Well let's say a Mercury for example. They offer a 20 HP and a 25 HP. The 20 is a 2 cylinder vs the 3 cylinder 25. The 20 is 5250-6250 RPM and the 25 is 5000-6000 RPM. They have the same fuel induction and ignition systems. The 20 HP has a carb, the 25 HP is EFI. The 20 HP weighs in at 115 lbs...