numbers you see catgorize engine size. Numbers in the hundreds usually are electric sizes. Blade is 300 size, trex 450 is 450 size, trex 600 is 600 size. Smaller numbers are usually nitro engines. Usually they have a dot (.) infront of them. Caliber 3 is .30 size, raptor 50 is .50 size, caliber 60 is .60 size and avant 90 is a .90 size. I think nitro engine size is cubic inches.