Bug 174 on 12lbs boost. Review please.

[41Sqn_Banks]

@Ospey I'd like to rephrase some sentences in the initial post. The changes sentences are in bold.

Description
Presently if the boost control cut-out is enabled on Spitfires and Hurricanes and full throttle applied there is no increase in boost above 6.25lbs. This results in reduced and inaccurate performance of RAF fighter aircraft.

During the BoB all frontline fighters had been modified to use 100 octane fuel in their Merlin II and III engines which allowed the engine to achieve 12lbs boost under strict limits. The Merlin XII engine fitted to the Spitfire II was designed to use this fuel from the factory. The approval and introduction of these changes gave the RAF fighters a performance boost in top speed, acceleration and rate of climb up to the Full Throttle Height (FTH) of about 18,000ft. At sea level a Spitfire was about 30mph faster when the boost control cut-out was enabled.

Merlin II and III (fitted to Spitfire Ia and Hurricane Ia)
The Merlin engines have a mechanical supercharger and can deliver up 17lbs to 20lbs of boost at S.L. with the throttle valve fully open using either 87 or 100 octane fuel however this would cause serious engine damage by pre-detonation in the cylinders. Because of this the Merlin II and III have a boost controller fitted which limits the boost to only 6.25lbs. A boost control-cut was fitted to disable the boost control and give the pilot direct control over the throttle valve. With modification to use 100 octane fuel the boost control cut-out was modified to limit the boost to +12 even if the boost control was disabled. The pilot subsequently could increase from +6.25lbs boost to +12lbs boost for emergency (= combat) purposes by enabling the boost control cut-out, which was the ‘safe’ power that the engine could produce when 100 octane was used. The use of emergency boost was limited to 5 minutes. The boost control cut-out was wired to indicate to the ground crew that it had been used and to make necessary checks. The Spitfire had a red lever at the front of the throttle control that was flipped and the Hurricane had a valve at the instrument panel which was pulled (“pulling the tit”).

Merlin XII (fitted to Spitfire IIa)
The Merlin XII did not require modification in order to use 100 octane fuel and had two systems to increase the boost above rated boost of +9lbs.

a. Boost control cut-out
The boost control limited the boost to the rated +9 lbs of the Merlin XII. The boost control cut-out was fitted to achieve emergency (= combat) power of +12lbs boost (see pilot notes) for 5 minutes up to full throttle height. The use of emergency boost is reported since 21 August 1940, 8 days after the Spitfire II entered service.

b. Gate control
In addition a gate control was fitted to achieve a special take-off boost. The gate control was enabled by pushing the throttle above the rated position through the gate. The gate control set the throttle valve to a predefined opening to achieve 12-12.5lbs boost. As the opening is not adjusted with the decreasing atmospheric pressure at altitude the boost will fall off very quickly. The take-off boost was limited to 1 minute and 1000ft altitude.