The XR-7755: The whole story

I was reading about the XR-7755 on the Internet and was wondering if you would clear something up for me. Most sources say there were two engines made, but I’ve found there were three versions. The first was set up for a single output shaft, a second had contra-rotating shafts and the third was set up with fuel injection. Are some of these versions the same engine or were there three engines?


Jersey Shore, Pa.

Thank you for your interesting question regarding the world’s largest reciprocating aircraft engine. The XR-7755 is very special to me because my Dad was a proud member of the engineering staff that designed and built this unbelievable engine.

From what I remember, my Dad always said there were three of these engines built and I would guess they were configured as you mentioned.

To my knowledge there is only one engine remaining and it’s on display at the National Air and Space Museum Steven F. Udvar-Hazy Center near the Dulles International Airport in Washington, D.C. I encourage everyone to pay this facility a visit if the opportunity arises. It’s my belief that the others were scrapped at the conclusion of the program.

I should go over some of the characteristics and specifications for this engine since I’d guess there are many people interested in aviation who may not be aware of this tremendous engineering feat during the mid-1940s.

First of all, its intended use was to power the giant B-36, which eventually ended up using six big Pratt & Whitney 4360 engines, plus four J-47 jet engines. The engine model designation XR-7755 stood for “”X”” meaning Experimental, “”R”” meaning Radial, while the 7755 was the cubic inch displacement for the engine just as the 320, 360, and 540 are for today’s common models of Lycoming engines.

The XR-7755 was a 36-cylinder radial consisting of four rows of nine cylinders each. These were in straight rows rather than in a spiral like the 4360 P&W. The reason for this is that the XR-7755 was liquid cooled, while the P&W 4360 was air cooled, which required the cylinders to be spiraled in ordered to get the cooling air across them. The actual bore and stroke of each cylinder on the 7755 was 6-3/8 inches x 6-3/4 inches, which gave a total displacement of 7755 cubic inches.

The XR-7755, rated at 5,000 hp@2,600 RPM, weighed 6,050 pounds dry weight. When you stop and think about it, the power to weight ratio on this engine was really quite good. If you compare it to a current production Lycoming 200-hp IO-360 series engine, which weighs in about 288 to 300 pounds, the big motor wasn’t anything to be ashamed of in terms of power to weight.

As an aside, the largest current production at Lycoming is the IO-720 series which, as you know, is 720 cubic inches, so that makes the XR-7755 more than 10 times larger.

The engine was 10 feet long and 5 feet in diameter, which was extremely large by aircraft standards of the day. I think one of the most interesting things about this engine was the fuel consumption at rated power. I guess all of us wish the fuel consumption on our current engines would be a bit better than, say, 10 to 12 gallons per hour, but this giant engine had a fuel consumption of 580 gph at rated power of 2,600 RPM.

For you engineering types who understand brake specific fuel consumption, this engine was capable of operating at brake specific fuel consumptions of .38 to .41 lbs./BHP per hour, at lower cruise power settings. That’s an economy rating that few current production engines achieve to this day.

Now for those of you who were a bit shocked by the fuel consumption, let me tell you about the coolant requirements for the XR-7755. At takeoff power the XR-7755 had to dissipate more than 95,000 BTUs, enough to heat an average home. To keep the engine within its temperature range limits, it was necessary to circulate water or other liquid coolant at a rate of 750 gallons per minute. To give you some idea of this capacity, you could nearly fill a standard railroad tank car in about 10 minutes at this rate.

When an engine is developed, it’s usually thought of as a growth engine that will, at some point in the future, get an increase in horsepower and the XR-7755 was no different. There are reports that indicate that if this engine had become a production unit, there was the potential for increasing the maximum power to as much as 7,000 horsepower.

Since the Lycoming factory is located in basically a residential area, you can imagine the distress of the neighbors when the engine was run at rated power. There are reports that a small neighborhood grocery store struggled to keep its canned goods from vibrating off the shelf. Lycoming, as always willing to resolve any problems, assisted the store owner in installing small raised strips on the shelves to keep the goods in place. I also was told by my Dad that all of the windows in a school building about a half mile from the factory would rattle when the engine was run at rated power. Apparently during those times no one even thought of complaining, which is a far departure from people’s attitudes these days.

With the end of World War II in 1945, the military no longer had a need for a new super engine and development of the XR-7755 stopped at the prototype stage. As the development of jet engines took place, there was no longer a need for a giant engine like the XR-7755.

As I sit here more than 60 years later, I still marvel at the engineering ability and rapid pace with which this engine was developed and built, even under the circumstances of that time.

Those of you who know me realize I’ve got a pretty good sense of humor, and when I think of this engine, I’ve often thought I’d love to approach the Smithsonian Air and Space staff and make a proposal. I’d like to borrow the XR-7755 from the museum and take it to the Reno Air Races this year and put it on display. I’d also like to have a huge sign that reads “”Orders Now Being Taken””.

Rebecca, thanks again for inquiring about this giant engine from the past. It’s a subject that is always special to me because of the small role my Dad played in this special part of Lycoming’s history.

Paul McBride, recognized worldwide as an expert on engines, retired after almost 40 years with Lycoming. Send your questions to:


  1. Paul Bartow says

    Would it be possible to mount the XR-7755 on a semi trailer, and actually start it up at air shows etc. ? Yes, It would take a lot of money to build a test stand like that, and is there any parts to build a second unit, because the museum will not give up the one they have ?
    Thank you for your time

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