[Graham Boak]

Kurfurst: Allied engines also went the route of increased displacement, for example Merlin to Griffon, Taurus to Hercules to Centaurus, Twin Wasp to Double Wasp. This in itself was not specifically a German approach: indeed the step from DB601 to DB605 was not linked with any major increase in displacement. As for which detail design route is the most efficient, the RR engines managed similar output to the DB designs despite considerably smaller internal volumes: that seems one measure of efficiency to me.

Kenneth: To some exctent this is just different ways of saying the same thing. Air pressure reduces with altitude. An engine has a maximum pressure it is designed to withstand (which can vary with time, hence the time limits on different power ratings). This can be above that obtained naturally at low altitudes. The purpose of the supercharger is to provide the engine with that maximum air pressure, by forcing more in . Each supercharger is designed to provide a given maximum boost, allowing excess to bleed away at sea level, and hence as the aircraft rises in the air the supercharger is providing a greater proportion of the pressure, until it reaches its maximum - the full throttle height (or rated altitude). Above this height the engines' maximum boost cannot be maintained, and the power output falls.

So at altitudes above the full throttle height the engine is short of the ideal amount of air - you may see this as a shortage of oxygen if you wish (as I'm sure the combustion specialists would) but it is normally expressed as a pressure shortfall. Using exotic additives at altitude is a way of adding more oxygen to the mix, but this saw little use outside Germany, which had specific problems with fuel supplies.