Up close and personal with DC-7 tankers: power plant specifications

As I was looking around on the web recently to learn some history of the Douglas DC-7's when I ran across a couple of references to power plant (engine) specifications for the four engines on the DC-7. The specification was as follows:

4× Wright R-3350 18EA1 Turbo-Compound radial piston engines, 3,400 hp (2,535 kW) each

Now because of the reference to a 3,400 hp engine, I suspected that these power plant specifications where for the DC-7C not only that but some of the other specifications (not listed here) did not match those that Larry had sent me earlier. They appeared to be for a slightly bigger and heavier aircraft. So, I sent Larry Kraus an e-mail, I include his response below. But first I want to thank Larry for the time he took to write me and walk me through the incorrect power plant (engine) specifications. I am now slightly better able to read power plant specifications in the future. Here is what Larry says:

Our engines are a mix of civilian and military engines.The civil engines are either DA4's or EA1's. With 115/145 avgas,the DA engines were rated at 3250 hp. The EA's were used originally on the DC-7C and had some localized strengthening of the crankcase and improved mounting flanges for the PRT's that allowed an increase to 3400 hp (the C model was larger and considerably heavier than the DC-7.I think that the max take-off weight for the C model was 142,000.

We also use R-3350-42's and a couple of other dash number Navy and Air Force engines that were used on WV-2's and C-121's.These were military versions of the Lockheed Constellation. The P-2 also uses very similar versions of the R-3350. All of these engines are the Turbo-Compound models with the PRT's (Power Recovery Turbines).There are 3 PRT's per engine. They are similar to turbo-superchargers in that they use engine exhaust to spin a turbine wheel,however,instead of the tubine wheel being attached to an air compressor (to boost the air pressure at the carburetor), the PRT's are connected directly to the engine crankshaft via a fluid drive something like an automatic transmission in a car.

The main difference between the DC-7 engines and those in the P-2 is that the DC-7 engines use direct fuel injection with an injector on each cylinder and the P-2 engines use a single large injector into the eye of the internal supercharger that's driven by the crankshaft. I don't remember all of the dash numbers for the 3350 military engines that we use,but if it matters.I think that I mentioned it in my WIX thread somewhere. These are large engines with two rows of 9 large cylinders (18 total). R-3350 means a Radial Engine with a 3350 cubic inch capacity (the internal volume of the cylinders). A large 8 cylinder car engine might be 350-400 cubic inches.

The main point being that all of these engines (DC-7 and P-2) have been de-rated to 2880 hp while using 100 octane fuel. I'll attach a cutaway of a DC-7 engine from the Douglas manual (above) that gives a view of the PRT drive. It's a bit complicated, but the PRT's really do recover horsepower that's just going out the exhaust. With the 115/145 power, each of the 3 PRT's is said to have been worth 150 hp at take-off power.

So, if I were to say anything about the power plant on Butler's DC-7's, I would say the following:

Four 2,880-horsepower Wright R3350 engines.

Thank-you Larry!

This just in, I am adding a couple of posts tomorrow on changing out engines on the DC-7. Stay tuned for more.

Note: double click on the manual page to enlarge the view for easy reading.