Prelude-88-91

S = Carb'd
Si = Fuel Injected

Prelude-92-96
S = F22 SOHC
SI= H23 DOHC
SI = Vtec

Basically the S, Si, Ex or whatever is the Trim Level....little extras like tilt steering, sunroof/moonroof, leather seats little stuff like that...and also engines are different in some trim levels.