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

[whoarmongar]

The figures I have from SpitIIA pilots notes are

Max climb 2850rpm @ +9 (+7)
Max rich 2650 @ +7 (+5)
Max lean 2650 @ +4 (2.5)
Combat 3000 @ +12 (7) 5 mins max

100 octane fuel, figures in brackets 87 octane

[Viper2000]

This thread is about Spitfire I and Hurricane I aeroplanes whose engines had not been modified for +12 boost.

[The Spitfire II has a different engine (Merlin XII) and was designed for +12 psi from the start; therefore its boost control cutout would have incorporated the modification and so operating the cutout would give +12 psi; doing this on 87 octane fuel would be a Bad Idea, and you'd be able to get to the 87 octane boost limit with normal throttle movement.]

In any case, the point is that operating the boost control cutout and advancing the throttle to the stop only gives usable combat power if you're using 100 octane fuel.

If you're using 87 octane then it breaks the engine; it'll just do it somewhat faster (+17 boost instead of +12, and a generally weaker engine) if the engine in question is an unmodified Merlin III than a modified Merlin III or a Merlin XII.

[41Sqn_Banks]

AP 1590B Merlin II and III Aero-Engines

In its unmodified condition, the boost control cut-out valve permitted the boost pressure to rise very considerably. Mod. Merlin 154, however, limits the pressure to +12 lb. per sq. in. for use under emergency condition (see para. 359A et seq.).