OK, DRock was right - you're over revving your engine.
The Spit 100 Octane was an early Mark of Battle of Britain Spitfire. It does not have a constant speed prop. Technically it's a 2-speed prop (Fine pitch/Coarse pitch), but the BoB Spit pilots figured out how to manipulate the prop pitch control to give intermediate pitch control. This sim models that manipulation. (The Prop Pitch control in the Mark 1 Spit that you're flying looks a bit like a bicycle pump plunger, compared to the Spitfire 1a & 2a Prop Pitch lever control).
What's happening is that when you start off at 85% prop pitch in your Mark 1 Spitfire on your take off roll your engine rpms are a bit less than the maximum 3000 rpms limit - which is good. But as you climb the air is getting thinner, so the propellor is encountering less air resistance as it turns - hence the engine rpms begin to gradually increase. Before too long you've exceeded 3000 rpms and you've blown the engine! To avoid this you must carefully monitor your engine rpms as you climb, gradually pulling back on your prop pitch (bicycle pump handle
) to coarsen the pitch and keep the engine revs below 3000 rpms. A bit of extra workload, to be sure.
Do yourself a favour: grab a Spitfire 1a 100 octane or a Spitfire 2a. They both have Constant Speed Propellors. In other words, when you set the Prop Pitch for 85% for take off (= about 2850 rpms), the engine rpms will stay at 2850 rpms as you climb. (The Constant Speed Prop automatically coarsens its pitch as you climb to maintain the 2850/85% rpms that you set on takeoff - hence the term "Constant Speed")
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