Mixture and Engine Pitch..

[PE_Tigar]

Just to chime in - in reality, Corsair had double the engine controls than we have at the moment in Il2. Aside from prop pitch, compressor stages, throttle, cowl flaps (that we have), it had more complicated mixture control, inter-cooler flaps, oil cooler flaps and alternate air control (not sure about the purpose of this one, some a/c have it for instruments, in case outside static gets frozen over).

As for the situation in Il2 - a/c with automatic mix perform "perfectly" - like they have a manual system that is perfectly managed. If F2 or F4 outrun you in level flight in Spit IX 25 lbs. you might be doing something wrong. Try the following - full throttle and trim the plane properly keep the slip indicator and climb speed indicator zeroed with trim and see what happens. One other thing that might be hindering you - cooler flaps on Spit IX are on closed/auto by default, so if you were running for a long time on full throttle they might be open, thus lowering your airspeed.