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).
Engine power analysis for naturally aspirated running, detailing the effects of valve timing and pipe lengths.
| Identifier | ExFiles\Box 133\1\ scan0110 | |
| Date | 17th September 1936 | |
| -6- Hs{Lord Ernest Hives - Chair}/Ed.{J. L. Edwards}1/KW.17.9.36. Naturally Aspirated Running. Power. As will be observed, the B.M.E.P. of this unit as delivered from the makers is considerably in excess of anything we have been able to obtain on car engines, there being an M.E.P. of 164 lbs/in.² at 4,500 R.P.M. This power is extremely susceptible to three items, all of which appear to be bound up one with the other, viz:- (1) Valve timing. (2) Exhaust Pipe length. (3) Induction Pipe length. We see from Fig. I that if the exhaust pipe capacity be reduced to 440 c.c., i.e., the length reduced from 53.5" to 18", there is quite a considerable drop in power, this being very prominent at the higher speeds. Again, by altering the induction pipe from 1" to 3¼", there is a considerable power increase after 3,200 R.P.M. There is a rather different story to tell when we use the valve timing given by modification No.1 camshaft. Here we see (Fig.II) that the exhaust conditions have really very little effect, whereas with induction pipe variation, the power variations are very large indeed (Fig.III). This modified camshaft was now replaced by one giving standard Bentley timing, and three cylinder heads were tried (Fig.V). It will be clearly seen that the penthouse and hemispherical heads are very nearly the same, but that the semi-hemispherical head appears to give a much lower power than the other two, at the higher speeds. This is very difficult to explain, so difficult in fact that it is suspected that the results are probably in error, and that actually there is very little to choose between the heads. This, it might be remarked, is borne out by the makers. A further test showed that if one exhaust valve were put out of action, reducing the exhaust valve area/inlet valve area from 0.81 to 0.405 - allowance being made for valve stem diameter - a very considerable power drop resulted, amounting to nearly 23% at the higher speeds (Fig.VII). | ||