1966 British Columbia Cup
Lake Okanagan, Kelowna, BC, Canada, July 17, 1966
Allisons and Rolls Royce Share the Spotlight
The engines used in unlimited hydroplanes are generally the General Motor produced Allison, a 12 cylinder 1710 cu. in. power plant, and the British Rolls Royce Merlin, a 12 cylinder 1650 cu. in. configuration.
The Rolls Royce is the same engine that powered the famous British Spitfire aircraft so successfully during the Battle of Britain. The Allison powered the North American P-51, a very successful portion of the United States first line of defence.
In recent years, particularly the past 10 years or so, the Rolls Royce engine has seemingly dominated the sport of unlimited hydroplane racing. Rolls installation in the famous Slo-Mo-Shuns, Hawaii Kai, the Thriftways and the Bardahls, have bred great confidence in the interest of the Rolls Royce purist.
However, though the Rolls has sustained many victories the past few years, the choice of Allison versus Rolls by owners and drivers is pretty well split down the middle.
The Allison enthusiast, with little regard for the designer's original intention, has produced some exciting performing machinery with the installation of fuel injection, auxiliary stage supercharging, duel Allison installations, turbo supercharging, etc.
Th. main difference between the Allison and the Rolls engine is the supercharging.
The Allison uses a single-speed, single-stage supercharger. Fuel is compressed and exploded only once through a supercharger that turns the same revolutions per minute as the engine crankshaft.
The Rolls has a two-speed, two-stage supercharger which permits a heavier fuel charge to be forced into the cylinders !under higher pressure. This results in an increase in horsepower.
The Rolls outweighs the Allison by approximately 400 pounds but enjoys a horsepower edge of about 300 horsepower.
The Rolls has lighter pistons and rods which allow it to turn somewhat higher rpm's than the Allison. The Allison, on the other hand, is not only lighter than the Rolls, but has a stronger "basement"—crankshaft, rods, and bearings.
There have been efforts to pull more horsepower out of the Allison to make it more competitive. Some that have been the most successful include the use of high-compression pistons, the adaption to the Allison of a Rolls Royce aftercooler, "off-stage" supercharging, etc.
The Rolls Royce, in competition, will turn a maximum of 4,000 rpm's through a gear ratio of approximately three to one. This provides a propeller speed of 12,000 revolutions per minute, and a boat speed. of approximately 150 m.p.h.
(Reprinted from the Kelowna Courier, July 15, 1966)
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