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).
Comparison of exhaust system configurations focusing on swan neck pipe diameters and resonance silencers to manage power loss and back pressure.
| Identifier | ExFiles\Box 179b\2\ img024 | |
| Date | 12th October 1932 | |
| -2- Hs{Lord Ernest Hives - Chair}/Lid.{A. J. Lidsey}/XT.12.10.32. With regard to swan neck pipe diameter, we lose 3 HP. above 3000 R.P.M. when using the 1.5" diameter pipe, as compared with a 2.0" diameter pipe, this loss however we have to accept in order to overcome a slight boom between 30 and 40 m.p.h. for which the larger pipe is responsible. The effect of the swan neck size is shown on Curve II. Actually the large diameter swan neck is never responsible for a greater loss than 1 B.H.P. We think that the small loss of 3 B.H.P. which amounts to very little car speed at high speeds, is justifiably sacrificed to obtain greater silence. In Curve I we show the power comparisons between the standard and the resonance silencer. The decrease in power loss to the exhaust system by using the resonance silencer is not very great. We do not know how to improve on it without sacrificing silence. In Curve III we show a comparison of back pressure between the two types of silencer without any swan neck; the resonance silencer produces 1.5 ins mercury less back pressure than the standard silencer. On this curve we also show the back pressure due to the manifold downtake pipe, front expansion box and intermediate pipe. We can call this the necessary back pressure because we have to use these parts whichever silencer we use. The increase over this due to the silencer only is 3 ins. Hg. in the case of the resonance silencer and 4.5 ins Hg for the standard at 3500 R.P.M. Thus the resonance silencer reduces the back pressure by 30% under this condition. The advantage gained here, however, is to a large degree negatived by having to use the small diameter tail pipe which produces almost as much back pressure as the remainder of the exhaust system (see Curve II in conjunction with III). We propose to standardise this scheme, and very shortly we shall be in a position to send to Cricklewood a silencer that can be fitted to a 20/25 HP. trials car so that it can be tested and criticised by London. Hs{Lord Ernest Hives - Chair}/Lid.{A. J. Lidsey} | ||