From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Modifications to an S.U. carburetter and supercharger to improve engine power and slow running performance.
| Identifier | ExFiles\Box 140\1\ scan0108 | |
| Date | 21th December 1934 | |
| -2- In order to improve the top speed power output we fitted an S.U. carburetter and the power curves taken are shown on B.V.61, it will be noticed that the top speed power is increased by approximately 6 B.H.P. but there is a considerable drop in low speed M.E.P. when compared with the Graham fixed throat carburetter, due to the S.U. being an expanding carburetter, so that the efficiency of the Supercharger is reduced at low speeds. The S.U. carburetter was then made to work as a semi-expanding carburetter by lifting the air valve .650 and under these conditions the low speed M.E.P. compared favourably with the Graham carburetter. To improve the general performance we turned the Supercharger through 60° and fitted the short intermediate pipe as shown on B.V.69 which has a water jacket for the majority of its length, with this arrangement using the water jacket on the intermediate pipe and no heat to the volute jacket and the S.U. carburetter, we obtained a maximum B.H.P. of 138.8 at 4,250 r.p.m. with a boost pressure of 6.250 inches and a temperature of 40°C. The low speed M.E.P. was increased considerably over that obtained with hot water to the volute jacket. This power curve is shown on B.V.61. The slow running using an S.U. carburetter with this pipe and water heat to the volute but no heat to the intermediate pipe jacket, was more even than with the previous pipe and the engine would "idle" indefinitely at 300 r.p.m. We then tried exhaust heat to the volute jacket and although the temperature of the volute jacket was much lower the slow running was much better, and the engine would run evenly at 250 r.p.m. with only a slight tendency to "build up" after a considerable time. With exhaust heat to the volute jacket and hot water to the intermediate pipe jacket we avoided the building up trouble, and did not affect the idling speed in any way. | ||