Diff opinions

[mikem]

Okay guys, working on the last piece of the puzzle, but thought I'd get some opinions from some who have been down this road. Putting a new diff behind an S50B32 w/ Group N DME (~8k redline), and the E36 six-speed (4.23, 2.51, 1.67, 1.23, 1.00, .83). Thinking 3.62 gears (210mm - euro housing) with a 40% lockup. Car is being setup to do auto-x, the odd HPDE, and general street use (think weekend car). Any opinions/other things I should consider?

Thanks,
Mike

[99MPower]

3.64 you mean? or is the 3.62 a Euro proprietary gearing?

I would think 3.62/3.64 would be perfect for auto-x/street driving..

anything more aggressive and it would get very tough to drive and with the extra power 1st ,2nd would be useless and just spin with 3.73 or higher I would think

I've driven a s52 with the stock 5 speed and a 3.64 and that would be about the max I could take on the street

[mikem]

3.62. Believe it is set up for e46 diffs (later e46 m3s came with 3.62 I'm told). But, I'm also not real clear on the whole 12mm vs. 14mm bolt thing, so the gears may not be for the e46 specifically.

Stock euro is still 3.23. This would be a custom build. So I also get to pick the lockup (really # of clutches in LSD)

Yea, thinking with 8k I can just stay in 2nd most of the time for auto-x, and I wouldn't be sliding the tires all over the place on the street. Used to having a (mostly) useless 1st on the M5 though, and I think this setup is going to be quite similar.

[Camdinans3]

Our M coupe has a 3.46. might be a good ratio for you to look at. with the 6 speed combo it would provide good acceleration with decent RPM's at highway cruising speeds.

[99MPower]

with the 6 speed... you would want to actually utilize the 6 speed, and I wouldnt go with anything less than 3.55 or something like that...

3.62 is the perfect ratio for a DD with the 6th speed...

I would go 3.73 due to being more track dedication or even more, but 3.62 is perfect IMHO

[mikem]

Yea, 3.73 is tempting, but revs a bit high at "normal" highway speeds. I'd like to be able to take this thing out to Bimmerfest, and maybe up to the NE at some point, so highway speeds are a valid concern. UHaul doesn't seem to want to rent me a trailer given that I'd be pulling the thing with the niner for some reason.

[99MPower]

they dont want to rent you a trailer??? WTF :shifty:

[Roland H]

Mike I sent you a PM, replace 3.64 with 3.62 in that.

And honestly, unless you track the car, I'd recommend lower gearing. 3.62 is a little high, even with a six speed. I'm at Calvin's house right now and on the drive to Michigan from Colorado I averaged 25MPGs with my 3.38/5speed combination. With my 3.23 I got 32 at the same speeds... food for thought.

[99MPower]

roland at my house FTW :rulaiz:

[mikem]

Originally Posted by 99MPower

they dont want to rent you a trailer??? WTF :shifty:

Yea, they have this database of "approved vehicles". Nevermind the fact that this thing has enough torque to pull your house off its foundation and down the street. Might be something with wheelbase, I don't know. But they won't do it.

[mikem]

Originally Posted by Roland H

Mike I sent you a PM, replace 3.64 with 3.62 in that.

And honestly, unless you track the car, I'd recommend lower gearing. 3.62 is a little high, even with a six speed. I'm at Calvin's house right now and on the drive to Michigan from Colorado I averaged 25MPGs with my 3.38/5speed combination. With my 3.23 I got 32 at the same speeds... food for thought.

Yea, this thing is going to be used for track/auto-x primarily. I've got the M5 for road trips, and it makes such an awesome cruiser that I won't be doing much cross country driving in the E36. That said, according to the handy rpm calculator spreadsheet I just built, my highway RPMs at 80 with the six speed and 3.62 are about the same as the 5 speed and my 3.15.

In other words, dropping the output ratio from 1.0 to .83 lets me go as high as about 3.80 and keep the same fuel economy, everything else being equal. But, I don't like the high revs on the 3.15 as it is, so I think staying down at 3.6x makes a lot of sense. 3.62 gives me 3230 at 80 mph in 6th. 3.73 has me up at 3328. 5th w/ 3.15 is 3472 FYI, and 3.23 is 3514.

Now that I'm playing with the numbers, 3.73 looks like it might be a real option. 7900 redline means I top out around 35 in 1st and 60 in 2nd, and its still livable at highway speeds. 190 theoretical gear limit too.

[Roland H]

Just bump up your redline to 8000 RPM and your set.

[vfr-rich]

6 speed seems kind of pointless for the uses you have decided on...

You have to ask yourself, how much power are you talking about? How much torque? Remember the diff is a torque multiplier...

A 3.73 will make 1st gear really useless, and really hurt you off the line. So much so that it make actually slow you down due to lack of traction. You are so busy feathering the throttle and clutch, you aren't accelerating. You have to get to 2nd before you can stay in 2nd... If you spin up 1st and then shift to 2nd, then you drop right out of the powerband, and you lose even more time... Just something to think about.

I really think 3.46 is the best ratio with a 5 speed, and maybe a 3.64 if you have a 6 speed and do a lot of highway driving. If not, stick with the 3.46. (that's a tough choice, you have to evaluate your specific traction limits and driving style.)

[99MPower]

he already has the 6 speed.... just deciding on rear diff he wants to put in .... so jealous Mike....

[PurduinaM3]

Mike's got a pallete full of sick ass rare parts that combined are worth more than my car :bow: