R975 Radial engines

[Hanno Spoelstra]

Originally Posted by Bob Phillips

Continental manufactured the R975 under licence from Wright and made many thousands of engines- far more than were ever made as aircraft engnes. The earliest tank versions were called R975-EC3 I believe, and rumour has it they used high octane (100) aviation gas - but none of my manuals confirm this. Hanno do you know?

Bob,

Thanks for posting this interesting info. I love the sound of radial engines! As you say they are not particularly well suited for use in tanks, but at the time aircraft engines were the only engines with the power output needed to propel AFVs.

Early M3 Medium tanks and Ram Cruiser tanks were fitted with the Continental R975-EC2, a licence produced Wright Whirlwind. My copy of Data Book: Tank Type Vehicles of Canadian Manufacture dated January 1944, states: "The R975-EC2 Series Engine requires 91 Octane Aviation fuel, while the R975-C1 operates on 80 octane motor fuel (the standard fuel used by the mechanized forces of the United States Army). This is accomplished by changing the compression ratio of 6.3:1 in the R975-EC2 to 5.7:1 in the R975-C1, by a design change in pistons and changes in spark advance and in the carburetor main metering jet."
It lists for the R975-EC2:
- rated horsepower: 400 at 2400 rpm
- torque: max. 890 ft.lbs. at 1800 rpm
The Data Book refers to: Hand Book Continental R975-EC2 Ordnance Engine. Continental Motors Corporation, Detroit, Mich., April 1941.

Regards,
Hanno

[Bob Phillips]

Thanks Hanno for confirming that early R975s had high compression pistons for use with higher octane fuels. Twenty years ago when I was actively buying R975 parts one item that every big dealer had was quantities of the modified pistons ( # 202050 I think) these were the downgrades to 5.7 compression ratio for use in the early C1 engines. Unlike aircraft pistons ( and later tank pistons) these downgrades were externally very similar in dimension to the aircraft piston and achieved the lower compression ratio by creating a concave centre in the piston hence a lower compression ratio. Unlike the other pistons it was of cast ( not forged) construction and did not have the heavy reinforcing grid/baffles in the underside of the cylinder head. I remember being told by one of the " old timers " that they were " no dam good " because the cast pistons tended to crack early in their life span and were soon replaced by another 5.7 ratio piston in which the piston pin was moved slighly closer to the top of the piston, lowering the comp ratio but also being of forged manufacture with reinforcing grids inside. There were also apparently oversize pistons available ( I have a cast one in .010 os, and several rings sets in .020) but I never have seen or heard much about them aside from the few examples I have.

[drcowie]

Thanks Bob for more info. How do I access those manuals to download - can't see anything on the Forum ? also could you help with contact details for Brian Asbury?

[BCA]

Hi Des,
Your R975 Base Shop Data Manual went out to you by airmail yesterday. Perhaps there is another person by my name you'd like to contact!?!
.......... Brian

[Bob Phillips]

I wanted to revive this thread to address the issue of using lower cost 975-46 engines as a parts source for tank engines. I have had the good luck to aquire a couple of these high horsepower helicopter engines and will share what I have learned about them. Please include your comments and experiences with these engines. There are still very nice -46 engines available for reasonable prices. These are helicopter engines, some of which can produce up to 550 hp ( take off) using high octane aviation fuel (100/130 or 115/145). These are the ultimate refinement of the R975 engine and were designed and built by Continental after WW2.

r975-46-.JPG r975-46--.jpg

The initial impression you get is that this is a much more heavily built engine. The crankcase is heavier, the rear crankcase is simpler and heavier than a C1 or C4 and the magnetoes are larger, the carb huge! In front a larger oil sump, a longer crankshaft (as per all aircraft engines and cylinders that are bigger than C1 but smaller than C4s. The thin web of a C1 or C4 crankcase is easily damaged either by rod damage (piston fails striking sides) or when bearing ejecta is smashed between the case and rotating crankshaft counterweights. The -46 case is solid where cylinders mount and much sturdier.
Inside there are several other differences. Pistons in C1 & C4 engines are 5.7 comp ratio (early ones cast later forged) while aircraft and the -46 are 6.3 comp ratio ( though higher ratios were used in prewar aircraft engines). All tank engines use "square" comp rings. The -46 and some aircraft variants use tapered keystone compression rings. The tapered rings are widely used on diesel engines and are currently used on the standard Continental 1790 diesel tank engine. A variety of oil scraper rings can be used. Note that all the forged pistons have reinforced ridges in the back, only the 202050 cast has none.

[Bob Phillips]

All early aircraft engines and C1 tank engines use a 6 7/8 inch impeller with 12 vanes. I do not have a C4 impeller but the manual shows what appears to be a larger diameter impeller with 16 vanes. Can anyone provide outside dimensions? The -46 impeller is 8 1/8 inches diameter and has 16 vanes. It also has holes through to the back.
Valves in the -46 and C4 have larger diameter stems than C1, and are not interchangable. I believe the -46 exh valve has an even larger diam stem than a C4.
Crankshafts, of course much longer in aircraft applications than tank engines but the C4 and -46 use a single nut and sleeve to hold the two front bearings in versus the earlier two nut method of the C1. Could you cut down a -46 to work in a C4?? Not sure.

pistonstop.jpg pistonsrear.jpg superchargers.jpg -46superchargerrear.jpg

[Bob Phillips]

Cylinders- I was prompted to provide this information by an internet photo of a C4 engine that appears to have been rebuilt using -46 cylinders. This seems like a great idea given the scarcity and high prices of C4 cylinders. Hopefully one of the forum readers will recognize the picture and provide details on this rebuild. I assume it will require custom baffles around the cylinders.

C4-46cyls.jpg r975-46cyl.jpg