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).
Analysis of engine performance based on modifications to choke size, valve lift, timing, and exhaust valve size.
| Identifier | ExFiles\Box 133\2\ scan0062 | |
| Date | 26th July 1934 | |
| -4- It will be noticed that the peak speeds of the B.M.E.P. curves have altered roughly in proportion to the inlet valve areas of the two heads. On Sheet 6 will be found a general comparison of the two heads (anti-detonating fuel). The motoring losses have been estimated from the curves given on EV{Ivan Evernden - coachwork} 21 (six-cyl. Bentley engine), from which the I HP. and I.M.E.P. curves have been computed. Choke Size & Effect on Power. The majority of the tests on this unit have been carried out with a 32 m/m.{Mr Moon / Mr Moore} choke in the carburetter, this being the largest available. The effect of fitting smaller chokes or removing the choke entirely is shown on Sheet 7 of the appended curves. Tests with both the Ramp and std. heads have been carried out. The carburetter is a Zenith triple diffuser type with a 42 m/m.{Mr Moon / Mr Moore} dia. throttle. Valve Lift. It will be noticed that the valve lift of the Ramp head is .025" less than std. We do not consider that this small difference in lift makes any appreciable difference to power output, as a test on the std. head with .025" less lift made no readable difference to the power. Variation in Valve Timing. A few tests have been carried out with various valve timings, but none has been found to give better all-round results than the normal "Bentley" timing. Tests have shown that valve size and timing duration are interdependent; the larger the valve the shorter the desirable period of opening. This is illustrated by the curves on Sheet 8. Exhaust Valve Size. A larger exhaust valve (1.425 dia.) has been tried out on the Ramp head, but no greater power output has been obtained. The result of these tests is shown on Sheet 8, from which it will be seen that the larger valve in itself causes a loss of power throughout the speed range. It was at first thought that the reduced clearance around the larger valve was restricting the flow of gas, hence the head was trepanned to an extra radial depth of .075" thus restoring the clearance to normal. This resulted in a still further loss of power. The next step was to reduce the duration of the exhaust valve lift by 13o (flywheel), which restored the power approx. to normal. | ||