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).
Valve timing and detonation characteristics for high-power heads compared to standard Bentley heads.
| Identifier | ExFiles\Box 134\2\ scan0017 | |
| Date | 27th May 1935 | |
| - 4 - Hs{Lord Ernest Hives - Chair}/Wd.{Mr Wood/Mr Whitehead}2/GB.27.5.35. above which speed the high power head leads. Sheet 6 shows a direct comparison, on the high power head, between the normal Bentley and the short duration timing (which has been found to give the optimum results with the high power heads). Sheet 5 shows the effect on power output of the silencer which is used for detonation work. VALVE TIMING. It will be seen that the optimum power is obtained with the high power heads when a very short inlet opening duration is used. The inlet opens at 2° before T.D.C. and closes 28° past B.D.C., thus giving 210° duration. The normal Bentley inlet duration is 235°. The necessity for the shorter duration is caused by the large area past the inlet valve(s) and the very free induction system of the high power heads. The exhaust valve timing is the same as that used on the standard head. The above valve timing has been evolved from experiments of variation in duration and phasing until the maximum power was obtained at about the speed of the peak of the B.M.E.P. curve. DETONATION. The high-power heads have a peculiar detonation characteristic, in that the most objectionable detonation occurs between speeds of 2500 R.P.M. and 3750 R.P.M., whilst at the lower speeds (1000 R.P.M. – 2000 R.P.M.) detonation is very moderate. This low-speed is regular but that which occurs at the higher speeds is irregular (sometimes only occasional) but usually very severe. The low speed detonation is instantly reduced in magnitude by slightly retarding the point of ignition but the high speed detonation is insensible to retardation of the ignition: even after a considerable fall-off in power (due to retarding the ignition) the detonation is as severe as before. The most promising development of the high power head for minimum detonation is the twin inlet head used in conjunction with a sodium cooled exhaust valve. A comparison of this combination with the standard Bentley head is shown | ||