I should start by saying this is not my first autocross, I'm a three-time regional class champ in a E36 M3. After a two day autocross in my new car I have some opinions about the capabilities of our beloved 135i as to its potential in this sport. The events were held on the largest venue we have available in New Mexico (Farmington) and therefore the course designs tend to be more open, faster layouts and a LOT of fun. top speeds by the faster cars were near the SCCA recommended limits of 65mph.
First the bad news. There is throttle lag with the turbos. It's my opinion that while there is hardly any lag in street use, the constant on-off-on-off throttle work of autocross simply overcomes the ability of the boost management system to react quickly enough to avoid noticable lag. This can be compensated for to some degree by earlier throttle application, and perhaps left-foot-braking (which is beyond my current level of expertise) while keeping just enough throttle applied to maintain boost. For those running in non-stock classes maybe aftermarket or modified diverter valves will yield siginificantly reduced lag--experiementation in this area may be beneficial.
The good news. Running stock wheels and OE runflat 'Stones at 41lbs front and 38 rear the understeer was limited and turn in was very good (once the stock springs and shocks got settled). I was VERY surprised at how good the car handled as I was expecting massive understeer similar to my unmodified E36 M3. The relatively short wheelbase is a big help in handling. I'm thinking a staggered wheel setup may be fine for the car and would like to see how 8.0-8.5" wide front and 9" (or even 9.5") rear with 235-245 and 255-265 tires would work in non-stock class autocross. This would require some fender 'massaging' I'm sure.
The eDiff. Not too bad, actually. Would it be better with a true LSD? Of course. 1st gear on-boost throttle while turning even slightly totally overcomes the braking of the inside rear wheel. In 2nd gear it works pretty well with less inside wheel spin. Larger and sticker tires in the rear would help a lot, I think.
The results. Well, I ran in D-Stock Street Tire and did well enough be make the
top-ten PAX both days, not bad for street tires. In fact, other members are whining about my class and want to see it raised siginificantly before the next event in three weeks. Oh well, I may wind up in B-Stock or even STU if I decide to do any mods. Over time, and as the car progresses in modification development, I think the 135i will prove to be a fierce competitor, especially in the modified classes, and even will do better in stock classes than previous 'MacStrut' BMWs. However, I was beaten handily by a well-driven A-Stock E46 M3 on street tires as the
Class Results show (scroll down to the bottom and note Raw times).
Here is a
video by a very fast Z06 Corvette that shows the relatively open and fast course.