Preferred differential ratios for M5s

[bigdaddy]

Just purchased an e28 with a S38 (M6) engine...diff is the original 535i unit...what is recommended as a good diff gear ratio for "spirited" weekend driving...not to be a DD or a track car.

thanks in advance.

[ecartoys]

Stock M5 and M6 rear diffs are Big Case LSD 3.91. I would assume that is the best all around ratio.

I have a low miles unit sitting in the garage. Less than 100K from a running 87 M6. Will love to trade for a Big Case LSD 3.46.

[Mark 88/M5 Houston]

The S38 is sort of "soft" on the bottom of the rev range; the NA M5's were heavier than the Euro versions so BMW used the 3.91:1 large case LS differential. The large battery and the large case diff also helped with the weight distribution.

I some times wish that I had a 3.25:1 LS, but the 3.91:1 ratio is great for in town and is only a little "buzzy" at an indicated extra legal 85 MPH cruise.

In your situation I'd look for a 3.64:1 or a 3.73:1 LS diff.

[Jeremy]

I drove an M5 with a 3.64 once. Despite stroking and head work, the gears just felt too long for what the motor wanted to do. 3.73 or 3.91 would be my suggestions.

Jeremy

[bigdaddy]

thanks for the input guys...looks like i'll be in the market for a 373 or 364 in the near future....someone mentioned big case 391...does that mean bigger than the standard e28 case or do you mean bigger than the e30 diff case?

[Dana R]

thanks for the input guys...looks like i'll be in the market for a 373 or 364 in the near future....someone mentioned big case 391...does that mean bigger than the standard e28 case or do you mean bigger than the e30 diff case?

The standard e30 and e28 cases are the same, just a different cover.

[cgraff]

thanks for the input guys...looks like i'll be in the market for a 373 or 364 in the near future....someone mentioned big case 391...does that mean bigger than the standard e28 case or do you mean bigger than the e30 diff case?

The standard e30 and e28 cases are the same, just a different cover.

There are 3 sized cases BMW used historically (but after the side-loaders). The 'big case' uses a 210mm gear, the 'medium' or 'small' uses a 188mm gear. There was an extra small one (dunno off hand what the gear size was) that for example came on E30 316s in europe.

As Dana said, the standard E30, E28 cases are the medium case at 188mm. We call them 'small' b/c that's all we deal with. The 'big case' diffs came for the US market only on M6s and M5s.

In europe, the 2.8 liter and bigger M30 engined models came with big case diffs. So for example, a euro 535i has a 3.25 big case, whereas a US 535i has a 3.25 medium (although we might call it 'small'). European M635s and M5s came with a 3.73, although US models came with a 3.91 for the aforementioned reasons in the thread. You may be lucky and be able to find someone with an M635 who wants the big case 3.91 and is willing to sell you the big case 3.73 that came in their car.

-Chris

[skip535i]

same car??

NICE SCORE

[LA]

Cool find. Can we see some pics? I hope you will still drive up to talk BMWs even though I suppose the b35 swap we talked about is probably off for now! If you stop by Vines please tell Ron I said hello.

[bigdaddy]

Yes, this is the car...really looking forward to getting it in the stable.

Although originally equipped with a big case diff, I believe this car currently has the OEM, medium case diff that came on the 85' 535. (the M6 donor car diff was not useable I'm told). With that said, I assume that I could put another 373, 364,or 346 med case lsd into the car, as long as the driveshaft length and coupler end mate up, correct?

[m-racer]

Congrats. Andy is a friend of mine and Greg, the guy who did the engine conversion, is a very talented guy. Should be a really nice car.

[cgraff]

Yes, this is the car...really looking forward to getting it in the stable.

Although originally equipped with a big case diff, I believe this car currently has the OEM, medium case diff that came on the 85' 535. (the M6 donor car diff was not useable I'm told). With that said, I assume that I could put another 373, 364,or 346 med case lsd into the car, as long as the driveshaft length and coupler end mate up, correct?

The interfaces between the diffs are identical. i.e. the same driveshaft bolts on to a big case diff as it does to a medium case. Also, the CV joints bolt to the different diffs just as well.

The only caveat is that an E30 uses different half-shafts, and thus the output flanges on the medium case diffs are different E30 vs E28.

In terms of fitment to the subframe and body, the diff case has the same bolt pattern, so that they both bolt up to the subframe. The only differences between E28s and E30s for instance is the back cover, which are different due to different body mountings.

So...you can use any big or medium case diff, with the right output flanges to the E28 halfshafts and the right back cover.

-Chris

[bigdaddy]

Very good...thanks guys....really appreciate the info