SPIT MK I/II and over boost

[41Sqn_Banks]

Originally Posted by jf1981

Quote "Another important difference between the Bf109E and the Spitfire Mk.IA lay in the supercharger design. The early Merlin engines were equipped with gear-driven single-speed, single-stage units. The supercharger had to be throttles back at low altitude to avoid over-boosting the engine. As altitude increased, more and more of the supercharger capability was used and engine horsepower continued to increase until critical altitude was reached, after which power fell off rapidly"

http://theairtacticalassaultgroup.co...s-Spitfire-MkI

This is wrong. Early Spitfire I were equipped with a fixed datum and later Spitfire I were equipped with a variable datum type boost control. Both limited the boost to +6.25 and therefore prevented over-boosting. +6.25 and 2600 PRM is "Climb power" which is allowed for 30 minutes.

Initially the boost control cut-out simply disabled the boost control to allow any boost up to around +17 at low level. Though the use of higher boost than +6.25 was only allowed when 100 octane fuel was used. (This is stated in a 1938 manual btw.!)

In 1940 when 100 octane fuel became available for operational squadrons the boost control cut-out was modified to disable the boost control but still limit the boost to +12 which was then allowed as emergency power for 5 minutes.

Reference: AP 1590B Merlin II and III Aero-Engines (October 1938, reprint October 1939).