Stage I kits are best for vehicles that are stock. - Performance 550 Motors with Internal Fans & Lightweight Pinion Gears
Stage II kits are best for vehicles that are stock or modified up to 18 volts. - Performance 550 Motors with Internal Fans and Heat Sinks, Steel Pinion Gears & Steel First Gears on Ball Bearings.
Stage III kits are best for vehicles running 18 to 24 volts on a tight budget. If you're tearing up first gears you'll want Stage II or IV. Performance 775 Motors with Internal Fans.
Stage IV kits are the most durable and powerful option for 18 to 24 volt vehicles. - Performance 775 Motors with Internal Fans, Steel Pinion Gears, Steel First Gears on Ball Bearings.