Hold Select while choosing an area selection at the map screen. Release Select when the auto-pilot engages. Start a race, and you will be able to use a boost. This can only be done with cars that do not already have a turbo.
Foggy weather
Select time attack, free run, or vs. battle mode and choose rain. While you are selecting the track starting point, hold L2 to make it foggy.
Quick start
When entering a race at any time and the clock is ticking down, when it reaches "1" press Select and punch the gas. You will take a fast lead against any other racer in the challenge. Note: After the race is over you will need to go to the garage to cool the engine down. This extra turbo boost heats the engine faster. Use this trick wisely.
Snowy weather
Select time attack, free run, or vs. battle mode and choose rain. While you are selecting the track starting point, hold L1 to make it snow.
Stormy weather
Select time attack, free run, or vs. battle mode and choose rain. While you are selecting the track starting point, hold R2 to make it thunder.
Switch engines
Get over 1242.8 miles (2000 km) on the odometer on a car to unlock the ability to switch engines on that specific car. Successfully complete 100% of the game to switch the engine of any car.