[Seadog]

Originally Posted by Crumpp

Read above it.....


"rpm limitations are directly associated with similar limitations on boost pressures also defined in relation to the operational condition".........

(vi) It may only be exceeded momentarily during aerobatics, fighting manoeuvers, or when diving.

Dowding:

"The use of the automatic boost cut out control enables the pilot to get an emergency boost of + 12lbs per sq. in, from the engine for 5 minutes when the circumstances demand it..."

(ii) Limit of 5 mins, for a maximum of 3,000 rpm at 6.25lb/sq. in. or more.
http://www.spitfireperformance.com/dowding.pdf