the reason supercharged engines can have a higher compression ratio is that there is a LOT less back pressure in the cylinders, as there is no turbocharger to restrict the exhaust gases. This has the effect of letting more of the exhaust gases out and so there is more "space" for fresh charge, and less chance of detonation Due to a lower adibatic compression ratio and lower combustion chamber temperatures. Superchargers suck the bobby, turbos are the future.