The compression ratio for blowing, depends on the blower. A turbo can run a lot higher boost on the 10:1 ratio than a positive displacement blower.
The difference is , how much boost there is at low engine rpms. The turbo does not boost at 1500 to 3000 or so. This is the danger rev range for boost, that causes detonation.
A PD type supercharger can provide 90% of its total boost straight off idle. So they need either a lower comp ratio, or a boost controller to limit the boost at these rpms.
The engine is most prone to detonation at low engine speeds, due to the time that the piston takes to move over top dead center. The combustion chamber is held at a fixed volume too long for the burn rate.
Harvey.
