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      07-14-2010, 03:58 PM   #1
Brigadier General

Drives: Jerez Black E90
Join Date: Sep 2007
Location: STL

iTrader: (3)

Autocross nerd thread: The Ultimate E9X A-Stock SCCA Autocross Build?

No, I'm not doing it. Wish I was, wheel to wheel is too much fun. But since I enjoy bench racing, I will post my thoughts and somebody else can decide whether they want to do it.

The goals:

As much front camber as possible, as much tire as possible, and as little weight as possible, while being able to get on the throttle as early as possible.

The car:

No options 9/10 or later build 2011 coupe (for the manual seats) with 18" wheels. Carbon roof obviously. That should put the car somewhere around 3600lbs with a full tank of fuel, stock. DCT seems like a wash, either way you're going to be in 2nd most of the time. Going to 1st will just blow the tires off 99% of the time. It adds weight but you'll have 1st gear more readily available...pick your poison.

I would not choose ZCP and there's a very good reason why - tire sizes. Hoosiers don't come in good wide 19" sizes for the M3. The wide tires are also taller, 295/30-19 is 26.1" vs. 25.3" for the 295/30-18, which makes fitting wider tires a potential problem and also hurts gearing, which needs to be as short as possible. EDC is also going to get tossed because you can substitute shocks, so no point in having that either.

I guess paint color and interior trim could be optioned up, so spend all that tech package and ZCP package money on an individual color like LSB or Hellrot or Technoviolet.

The parts -

CCW classics in 18x8.5 and 18x9.5. This gives you lots of flexibility to build a wheel to suit the tire you can fit under the car just by changing rim halves, since tweaking the offset within the rules (+- 6mm) may be necessary to get the tires in there with no rubbing.

285/30-18 front and 295/30-18 rear A6's for starters. I would start with this fitment because it would have a good shot at clearing everything and then go to a larger rear tire if possible, perhaps a 315/30. The shorter diameter on both will help with clearance, and you'll also drop 2mph off the top speed in gear (70mph at 8300rpm with a 26.3" stock rear tire, 68 mph at 8300rpm with a 25.3" 295/30-18).

Straight pipes from the x-pipe back. Probably as easy as buying somebody's stock exhaust and using the stock connecting pipes with some 2.5" pipe welded on as extensions. If you want to be fancy put a 3" turn-down on the ends. It'll be loud but no louder than a CP car and you'll lose something like 40lbs. Now the car weighs about 3530 lbs with a full tank between the weight loss from the wheel/tire package (30lbs) and the exhaust (40lbs). Or maybe that Bastuck "sports" exhaust that just has the 3" bullet mufflers, but that's pricey for something you're going to take on and off for events. Plus the straight pipes could be stuck in the trunk and swapped out in about half an hour.

For shocks I'd go to a Penske dealer and have something custom made, but Motons are also available for the chassis already. Remote resevoir with nitrogen pressure adjustability would be a good thing since this car is still pretty softly sprung. The bodies are already aluminum and you're adding remote resevoirs so you're not going lose any weight here.

I'd have to look at swaybar options, but I believe RD Sport offers a 32mm hollow bar that would do the trick for starters.

I'd leave the brakes stock until they gave me a reason to change them. I have a feeling even autocross might be too much for the stock pads but who knows.

The setup -

I'd pull the pins for more camber up front and try to find documentation and/or convince BMW to give me documentation that it's an approved repair method, and zero the toe. Then I'd start with about 1/8" toe-in in the rear with maybe -0.5 degrees of camber. This car is not going to want to put power down and it will need all the help it can get, so keeping the tires square to the pavement will help get on the throttle earlier, along with a good amount of rear rebound. The nitrogen pressure can be used to tune balance at individual ends of the car without upsetting the whole works. This is going to be a slow-in, fast-out car because of the weight and lack of tire but it's going to put some distance between itself and the competition wherever the throttle is applied because it's higher up the power/weight totem pole than anything else in AS.

Anybody have real-life experience in a serious effort yet with one of these?