Effect of boost control cutout prior to +12 psi boost modifications

[Viper2000]

At the moment, we've got early Spitfires & Hurricanes with combat ratings of +6¼ psi boost at 3000 rpm, and a Spitfire II with a +12 psi combat rating.

We already know that the use of +12 psi required various modifications to the engine, as well as the obvious switch to 100 octane fuel, and that one of these mods was associated with the boost control cutout:



I was left somewhat unsure as to what would happen if you operated the boost control cutout and firewalled the throttle prior to the incorporation of the boost control cutout mod; I just found out:



So it turns out that if the engine hasn't been modified to give +12 psi, operating the boost control cutout at sea level should deliver +17 psi boost. Clearly on 87 octane fuel this would lead to detonation which would destroy the engine in short order. Equally, if the aeroplane was operating on 100 octane fuel then the engine would almost certainly have been modified for +12 psi operation because otherwise it would just be a waste of fuel.

I therefore conclude that the most likely reason for providing a boost control cutout originally was fear that failure of the automatic boost control unit would excessively close the throttle, effectively causing engine failure, and so the cutout was provided in order to guard against this possibility rather than to provide some additional combat power, and that its later use for this purpose was essentially a hack, rather like the modification of the Dh 2 pitch prop for constant speed.

This would also explain the fact that the Pilot's Notes for the Spitfire make no mention of operating the boost control cutout to attain any kind of combat power on 87 octane fuel; the 87 octane combat rating is simply +6¼ psi boost and 3000 rpm, which is reached with the automatic boost control engaged.

Therefore, operating the boost control cutout for combat power should be a 100 octane fuel only procedure; on aircraft not modified for +12 psi combat power it should almost certainly be an emergency procedure for use in case of ABC failure, carrying with it the risk of over-boosting the engine, causing detonation and rapid failure.