4.10 instead of 3.73 on Autocross?

[B1mmer]

How would the scrutineers know which diff your car came with? The 325i came with both 3.73 & 4.10 ratios from the factory.

They did, but you can only get the 4.10 with an automatic equipped car. At the National level, it would get noticed, but like the previous post said, it wouldn't be competitive likely anyway. Now if the person wanted to do it legally, they could run DSP. - AB

[hoyabmw]

In DSP you have the 2.93, 3.73, 3.91, and 4.10 as legal options. I wish the 3.46 was legal. Don't do the 4.10, you run out of gear way too soon. Unless you decide to run 285/18s, stay with the 3.73. Even then, don't go past 3.91.

In STX, you're stuck with the 3.73, but that's not so bad. With the MAF, you're in DSP anyway as was pointed out.

[Kev]

Shouldn't have an issue with a 4.10 and m20.

[SmokeE30]

You could always change tire sizes to get your gearing where you want but I would want all the second gear I could get for most courses

[hoyabmw]

With a 3.73 and 275/35/15s, I'm topping out at 61 at 7300. I wish the 3.64 was legal in DSP. I could do more with 285/35/18s, but that's a lot more weight to turn.

[Artful Dodger]

I run an e30 in the Atlanta Region SCCA. We have a stroker engine. We run in Street Modified. This group is fairly open as far as mods go. We have tried a variety of diffs in an attempt to optimize the setup for better times. I have been a follower of shorter diffs for better acceleration. but what we have found is a taller diff lets you can just stay in 2nd.

so stock the car has a 3.73. we swapped in a 4.10. it has better accel but we are in between 2nd and 3rd. so not enough power to pull in 3rd in the slow sections and 2nd tops out on the long slaloms.
now we are running a 3.25 diff from a 5 series. We have dropped another .5-1 second off a 50 second course.

this is just what I see. your driving style may not result in such gains with a taller diff.