sun5.JPGMany fans of Unlimited hydroplane racing grew up with the piston powered days of the sport. The roar of the Allison or Rolls Royce motor is what drew them into the sport. The only team left with piston power is the Ed Cooper owned and Jimmy King driven Go3 Racing Team. As we travel the Unlimited tour, our Allison engines always draw the biggest crowds and the most questions. The fans are still fascinated with the piston powered Big Red Turbinator. We've gathered lots of information, facts and the history on the Allison engine and will share it with you in this two part series.

Allison, which became a part of general motors in 1929, invested private funds in the development of a liquid-cooled V12 engine at the urging of Allison General manager Norm Gilman. While the U.S. Army Air Corp (U.S.A.A.C.) showed no interest, the Navy supported development of an experimental engine to test the basic design as a precursor to a reversible airship engine. The successful V-1710-A was test run in 1931 and delivered 650 hp at 2,400 RPM on 80-octane fuel. It featured the same 5.5" bore and 6.0" stroke as all succeeding V-1710's, weighed 1,010 lbs, had a 8.25" supercharger turning 7.3 times faster than the crankshaft, and had a compression ratio of 5.8:1. The engine featured a distinctive internal-spur gear propeller reduction drive (initial ratio was 0.66) which in late series models was replaced with more conventional external spur gearing. Modification were required half-way through the first 50 hr test run, and Allison took this opportunity to increase performance by increasing the supercharger gear ratio to 8.0:1. This engine completed its 50 hour test run in 1932 at a rating of 750 hp at 2,400 rpm.

Both the Navy and U.S.A.A.C. were now interested in the V-1710, the Navy placing the anticipated order for reversible airship engines designated V-1710-B and the U.S.A.A.C. designated V-1710-C. The Navy engine eliminated the supercharger (rotary induction blower) in favor of two carburetors placed in the Vee of the engine. The engine was designed to reverse from full power one direction to full power the opposite direction in less than 8 seconds, while driving a remote mounted propellers mounted on outriggers equipped with swiveling heads which allowed thrust to be directed vertically or horizontally. The engine power would be transmitted by 16' driveshafts to the remote transmission and gear arrangements in the swiveling heads. Allison had already designed such drive systems and transmissions for use in the Navy Airships USS Akron and USS Macon. The U.S.A.A.C. version was designated V-1710-C and featured a supercharger impeller enlarged to 9.5", a 2:1 propeller reduction drive in a longer nose casing, and a stiffened crankcase. Additional changes were required to overcome severe harmonic vibration problems caused by the long nose and change in reduction gearing which caused damage to crankshafts, reduction gears and cylinder blocks.IMG_5612.JPG

A significant redesign was undertaken by R.M. Hazen in 1936 which lead to an increased compression ratio of 6.0:1, improved combustion chambers which reduced the length of flame channel, improved piston and rings, and changed manifolds for better air-fuel distribution. This "C"-model passed its 150 hour type acceptance test in 1937, establishing a rating of 1,000 hp at 2,600 rpm at sea level. The definitive Allision which was the foundation for all future engines was now established -- penthouse-type combustion chambers with four valve per cylinder, overhead camshafts in each cylinder block with forked roller cam followers actuating pairs of valves in each cylinder, blade-and-fork connecting rods. Subsequent engines differing primarily in induction system and reduction gearing. A number of incremental improvements were made during the life of the "C"-model including increase in compression ratio to 6.65:1, eventually leading to "C"-models with takeoff ratings of 1,150 hp at 2,950 rpm and supporting 3,500 rpm for overspeed during dives.

The "D"-model featured a remote reduction gearbox in a pusher configuration for powering the Bell Aircraft XFM-1 Airacuda twin-engine fighter.

The power unit on the "E" and "F" engines were identical, with crankshafts, connecting rods, pistons, cylinder blocks, valve gear, and intake manifolds among other components completely interchangeable. The "E" featured a remote propeller reduction gearbox for the Bell P-39 Airacobra (and P-63 Kingcobra) driven with a 10 ft extension shaft turning at crankshaft speed between the engine and reduction gear. The "F" had a conventional integral tractor propeller reduction gear Lockheed P-38 Lightning and Curtiss P-40 Tomahawk. A turbocharged V-1710-F17L/-F17R engine equipped with ADI produced a WER of 2,300 bhp at 3,000 rpm with 90 in hg,, developing a BMEP of 355 psi. The "E" and "F" engines were the bread&butter Allisons, with these engines used in large volume on several major fighter programs of WWII. A total of 66,658 "E" and "F" types were built -- 18,998 "E"-type extension shaft engines were shipped, mainly for P-39 and P-63 aircraft, and 47,660 "F" models were shipped, mainly for P-38 and P-40 airplanes.

The ultimate V-1710 was the "G" series which brought together all of Allisons design and manufacturing experience to produce an outstanding engine in almost all respects from earlier engines. The V-1710-G was incorporated a number of desired improvements identified in 1943 by Wright Field (U.S.A.A.F. engineering/procurement organization) and improve performance to 1,725 bhp at 3,400 rpm. To improve performance, the 12-counterweight crankshaft developed in late 1942 was used to increase the maximum rpm to 3,400, the induction path was improved to increase flow, and improvement were also made to the supercharger, cylinder heads, and accessories section. One interesting characteristic of the "G"-series was the large proliferation of features and configurations -- short-nose integral reduction gears or extension shafts, with and without auxiliary superchargers, both 9.5" and 10.25" engine supercharger impellers, one or two-speed supercharger drives, a range of supercharger drive ratio's, three compression ratio's, pressure (injection) carburetor, speed density injection, or port fuel injection, and both left and right hand rotation. These engines became the most desirable to racers because of the high-revving crankshafts and improved detail design. A total of 763 were produced -- a batch of 750 V-1710-G6R/-G6L consisted of virtually all the "G" series production. When this batch of engines were declared surplus in the 1950's, they were said to to have been the engines which powered Unlimited Hydroplanes into the 1990's (Daniel D. Whitney, "Vee for Victory", pg 278). This same source describes how the Unlimited Racers traded performance for engine life, modifying engines to deliver as much as 4,000 hp.

The most remarkable engine was probably the V-1710-E27 experimental turbo-compound engine. This unique engine was the first turbo-compound or "power-feedback" engine and was way ahead of its time. Based on an E-22 power section with auxiliary stage supercharger, it used a General Electric CT-1 power turbine which was adapted from the exhaust turbine of a CH-5 turbosupercharger. The turbine drove the crankshaft through a 5.953 reduction gear. The engine compression was reduced to 6.0:1 to facilitate high supercharger boost pressure of 100 in hg (35 psig). Using 115/145 PN fuel with ADI injection, this engine was able to develop 2,980 hp at 3,200 rpm and 100 in hg boost from sea level to 11,000 feet. In addition to the remarkable power output, the engine demonstrated a 19% improvement in specific fuel consumption during cruise. Had the inlet exhaust temperature not been limited to 1750° F, it is certain even more power could have been developed. The temperature proved to be easily exceeded during full-power operation, so the engine was never placed into service with the P-63 as intended. IMG_5256.JPG

Key Specifications (Prior to racing modifications)

Note: Many Unlimited Hydroplane Allison's originated from a batch of 750 V-1710 G6L/R engines intended for the North American P-82E/F. These engines were surplused in the 1950's by the Air Force. These engines differed from other Allisons primarily in the areas of lower compression ratio, an auxiliary supercharger and speed/density fuel injection. The boat racers routinely increased the compression, and discarded the auxiliary supercharger and injection system in favor of the more available Bendix-Stromberg Injection carburetors.

Reprinted from the  Hydroplane and Raceboat Museum website and from unlimitedexcitement.com.