There are four basic classes for Time Attack on this game. These have been based closely around the classes set forth by Redline Time Attack.
Enthusiast
Engine Mods: Stock engine only, no aftermarket turbo kits/parts. Camshaft (Engine Part B) and muffler upgrades allowed.
Suspension: Up to Level 2 only.
Brakes: Up to Level 2 only.
Tires: Up to Street Level 2 only.
Chassis/Aero: Stock body only; no reinforcement, weight reduction, carbon fiber parts, aerokit, or GT wing.