Remember the 1980s when turbocharging first became prevalent in cars? Manufacturers—many of whom were not even building cars—were quick to slap “Turbo” on anything that was supposed to be an upsell. Long before the Reagan Administration, however, airplanes were using turbines to make horsepower. A number of World War II airplanes, like the P-38 Lightning, were powered by turbocharged engines to allow performance at high altitudes. This 18-cylinder, two-row Wright 3350-89 radial engine was not used in a World War II airplane nor is it turbocharged in the traditional sense, although the earlier-series 3350 powered the B-29 Superfortress.
Instead, this is a turbo-compound engine. While it still adds horsepower by putting exhaust gases to work like a turbocharger, the turbo-compound instead uses the exhaust to spin three turbines that act like secondary engines. These turbines are turn the crankshaft via a fluid coupling that also serves to dampen vibration from the turbines spinning at more than 20,000 rpm. An old Wright brochure shows that the turbo-compound system could produce nearly 3,400 shaft horsepower at take-off, which was almost 600 more than the same engine without the turbines operational. The turbo-compound engines did have a propensity to wear out engine components like valves, but the overall increase in efficiency offset the maintenance costs, largely.
It’s a pretty trick system that was developed during World War II, though not implemented until after, and largely applied to the great piston-engined airlines that just preceded (and overlapped slightly) the Jet Age. The non-turbo-compound Wright 3350s are still badass engines, making 2,500 horsepower easily. The Rare Bear Grumman F8F Bearcat that won piles of air races and also set all kinds of piston-engine aviation records gets its power from a non-turbo-compound 3350.
Enjoy this engine-stand runup of a turbo-compound Wright 3350-89 and, hell, watch the Rare Bear rip around Reno at 473 mph in 2013 if you’ve got the time.
Whilst developing their belated answer to the turbo revolution in F1 and under considerable pressure from Ford, Cosworth actually considered building a compound turbo motor. How the FIA would have allowed it to race is open to debate as it would have a significant power advantage.
I’d like to see one of these in a huge streamliner break the 500mph barrier at Bonneville next year and continue tradition of using aero engines to break land speed records. What a sight (and sound) that would be!
Yeah, I was previously unfamiliar with the turbo-compound engine and F1/WEC was my first thought. Not surprised that it was considered for use, would love to see someone run one in…anything. Super-cool engines.