4.10 instead of 3.73 on Autocross?

[AlexHale]

Car has a 3.73 LSD, slight engine mods, Miller Maf with 17lb pound injectors and KA Motors intake. Full suspension build. How would a 4.10 probably affect times?

[matthugie]

What class are you running? I'm pretty sure that changing the drive gear would bump the car to a modified class.

As for your times, it depends on the course. Acceleration will be a bit better, but if you have more open courses you might run out of gear. If your area has tighter courses then you might pick up some time being farther up the power band.

Case in point, there's a guy locally who runs a 318ti with a supercharged S52. Stock gearing for the trans is a 3.23 (or 3.15), but last year he was running a 3.46. In the tight courses at smaller lots he was extremely fast, but we have a large fast lot where he was stuck between being in 2nd and 3rd and didn't run well. This year he switched to a 3.15 and has picked up a couple seconds on the big course.

[AlexHale]

Still trying t figure my classing out, I'm shooting for street touring. I don't know if that would kill that, the Scca rule book is too wordy. So I theory, have a normal and taller doff to swap for different tracks?

[tinkerputzer]

STX doesn't allow gearing changes, but you are allowed to modify other aspects of the diff such as clutches.

Yes if able you would have gearing optimized for different tracks.

[AlexHale]

Actual clutches or the packs inside the diff. That control lockup- can they be modified from 15% ?( to my memory)

[tinkerputzer]

Both from what i understand.

STX, STU, STR – Only standard (as defned in Section 12.4) limited slip differentials (LSD) are allowed on AWD vehicles. For AWD vehicles that did not come with any type of limited slip differential (including center differential or transfer case), a single aftermarket mechanical LSD may be added. 2WD vehicles may use any mechanical LSD unit.

[Nsquared97]

2WD vehicles may use any mechanical LSD unit.

But you have to retain the stock ratio (3.73). In theory you could run a really fancypants torsen if you had the cash as long as it was the correct ratio.

That said, even if it was allowed I definitely would not switch to a 4.10. 3.73 is almost perfect for a lot of places I run at, and at times still bit too short for what I would like. Really comes down to where you run at and the course design, but it would have to be some pretty crappy tight courses to make me want to switch to a 4.10.

[tinkerputzer]

In theory you could run a really fancypants torsen if you had the cash as long as it was the correct ratio.

A lot of top str and stx guys around here run os giken lsd units.

[Bimmerman325i]

You converted the AFM to a MAF. That means you are in Street Prepared at a minimum.

Diff is wide open for you, but just so you know, a 4.10 may likely be too short depending on your course. If the course is very tight, it will likely be fine, but on more open courses you may be hunting between gears or riding the rev limiter in second.

[lcoleman]

Wrong direction imo. I'm thinking if your region has a lot(s) of any appreciable size you should be at or on the limiter in 2nd with a 3.73. Personally I'd even consider stepping mine up slightly, but I'm running ST tires with a Miata OD (smaller).

Can't help on classing. I'm running in our local STM class (run whatcha brung with 140+ tw) this season and doing well with similar mods. E30s just don't fit easily into any official SCCA class. DSP...maybekindasorta, if you're willing to run 10" wide wheels/tires.

[e30bulldog]

Personally I would not run a 4:10. I currently run my car in STX. I bang the limiter all day, even on smaller courses (58 mph @ 7k with my tires). I'm in the transition to DSP, and it looks like there are 2 routes, use 2nd gear with a 3.46 (not a big improvement) or use 3rd a lot with a 4:27. 4:10 would suck

[tinkerputzer]

I didn' think you could alter diff ratio's in dsp either.

Out of those choices i'd go with a 3.46 though.

[e30bulldog]

You can in DSP

[lukeADE335i]

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

[matthugie]

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

The same way everyone checks the ratios, the tag on the diff. The scrutineers are car guys, they know what to look for, and a lot of people are familiar with these cars.

That said, it really doesn't matter. At local events no one is going to care and at a national event an e30 probably isn't going to be competitive. Unless you're running something blatantly out of class and killing everyone (like the local kid who was trying to run a car with a motor swap and carbon fiber hood in STF) no one is going to care.