During June 1937 a Merlin II, mounted in a Horsley, began a 400-hr flight endurance test at Farnborough, and a specially rated "racing" engine was developed from it with a view to installation in the special Speed Spitfire, with which an attack on the world's speed record was contemplated. The engine used was a Merlin III, which differed from the Merlin II in having a standardized de Havilland/Rotol airscrew shaft and dual accessory-drive. It was taken from stock and was fitted with strengthened pistons, gudgeon-pins and connecting rods to withstand the extra load. "The power output of the standard engine," writes Harold Nockolds, "was 1,030 b.h.p. at 3,000 r.p.m. at 10,250ft with plus 6i lb boost. "Solely by opening the throttle, raising the supercharger pressure, and using fuel of a higher octane," he goes on [the petrol normally used at that time was 87 octane], "the engine was made to develop no less than 2,160 b.h.p. at 3,200 r.p.m. with the supercharger giving 27 Ib/sq in boost. This was a phenomenal performance, for it meant that a power to- weight ratio of 0.621 lb per horsepower had been achieved — a considerable improvement on the 0.71 lb per horsepower of the 1931 R engine.
"This tremendous output, which was admittedly only attained for a short period, nevertheless gave ample proof of the inherent possibilities of the Merlin. But Elliott and Hives were perhaps even more satisfied with a 15-hr endurance run at 1,800 b.h.p., 3,200 r.p.m. and 22 lb boost accomplished during the development period. After this they felt perfectly satisfied that the Merlin would be capable of meeting all the demands that might be made of it. How right they were!"
15 hours at 22lb boost in 1937!!!
|