Brian, what seems to the cause (or causes) of the performance loss?
Thermodynamics and sub-optimal engineering Dan.
Liquid cooling has advantages and disadvantages.
The disadvantages are clear - added complexity, added failure points, possibly added weight.
The advantages include better temperature control and thus the possibility of running closer engine tolerances. So less oil consumption, less blow-by and thus longer lasting oil. Furthermore no shock cooling, and - and here’s the kicker - the possibility to transport heat to have it removed from the system to a more suitable place than the up-front-in-the-nose location that you are forced to with air cooling.
Turns out that right behind the prop is not the best place for cooling. No good flow due to disturbances from the prop, no room to create a smooth path for air to flow without having to navigate all sorts of turns and around objects. As a result, with our air-cooled engines, cooling drag can form a significant part of the total drag of the airplane. As high as 40%.
The P51 Mustang is also known for its “Meredith effect” of its cooling system, which according to many actually produces a little thrust. While this is debatable, what its cooling doesn’t do is create 40% of the total drag of the Mustang.
The Spitfire and BF-109 with their wing-mounted radiators don’t do so bad either.
The secret is the smooth ducting that they can have, slowing down the air to hit the radiators at a significantly reduced speed, and then once through the radiator, the air can be accelerated again to exit at roughly the same speed as it entered. Thus posing very little drag. And enabled by liquid cooling, which transfers the heat from the engine to that place in the rear fuselage or wing.
One thing that air cooling has going for it is that you need less air over an engine than you need through a radiator. Simple thermodynamics - heat transfer is proportional to the difference in temperature between the media. And the cylinder heads of an air cooled engine get hotter than the coolant in a radiator. And so you need more air through that radiator than you would over an equally powerful air-cooled engine.
Now for the Eggenfellner.
Presumably for reasons of simplified installation, Eggenfellner chose to not take advantage of the option of moving the heat to a more suitable place for cooling. In stead, he stuffed the radiators in the nose. Worst of both worlds - bad cooling location, no way to create a smooth airflow with an engine sitting right behind the radiators, and to add insult to injury you need more air through them than you would have needed with an air cooled engine.
Result - massive air resistance and mediocre cooling.
And there’s your answer…
As an aside - an excellent read on this subject are the (now ancient but still valid) Hans Mayer articles:
http://www.glasairproject.com/GlasairI/AirSig/CoolingSystems/cooling1/CoolingSystems1.htm
http://www.glasairproject.com/GlasairI/AirSig/CoolingSystems/cooling2/CoolingSystems2.htm
http://www.glasairproject.com/GlasairI/AirSig/CoolingSystems/cooling3/CoolingSystems3.htm
http://www.glasairproject.com/GlasairI/AirSig/CoolingSystems/cooling4/CoolingSystems4.htm