Black 6 Thu 19th Apr update re Spit II

[41Sqn_Banks]

I've found a description of the "Gate Control":

Gate Control
On gated engine a high take-off boost is obtained by going through the gate, the pressure obtained may be of the same nominal value as emergency boost, +12 lb, but the method of control is entirely different. With pilot's lever at gate, relay piston has moved almost fully rearward to control at Rated Boost - +9 lb. The action of going through the gate opens the throttles a positive predetermined amount, causing boost to rise. Regulator piston will then complete its rearward travel in an endeavour to hold boost down to 9 lb, but has insufficient range to do so and has no effect.

As through the gate is a fixed throttle position, boost obtained will vary in ratio to the day's atmospheric pressure, and will fall with altitude, the regulator having no control at all under these conditions, although it would maintain the boost when value has fallen to +9 lb. This condition should never arise, as "through the gate" should only be used to 1000 ft.

http://www.enginehistory.org/Piston/...erlinABC.shtml

According to this description the boost obtained by the gate control may vary depending on atmospheric pressure, this would explain why the maximum take-off boost is given as +12.5 in the later manual, as this would possibly be achieved on days with high atmospheric pressure.