Choosing the right ratios, E28 track car build

[d3viCe]

Hi
I'm in the process of sourcing parts for my E28 track car build, but the question is not model specific. I need advice on choosing the proper gearbox and diff ratios. If I decide only on the basis of calculator-supplied values then I am shure I would choose a good configuration on paper, that would be completely undriveable in the real world The gearbox will be a 6-speed Samsonas (multiple ratios in their specification, I have not decided seq or H-pattern, kinda heart vs wallet battle ) and the diff will be a custom billet aluminum racing unit, with ratios anywhere from 3.2 to 4.3. Is trying to mimic the ratios of an existing M model car a viable starting point? Do you know of any literature I could study that would help me choose a setup? Sadly I have a maximum of 1 month to make the decision and cannot read books which weight could kill a person if hit I'm a driver-investor and not an mechanic engineer sadly. The car will be a weekend driver. I do not plan on using it for drifting, or for any professional racing. I would appreciate any advice on the topic.

[circuit.heart]

If the car is a weekend track/hobby car, then the ratios don't really matter and it's not really even worth it to spend the money on a Samsonas, which needs regular maintenance.

To race, gear ratios should be chosen on a per-course basis to maximize the engine's torque curve and avoid awkward shift points during the lap. There is no rule of thumb except data. Get your maximum and minimum speeds at the track, using the tires you intend to set the car up with. Set the gearing such that at your min speed, you are just under the torque peak in the gear you're in. The max speed should be just under redline in the gear you're in. There are a thousand ways to do this. I use a 4.10 diff on an OEM 6-speed transmission to use gears 3-4-5 at Sonoma. At Buttonwillow, a 3.46 is more ideal to let me use 2-3-4 instead.

[d3viCe]

Solid advice, thanks.

Yes I know of the maintenance and I am prepared for that, part of every hobby car I guess. I was aware of how complicated this project will be when I started it. For the record it's a drysump built M106 turbo on ethanol (powered by Ecumaster with many parts from VAC Motorsports), so there was much investment already, it doesn't make sense to stop halfway.

[aeronaut]

... it doesn't make sense to stop halfway.

HAHA!!

I think we need more info on this car, and pictures!

[d3viCe]

HAHA!!

I think we need more info on this car, and pictures!

If you want photos of the boxes of parts piling up in my garage then shure I can post them Apart from that I will post a build thread once I have everything to at least finish a first step in the shop (car standing on it's own). Rome wasn't built in a day you know. In all seriousness I did other way smaller projects and starting without parts was the biggest mistake back then.

[aeronaut]

If you want photos of the boxes of parts piling up in my garage then shure I can post them Apart from that I will post a build thread once I have everything to at least finish a first step in the shop (car standing on it's own). Rome wasn't built in a day you know. In all seriousness I did other way smaller projects and starting without parts was the biggest mistake back then.

Looking forward to the build!

[M_US_E30]

here is a spreadsheet that will help you with seeing the numbers. I used it for my GrpA/DTM E30 M3 with 2.7L S14 and GrpA 5 speed transmission. I tried a 3.73, 3.91 and settled on a 4.10 and one larger size rolling circumference tire.

https://www.dropbox.com/s/v5kmpo3pv6...hart.xlsx?dl=0

T