There are 4 kinds of boosts: DOUBLE CREDITS: The Double Credits boost doubles the amount of credits you earn at the end of all races for a limited time. NITRO STARTER: The Nitro Starter boost fills the nitro bar at the start of all races for a limited time. TUNING KIT: The Tuning Kit boost improves the performance of all the cars for a limited time. (The effect is the equivalent to that of buying one upgrade of each type). EXTRA TANK. When enabled, it doubles the quantity of nitro your cars can hold. Activate the Nitro Starter booster as well and start all the races with a full double-nitro bar!