The Royal Air Force Fighters in this game are equipped with three different types of engines, all Rolls-Royce Merlins, but different in their characteristics.
Both the Spitfire I and Hurricane I DH5-20 are equipped with the Merlin II engine, which is an earlier version of the Merlin III. This engine is slightly less refined than the Merlin III and is more prone to overheat and damage when stressed. Pilots need to be aware of their limits.
The Merlin II was originally built to run at a maximum of +6 boost Manifold pressure on 87 octane gasoline, but advances in Gasoline refining technology produced 100 octane gasoline in time for the Battle of Britain. With 100 octane fuel, the Merlin II was capable of +12 boost pressure and greatly improved horsepower. However, as mentioned, this is an older generation engine, and needs to be treated with care when using high boost and rpm. Pilots should be cautious of using +12 boost and 3000 rpm except in all out high speed level flight. Use of these ratings in low speed maneuver or steep low speed climbs will cause rapid overheating.
Like the Merlin II, the Merlin III was originally built to run on 87 octane Fuel. It had a number of improvements to engine reliability over the Merlin II, and therefore was more capable of sustaining the high power generated at +12 boost, but still needs to be treated with care. Like the Merlin II, Pilots should be cautious of using +12 boost and 3000 rpm with the Merlin III except in all out high speed level flight. Use of these ratings in low speed maneuver or steep low speed climbs will cause rapid overheating.
The supercharger on the Merlin II and III were not capable of achieving full +12 boost to their original 16,250 ft Full Throttle Height. While nominally the Merlin II and III on 100 octane have ratings of 1310 hp, that is only achieved to a Full Throttle Height of 10,500 ft. By 16,250 ft, both engines are rated at 1030 hp, and from that altitude up, their performance is no better than an 87 octane fueled Merlin III.
The Merlin XII was a newer generation of engine, and had a number of important improvements.
First, it was designed to run on both 87 and 100 octane fuel, and was a stronger and more durable engine.
2nd, while has a lower maximum horsepower rating of 1175, it was capable of sustaining 1090 hp up to its Full Throttle Height of 17,550ft.
3rd, and very importantly, the Merlin XII had a newer design Radiator and cooling system, which was fully pressurized, an advantage over the partially pressurized Merlin III or the unpressurized Daimler Benz engines. The Merlin XII used a more efficient mixed glycol/water coolant system, compared to the full glycol systems of the earlier Merlin and the Daimler Benz. As a result, the Merlin XII is capable of maintaining +9 boost and 2850 rpm for 30 minutes. The limit for the Merlin II and III using 100 octane is 2700rpm and +6 boost for 30 minutes.