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 performance, including valve timing diagrams, camshafts, and exhaust systems.
| Identifier | WestWitteringFiles\W\May1931-August1931\ Scan034 | |
| Date | 11th July 1930 guessed | |
| T.DC. 11 112° 49 49 .013 tappet clnce. 7° 116° 54 48 Inlet duration 235° Exhaust duration 227° Inlet Cams to Sch.1153. Exhaust Cams to Sch.1158. On the other hand with any sort of silencer the power appears to be very sensitive to the exhaust valve tappet clearance, .030" increase losing 3 BMEP at 1500 RPM. and 7 BMEP at 3500 RPM, even with a 40/50 system. Since the 25 HP. with the 112° camshaft and 40/50 silencer and twin carburetters gives a low speed MEP. of 128 and a high speed output equivalent to 180 BHP. on the Phantom, we do not think this unit could be accused of being lacking in power if we were turning out cars to this specification. With regard to a sports engine, we have already had more than 95 HP. out of a Japan unit with a 5½ compression ratio and think that we could without difficulty reproduce the Bentley high speed BHP. curve; we should then expect to be seen in M.P. only by the difference in engine dimensions, i.e. 12%. We understand that now we can use the 3" propeller shaft with the hypoid axle, sch.1462, which will raise the whirling speed above 75 M.P.H. with the existing top gear ratio, previously this has been a problem preventing high speeds on the 25 HP. We note you are worried by the drumming exhaust on 16.G.IV. This car is not fitted with a standard silencer and we believe this is causing the trouble and not the absence of the front expansion box as we at one time thought. We have a Japan unit running in 18.G.IV. which is entirely free from exhaust booms both with a 1.3/4" and 2" straight through downtake. PN.{Mr Northey} commented very favourably on the silence and absence of booms on this car when he tried it recently. Since then we have carried out various tests opening up the silencer and fish tail, and are still free from booms. HS{Lord Ernest Hives - Chair}/Bn.{W.O. Bentley / Mr Barrington} | ||