The purpose of the boost control cut out is stated in AP 1564A Hurricane I Aeroplane, Merlin II engine, March 1939 (prior to adoption of Mod. No. Merlin/154 - Modified boost control cut-out valve).
Section I, para. 15: Emergency Control – Cut-out for automatic boost. – ... It is intended for use should the automatic boost control fail in flight or should it be necessary in an emergency to override the automatic control for increase of boost. (See attachment)
The effect was described by W/C R. C. Wilkinson who flew with 3 Squadron during the Battle of France.
The original Hurricanes fitted with two blade props had about 6.25 P.S.I. (pounds per square inch) and when the throttle was pushed “through the gate” to give full boost, it would put on 16 P.S.I. This was extremely hard on the engines so the regulator was modified by drilling a 1/32 inch hole in it, this would bleed off the excess pressure, so it would only put out 12 P.S.I.
When the three blade steel props were fitted to the Hurricanes and the throttle was pushed through the gate it would run about 2,600 R.P.M. with a 12 P.S.I.
Wilkie simply put a wooden match stick in the 1/32 inch hole thus his two blade wooden prop would run at 3,200 R.P.M. with 18 P.S.I. boost when it was pushed through the gate. He definitely had the fastest aircraft in the Squadron but he was extremely hard on engines. During the Battle for France Wilkie used nine aircraft in ten days, amazing what a match stick in a bypass line will do. However, it should be noted, Wilkie was always the last in the air as he needed more runway to build up speed.
From: Spitfire RCW, The Wartime Exploits of Wing Commander Royce Clifford Wilkinson (See attachment)
Spitfires were cleared for 100 octane in September 1938, the month following their introduction into operational service.
Memorandum dated 24th September 1938 from the Air Vice-Marshall, Assistant Chief of the Air Staff.
1. I am directed to inform you that in order to improve the take-off performance of Spitfire aircraft, the use of 100 octane fuel by squadrons equipped with this type has been approved.
2. Improvement in the take-off performance of Hurricane aircraft will be obtained by the adoption of C.P. airscrews but there will be a period of some months before this modification is introduced. In order, therefore, to obtain some immediate improvement in take-off performance, the use during this period of 100 octane for Hurricanes has been authorized by squadrons at those stations where you consider it to be necessary.
It seems apparent that in order to make use of 100 octane for take-off from certain fields (with early, not constant speed propellers) it would be necessary to operate the boost control cut-cut.
Since 100 octane was introduced in Fighter Command before the Battle of Britain and before the Battle of France for that matter, the whole subject of the effect of operating the boost control cut out with 87 octane fuel is rather irrelevant to a Battle of Britain simulation; or perhaps at best merely of academic interest.