Throughout the WW2 period, the RR Merlin was used on a wide variety of aircraft, and it enjoyed a huge development period that saw its power range double by the end of the war.
One of the things that is most obvious on later aircraft as compared to earlier aircraft is the exhausts - the Spitfire for example goes from three on each side to six. Up until now, having no real in depth knowledge of the engine beyond the famous stuff, I always had assumed that the reason for the increase in exhaust horns was due to increase in cylinders on the engine - which I now know to be completely wrong as the Merlin was 12 cylinder right through the war.
So why the doubling of exhaust horns then? Anyone know?