turns 4k RPM at 80mph in 5th with a 4.10LSD.

I honestly think for a road coarse and DD car, a 3.91 would be a better choice. For an autocross car, a 4.10 would be nice. I usually start my car from a stop in 2nd gear with the 4.10 during DD use.

My car would be the same. 4k on the tach and an indicated 80mph. But it might only be really going 75mph. (too lazy to calc)


a longer diff would allow you to have more speed in each gear, but so would a high redline chip. it's really down to personal preference.

[Low Level E30]

Thanks for all the input - probably going to be a 3.91 for me!

[Scott314]

clutch type 4.10 vs. viscous?? 3.91

clutch type hands down. and 4.10 because it is fun. fuck, my daily-driving 325is has a 4.10 and 3% undersized tires. 13% shorter gearing is fun

Would shorter gearing be more of a pain in the ass in an autocross? It would probably mean lots of 2-3 shifts vs. staying in second gear, wouldnt it?

[Digitalwave]

I think the increase in acceleration makes up for any more shifts you'd have to do.

[Low Level E30]

I think the increase in acceleration makes up for any more shifts you'd have to do.

I think that would depend on the shape / length of the course. A long course w/ plenty of room to run the car through the gears would be great --- but if you're on a course where you're going to JUST fast enough / just slow enough to need to keep changing gears, you're going to lose a lot of speed. Which is obviously why dedicated professional motorsport teams have custom gearbox ratios etc. for each track.

[Widm3r]

I am running 4.11, i start up in second all the time cause first gear is just always silly, and gets me in trouble on the streets, but gas mileage is shit, but hey, what can you do, i knew that when i bought the car and still wouldnt change it for anything else...