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).
Summary of development and performance testing on an O.H.C. conversion unit.
| Identifier | ExFiles\Box 113\5\ scan0209 | |
| Date | 13th February 1939 guessed | |
| O.H.C Conversion Unit. Test having been discontinued on the above unit, we give herewith, a summary of the development up to date, as a useful starting point in the event of further work on this unit. Power (Maximum and non detonating). The free breathing for which this unit was designed, is confirmed by the resultant open exhaust power obtained. The attached curve S 20 gives the comparison of this unit with that of a standard Bentley 11. It will be seen that the maximum O.H.C. power is 158 B.H.P. at 4000 R.P.M. and 134 lbs per sq. in. MEP at 2500 R.P.M. as against 125 B.H.P. at 4000 R.P.M. & 120 lbs per sq. in. MEP at 2000 R.P.M. on the Bentley 11 unit. Both of these units were at the same 6.5:1 compression ratio, but the O.H.C. engine has a large induction system with 2-1 5/8 S.U's carburetters as compared with 2-1 1/8 S.U's carburetters on the standard Bentley. Owing to the better valving of the O.H.C. engine, however, the induction pipe depression of the two engines is approximately the same figure, so that we consider this basis is a fair comparison of the two engines. With a standard Bentley exhaust system, the O.H.C. power is reduced from 158 BHP at 4000 R.P.M. to 122 BHP at 3500 RPM or 23% drop. The same system on the standard Bentley reduces the maximum open exhaust power from 125 BHP at 4000 RPM to 112 BHP at 3000 RPM or 10% drop. A considerable difference in these losses will at once be observed and there seems little doubt that the greater power loss of the O.H.C. unit is responsible for the disappointing track and road performance figures of this unit when compared with the standard car. Alternative exhaust systems have been tried on the O.H.C unit, and it was found that by separating the engine into two banks of three cylinders, and providing each with a separate exhaust system, the power loss could be reduced to about 5%. Improvements over the standard system could also be effected by the running of twin downtake pipes into a common expansion box or duplicating the system as far as the first silencer. No tests on comparative systems have been tried on the road, and it is quite possible that some improvement in performance could be effected by suitable modifications. Detonation. The audible detonation is not comparable with anything previously | ||