Which Diff Ratio to Use??

[Rigmaster]

Wow, lots of confusion in this thread, and an amazing number of people recommending the OP change the tire size after he already said they are limited to the tire size they are using (class rules anyone??).


Medium case 4.10 diffs are found in ~88-up 325i/is with automatic transmissions. All 325is' had LSD and many 325i's came with LSD as an option, or part of an option package. From what I've seen, there's no sure way to tell if the car has LSD just by looking at it (other than 325is) but I've seen alot of 4 door automatics with leather interior that had LSD's. (of course, you can check the tag to confirm LSD or not)

3.91LSD is hard to find in the E30 medium case, because they didn't come in any E30's. You'd have to build your own with the ring + pinion from another car, or I *think* there might be some E28/E24's that came with that ratio and those diffs bolt into the E30 with a change of side flanges and rear cover.

[ForcedFirebird]

And the story is even more complacated in this side of the pond.
If 4.10 medium are only M3, that makes it either impossible to find or too expensive for scarcity.
Same story with the 3.91. This makes me hate my 240 gearbox.
Only LSD I can find easier are the ix viscous ones (also 3.91), but I'm not sure if the gradual lock is good for track. Though I've read that the positive side is it has a higher lock percentage than most of the others, provided it is in good shape.
Can't we use something E34 or 36 with a similar ratio? Medium case E30 = small case E34, no?

z3m?

[Fusion]

And the story is even more complacated in this side of the pond.
If 4.10 medium are only M3, that makes it either impossible to find or too expensive for scarcity.
Same story with the 3.91. This makes me hate my 240 gearbox.
Only LSD I can find easier are the ix viscous ones (also 3.91), but I'm not sure if the gradual lock is good for track. Though I've read that the positive side is it has a higher lock percentage than most of the others, provided it is in good shape.
Can't we use something E34 or 36 with a similar ratio? Medium case E30 = small case E34, no?

[ForcedFirebird]

Or a convertible automatic.

My sedan, a client's coupe and a vert I work on all have factory LSD behind the auto's - factory optioned. Funny thing is, one local bought his MT2 vert new, and he has an open. Pay all that money back then for looks, but not have an LSD.

[ForcedFirebird]

Sorry, but this is not correct.

We bought my 318is brand new from Maywood BMW in 1991. Ticked the boxes for 3 of the 4 avilable build options: LSD, sunroof, and fog lights. The only one we didn't get was metallic paint. Then my dad paid extra for body-colored wheels (don't ask!) and Webasto sunroof vent from the BMW Accessories catalog.

That's the car I started doing BMW CCA events in and slowly modded into a dedicated track car. So, I have a pretty clear memory of the car as it shipped and what we had to upgrade along the way.

It absolutely did come from the factory with a small-case 4.10:1 LSD. Just like all 318is models. I'm 100% certain about that.

I stand corrected.

I DO know the medium case 4.10LSD came in my auto, though :) Actually 2 of the last 5 autos that came in had LSD's, I thought they were more rare than that.

[Emre]

Any 318is that was originally purchased with the optional LSD came in medium case...

Sorry, but this is not correct.

We bought my 318is brand new from Maywood BMW in 1991. Ticked the boxes for 3 of the 4 avilable build options: LSD, sunroof, and fog lights. The only one we didn't get was metallic paint. Then my dad paid extra for body-colored wheels (don't ask!) and Webasto sunroof vent from the BMW Accessories catalog.

That's the car I started doing BMW CCA events in and slowly modded into a dedicated track car. So, I have a pretty clear memory of the car as it shipped and what we had to upgrade along the way.

It absolutely did come from the factory with a small-case 4.10:1 LSD. Just like all 318is models. I'm 100% certain about that.

[Wh33lhop]

Any 318is that was originally purchased with the optional LSD came in medium case.

I think you are wrong on this. Medium case 4.10s can be found on (some) auto 325is and M3s. 318is all came with small case 4.10s, some open, some fun edition.

[M-technik-3]

Small case (from the '91 318is) won't survive an M20, let alone an M30 or M50. Ask me how I know!

If you want to swap, you'll need a medium-case 4.10:1 LSD from an E30 M3.

Or a convertible automatic.

[ForcedFirebird]

Small case (from the '91 318is) won't survive an M20, let alone an M30 or M50. Ask me how I know!

If you want to swap, you'll need a medium-case 4.10:1 LSD from an E30 M3.

Any 318is that was originally purchased with the optional LSD came in medium case, same with the automatics. My auto was ordered with LSD (auto came with 4.10), swapped in a manual and am using it on the track. Homestead is a shorter track than the above mentioned ones, so difficult to give advice.

[Emre]

That being said OP I would run a 3.74 with a smaller sidewall to make a 3:9x ratio.

Agreed. The OP should keep his 3.73:1 LSD and run 225/45-R15 tires on 15x8" wheels.

[Emre]

Which 4.10 will take the M50 or M30 power? I hear the small case LSDs will pop.

Small case (from the '91 318is) won't survive an M20, let alone an M30 or M50. Ask me how I know!

If you want to swap, you'll need a medium-case 4.10:1 LSD from an E30 M3.

[Fusion]

Which 4.10 will take the M50 or M30 power? I hear the small case LSDs will pop.

[Jparkr]

Go with the shorter tire and 4:10, the shorter sidewall should give you a little more response/feel in the corners/braking zones.

Our pro3 15s with 50mm sidewalls keep the 4:10 ratio, when we run on 45mm sidewalls it turns into a 4:27 if I remember the rule meeting convo right.

That being said OP I would run a 3.74 with a smaller sidewall to make a 3:9x ratio.

We also have an M30b35 chumpcar and run a 4:10. More than happy with it Straight away speed is ok but power exiting corners will make you much more competitive.

[yeaseth]

my orignal ratio was 3.73, then I ran a 3.91 for a while and didnt like how short it was so I switched back to the 3.73

[djjerme]

Go with the shorter tire and 4:10, the shorter sidewall should give you a little more response/feel in the corners/braking zones.