As always the argument for liquid-cooled versus air-cooled engines is not simple. There are three components to an answer, immediate power transfer to the airscrew and how efficiently the heat transfer to the outside environment are the two major components that usually come to mind, but mechanical supercharger stages from the crankshaft versus the engine exhaust driven staged turbocharger stages often combined with a mechanical supercharger stage intrudes severely on those two qualities. I would argue that the British, and the Italians did best in the engine aspiration problem (one of the reasons that I like the British enfines so much) and that the Germans and the Americans were the worst when it came to that part of the engine design problem. In fact the Wright engines were positively horrible to aspirate either mechanically or with an exhaust driven turbocharger stage.

The R3350  being the bastard example in point, that PoJ was an in flight maintenance nightmare that cost the US many a B-29 from mission turnback due to engine failure or crashes on takeoff due to 'unexplained'^1  engine failures.

^1 Having the turbocharger overheat or the oil reservoir burst and the engine explode is 'unexplained'? That was one of the major problems the 'Battle of Kansas' identified. The R3350 at the time was a piece of CRAP engine.

H.         

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The Jumo 213 was a rework of the Jumo 211 and was a Junkers revisit that copied the DB 601's pressurized cooling jacket design. It was a GREAT engine, but timing and aircraft selected  is everything when it comes to effectiveness. The Jumo 213 needed a LOT of supercharger work to get it to work properly so it was late in service and it was mated to some decent bombers (the 88 series bombers ) but  the selected fighters it was tried on were mediocre (with the exception of the FW-190)

The DB603 was about 1.30 kW/kg. The Jumo213 was about 1.35 kW/kg . By comparison, the standard Allison V-1710 was 1.75kW/kg! Aspiration was EVERYTHING in those WW II engines. 

The German engines were heavy and this hobbled their aircraft. Still the engines were tough and with late war superchargers these were effective designs in the lightweight German aircraft frames even with the low octane gas the Germans used. 

I don't know how much people like to hear it, but even though the Junkers and Daimler Benz engines were flying Swiss Watch quality engines with 21st century engineering in 1930 engines, as aero-engines they were just not Merlins or Pratts. Shrug.

Its one of the reasons I tear my hair out about the damned Allison. That engine should have been properly aspirated and developed with anything and everything that US automotive engineers knew how to do. 'Modern' Allison V-1710s at sea level with modern two stage superchargers do well above 2.0kW/kg.  That is how good the Allison potentially was/IS. 

H.   

German engines usually has lower power to weight ratio. Part of that reason is inferior fuel, but mostly due to heavier engine frame derived from DB601 . I have not seen the particular design, but German engines usually have slight inferior two-stage supercharger with the notable exception of DB series. Eventually they caught up and installed some decent superchargers on Jumo engines used in Ta-152, but that was too late.

The Germans were very clever in much of what they tried to do. They often tried to design universal engine mounts and leads in their aircraft and tried to get their aqircrafy manufacturers to design to those mounts so that the Luftwaffe could swap Jumos for DBs as to type available. The 601s were the pattern setter. Not until the power-rgg did a srensible four lug quick connect yoke system come into general German use.  Once they stumbled into it, the Allies (especially the Americans with their radial dominated force, switched to copy. The Germans before used rails and lugs and that cost weight in the block and completely screwed up plumbing paths. The Jumos supercharged to starboard while the DMs supercharged to port. (I may have that backwards) but the point is that they plumbed opposite to each other.             

Even at the higher octane ratings, the German engines suffer lower watt/kg ratios from their average higher block weights compared to Allied equivalents. The Germans were clever that they could use injection boost at the cost of burned out enfines to ramp up acceleration during key minutes of air combat. THAT made the FW-190 deadly during the two or three minutes that the power and acceleration was needed in the dogfight. The Hermans pioneered that concept of  'critical boost'.       

Had they made good supercharger work with BMW 801 and had some decent Fw-190 capable of high altitude fighting, it would have been more nasty for the Allies.

Same could be said for the Pratt R-2800.  If the supercharger stages had been rectified for it earlier and the engine redesigned as a lighter power-egg, then the Hellcat would have been deadlier than it was. That would have been more than a match for an FW-190.

Contrary to Herald said, I believe that Italians may have good supercharger design, but they didn't have two-stage supercharger deployed in large numbers.

I point out that the Germans made a lot of engines without  two stage superchargers and sent them to theaters (Russia and the Mediterranean) where high altitude combat was not the norm. The enfines were mated to the theater need Their remarkably few high altitude band tuned engines were reserved for speciulized recon birds, and their home defense fighter forces. This was common practice among all the combatants. For example, American medium bombers generally used engines that were aspirated for 3000-6000 meters effective. It was only the heavy bombers that received the expensive high altitude super chargers and turbochargers. This made sense. You use the type engine to fit the intended use altitude band. 

H.

I have to agree. The Army Air Forces wasted millions on the hyper-engine project in the 1930s. The Allison started as a private venture and was remarkable in that it almost achieved in an LC engine what those idiots in their machine shops at Dayton failed to accomplish. The stupid decisions with turbo-chargers was a failure to design and figure out the lube, heat compression and gas flow problems. A two stage mechanical supercharger might have been simpler, but then the Allison was NOT designed to have one built into the block. That had to be added as an afterthought. The only convenient place was where? Aft, and then its portal diameter was SMALL.

allison-v-1710-supercharger-im...

There are only so many kinds of STUPID, to be seen to be believed, but the Packard engineers must have laughed  at their GM counterparts when the first Merlin arrived at Packard to be reverse engineered and  IMPROVED from the rather shoddy RR workmanship to Packard standards. The British had thought about aspiration first, so the Packard engineers had an easy time to fix the basic carburetor and crank faults in the Merlin design.

Nothing was going to help the Allison in 1942-1944 except a major leap in the understanding of gas flow in an ICE's manifold geometry from supercharger stages to cylinder heads. That would not happen until the later 1940s, postwar. Until then the math was not there. It was all an art and intuition-more trial by error to know how to squeeze pipe diameters down over the different run lengths to maintain constant feed volume to all the cylinders, with the years of work that the Allison never got until postwar when hobbyist and aircraft racers finally SOLVED that plumbing nightmare.             

It has to do with what kind of people are running the military in the US in early 20th century. Sorry, but the most brightest and smartest in the US usually go for business for big bucks. The military cuts in the 20s and 30s has transformed the military into a stagnated group of bureaucrats. The problems were only rectified after bloody natural selection under the bath of war, where only the superior ones can survive and excel.

Well, let's not blame the people who worked on the hyper-engines. Both private companies and Army engineers produced no less than three candidate engines that bench tested to the idiotic designed specifications. Trouble is, the program managers kept changing their damn minds and nobody was smart enough to settle on a SINGLE engine and a single standatd. America did not have the time, talent, or money to screw around with four government engines. There was only enough resources for one. That should have been either the Allsion or an Army Air Force LC hyper that could produce 1500 kW at 1.7kW/kg in an aspirated version that could be either single stage supercharged at 3-6000 meters or two staged at 3000-9000 meters. Auto controls would have been a necessity for such an engine then to simplify manufacture costs . That is the Americans would have beaten the Germans to the power-egg by half a decade through sheer Depression Era necessity.               

The radials needed the superchargers and turbo-charger combinations . This was especially true of the damned Wrights. A Pratt could get by with mechanical superchargers, but rob a Wright of too many watts from the front end and watch that radial engine burn up.    

Hermans wanted to make sure that their tear downs and rebuilds lasted 2,000 h