What part do you eventually earn in Sega GT?

[BullitNC]

So far I'm into level 4 parts in weight reduction, intercoolers, and tires. Getting the race tires seemed to take forever, and now I have super race tires. What other parts will still level up? I think those 3 are the only ones I have at the 4th level now.

So far: = progressions

Turbos= 2
Tires= 4
Intercoolers= 4
Weight reduction= 4
Brakes= 2
Suspension= 3 (or 4) is full race 3 or 4? Sport, semi-race, race, is that the progression?
ROM= 1
NA Tune= 2
Filter/exhaust= 3

[nemesYs]

... i don't think i even have those.

[Comic Book Guy]

Suspension - You'll get "original tune" or something that kick ass.
Exhaust - I think only one more but it only gives me about 8hp and nothing else
Brakes - I think there is only one more upgrade because I only have 3. My Lotus Elise Exige can stop on a dime!

And I don't know about any more.

[Carnage]

apparently turbine can be leveled to 3? I finished seaon 2 and didnt get it, but i remember someone mentioning they had it..