By driving up close on civilian cars you build up the boost meter. Instead of trying to approach them without direct contact (which can be hard to perform) simply crash into a car that comes your way (it must not go in the same direction as you do). By doing so you will only lose about 40 km/h of speed but gain an additional full turbo boost.

Win all the Road Challenges in arcade mode.
Win all Evolution Championships in first place.

Win the following races to unlock the corresponding car or part.

Saleen Qualification: Hummer H2
Colossus: Nissan 350Z
Urban Sports Cars: Audi TT
Nissan competition: none
V6 Engine: Sport Tuning Kit Evo 1
Urban Performance Cars: 2005 Corvette C6
Low Displacement Challenge: none
Chapman tribute: none
Chevrolet Fighting: Sport Tuning Kit Evo 2
Aero 8 Trophy: none
Mythical Elanor: Morgan Aero 8
Barchetta Challenge: none
Under 500 hp: Performance Tuning Kit Evo 1
GT Championship: Nissan Skyline GT-R R34
Aston Challenge: none
German Power: none
TVR Challange: none
Gentlemen Drivers: Performance Tuning Kit Evo 2
Sport Racing Cars: Lamborghini Gallardo
V8 Fever: Unknown
V10 Rage: Evo Kit 1
Propulsion Challenge: Unknown
Performance Racing Cars: Unknown
V12 Only: Unknown
American Muscles: Unknown
4x4 Competition: Unknown
Carol Shelby Challenge: Unknown
Ultra Race Cars: Lamborghini Murcielago
Professional Challenge: Unknown
Italian Affair: Unknown