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Is it because of the inherent limitations of cooling the cylinder walls, only 2 valves per cylinder, relative ease of scaling up instead of dialing up the boost, or some other reason?

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  • $\begingroup$ The Merlin is .8 and the R2800 .89 lb/hp. But does the Merlin's number include the coolant and radiator? $\endgroup$ – John K Jan 24 at 16:19
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    $\begingroup$ Radials do not produce "much less" HP/L. By the end of WW II, radials and inline engines were all very similar in horsepower to displacement numbers. Most had max horsepower numbers approaching up to 1 horsepower to 1 cubic inch. $\endgroup$ – Mike Sowsun Jan 24 at 18:18
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A liquid cooled engine doesn't necessarily expose all its cylinders to the airflow, which reduces drag in comparison to a radial engine. Also, water is a much better medium for distributing heat from an engine so it takes much more heat to have a detrimental effect on LC engines (aka temperature above operating levels).
Additionally, the design of a LC engine (inline or V) require the cylinders to be arranged in banks where one camshaft can be used to actuate valves in multiple cylinder heads (thus producing more HP).

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  • $\begingroup$ On the other hand, rigidity is very important for a military aircraft. While radial engines can lose some cylinders and still be operational, LC ones will fail soon after getting coolant leakage. $\endgroup$ – Eugene Jan 24 at 21:37
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    $\begingroup$ Well, LC cooling doesn't require cylinders to be organized as banks they could still be seperate blocks - it rather enables it by removing the need to expose each cylinder to (sufficient) airflow. $\endgroup$ – Raffzahn Jan 25 at 0:01
  • $\begingroup$ The aspects you mention may affect power to weight and overall efficiency due to drag from the engine, but none of them have any effect to power to displacement ratio! $\endgroup$ – Jan Hudec Jan 26 at 21:45
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I read somewhere once, maybe I can find it again, but it is because of the inherent limitations of cooling the cylinder walls. Air cooling is not as controllable, because the temperature of the air varies with altitude and season. Hence, the temperature of the cylinders in an air cooled engine are not as tightly controlled as a liquid cooled engine. So, the cylinders need to be able to operate over a greater temperature range. Since metal expands with temperature, that means the gap between the cylinders and the bore is typically greater in an air cooled engine than a liquid cooled one. So, this greater gap, even with piston rings means some of the cylinder head pressure after combustion escapes rather than act on the piston. Makes sense to me...

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