Not sure if I should make this topic a new thread or not but will begin it here-
After a full season of using the GTR as an autocross car here are some learnings.
A. For the person just starting to autocross:
The GTR is not legal for any stock class in SCCA Solo events of Autocross. It is currently in A Street Prepared. Read the rules as to the limits of legal upgrades.
The GTR is a good platform but it is heavy. There is more performance available than most new drivers can harness and you are limited only by your level of driving skill.
Expect to finish as high as top 20% of drivers based on raw time but with the class that the GTR falls in you'd be lucky to finish in the top third on PAX overall.
Within ASP class in SCCA you can do well but not with street tires- maybe if your competition has worn R compounds you can try to keep up.
You can run the GTR completely stock, there is enough stiffness in the stock suspension to limit body roll but the smoother you steer and use the brakes and throttle the better control you will have and the less you will tend to understeer and oversteer. Too much speed entering tight turns will result in moderate understeer and too much throttle on exit of turns can lead to oversteer especially with runflats. The stock front 255/40-20 tires are fairly narrow for the GTR and barely keep up, while the rear tires seem to be OK with most of the wear on the front outer and inner edges.
Stock alignment can be adjusted and made more suited for handling but will result in more tire wear. Front and rear camber can be made -1.8 degrees, front toe 1/8" out and rear toe 1/16" toe in.
You can also leave the alignment at stock track settings and it would be OK. Expect tire life to be shortened by about one third vs street only use. The runflats will work in any weather dry or wet.
Use tire pressures that are about 33-34 psi in front and 31-32 psi in the rears vs stock 29 psi.
The car number magnet will work on the rear quarter panel behind the front door which is aluminum. A clear bra will help to reduce any damage from rocks. Tape is possible but a hassle.
Be sure to use a clean air filter for events.
B. For the experienced autocross driver new to the GTR:
Try to start with a stock GTR and later you can modify it. This will give you greater appreciation for the upgrades you did. Since power is sufficient you can start with suspension upgrades. Wheels and tires are first then shocks and springs followed by swaybars. Aggressive alignment is possible once ride height is lowered. About -2.4 degrees front and rear are possible with a one inch drop in suspension.
Lowering springs will improve handling and keep the stock shock console adjustments, street comfort can be better than stock. Coilovers or sleeves will allow for any ride height and slightly better handling but costs more. Labor is the same for springs or full coilovers.
Wheels can be 18" or 19" if you want to install R compound tires. Hoosier A6 (soft rubber) or R6 (firmer rubber better for two driver autocross or track) come in 18" or 19" but not 20". Kumho V710 only comes in sizes up to 18". Hoosiers are their best for about 6+ events while Kumho V710 can be good for about double that or one full season/year of events. You can use any tire longer but it tends to heat cycle and get harder and less grippy with time. Despite this some drivers will be able to do well for two full seasons even on worn tires due to driving with more skill. At this time there are no really good autocrossing tires in 20". The stock 20" rims are not bad so if someday there are better 20" R compounds then this would be an option.
18" wheel choices are limited by the large front brake calipers. Enkei GTC-01 18x10 +22et are a proven wheel that fits front and rear. Tires can be 305 or 315/30-18. You can also get custom rims and go wider in the rear. Typically you want the widest possible tire and widest possible rims. A wider front wheel and tire will help to reduce some understeer.
19" rims are better than 18" but can cost more, Volk RE-30 19x10 in front and 19x11 rear is good running 295mm front and 315mm rear tires. Spacers can be run front and rear from about 10-15mm in front and 15-20mm in the rears. Either longer studs and hubcentric spacers can be used or Ichiba version II spacers that use the stock studs.
Upgrading the wheels and tires while using stock suspension will be an improvement but without making the supension lower and stiffer it's difficult to get the full benefit of the better tires. The advantage is you do get some benefit and after you change wheels back to stock you get your full warranty and street configuration for daily use. It is arguable that a good suspension upgrade feels better than stock for daily driving espeically if it is adjustable or if it is not overly stiff. Lowering springs by Swift, Eibach and Tanabe will work both for autocross and street use.
To reduce understeer you can add either front and rear swaybars adding stiffness to both with slightly more stiffness setting in the rear or add only a slightly stiffer rear bar such as one by Whiteline.
Swift swaybars are designed to be less stiff and may be suitable for autocross and street use while many of the other bar sets are quite a bit stiffer suitable for track use but your results may vary with your driving style and choice of suspension upgrades and alignment. I think if you can minimize understeer that will help the most, and overly stiff rear bar can lead to more oversteer which can be more difficult to control. You can adjust tire pressures to help with handling, try slightly higher front tire pressures to reduce understeer. If you have an adjustable suspension try slightly softer front shock settings to reduce understeer.
I have also used B Woody endlinks front and rear and while they are stiffer than stock parts they are not essential for improving handling. I would say there is minimal improvement that I was able to appreciate. So far the only adjustable endlinks are from the Stillen front and rear swaybar set- these are good if you need to do corner balancing as part of a full coilover install.
C. For the experienced autocross driver that has driven their GTR and are ready for more:
See the upgrades listed above. Add upgrades to the exhaust midpipes and intake at least. Add Cobb AP with a canned (version 2.0) or custom tune. Read the rule book on what is legal for the class you are in or wish to compete in. ASP class does not allow upgrades to the turbos, BOVs, or injectors. You must have a full interior. A racing harness is nice to have, a CG lock for the seatbelt is workable.
Custom extra wide rims and wider tires are a plus but you may add more weight, tighter fit and higher cost. Really wide rims may need wider fenders.
Be careful on the extent of your upgrades as engine mods can bump you out of basic classes like Street Modified and ASP, read the rules. You can run any plugs, any fluids, no race gas only premium pump gas. Some regions are more strict about exhaust loudness so if you have a variable valve exhaust that might help vs one with no mufflers or resonators.
D. For the crazy GTR owner ready for anything:
This is more for fun than anything else. Probably 500-550 whp is enough for any autocross GTR. Max out the suspension. The eligible legal class PAX handicap will be difficult to overcome.
You can run a track ready GTR at autocross but the runs will be short. Save some tire for funruns.
OK, that's plenty to get you started and going to driving school is always good no matter what car you drive and what class you are in. If you want to understand autocrossing up close then get involved with the behind the scenes aspects of the sport such as- course design and setup, timing, registration, tech inspection, training and supervision of course workers, or be a safety steward.
Stock wheels and tires, swift springs with 1" drop.