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).
Secret internal memorandum discussing 20HP car performance improvements by increasing the engine bore size.
| Identifier | WestWitteringFiles\T\March1929-December1929\ Scan073 | |
| Date | 15th April 1929 | |
| R.{Sir Henry Royce} FROM E.{Mr Elliott - Chief Engineer} C. to SG.{Arthur F. Sidgreaves - MD} WCR. " FH. HS.{Lord Ernest Hives - Chair} " DY.{F R Danby} EF. " DA.{Bernard Day - Chassis Design} SECRET. E1/M15.4.29. 20HP. CAR PERFORMANCE. [Handwritten annotations] X5770 X4538 X5050 X5040 X5010 We are quite in agreement that it is highly desirable to reduce the overall weight, but we think that even with the weight reduced as much as possible, the compression ratio increased, and perhaps the gear ratio lowered, the performance will still not be good enough. We do not believe that an easily operated change speed will make up for lack of top gear acceleration, although no doubt it will help matters somewhat. We would like to suggest that at the same time as we alter the engine to fit the new head, the bore shall be increased to say 3.15, or even 3.2, thereby increasing the capacity to 10% or 14% respectively. This can be done we think without any increase in the number of parts we shall have to alter to fit the new head except for pistons and piston parts. We attach herewith a print of LeC. 2694. shewing the alterations proposed. The 20HP. engine was designed over 8 yrs. ago as the first of the detachable head type to be adopted by RR., and has not been materially altered since. From a design point of view we believe the engine will easily carry a bigger bore. On the present 20HP. the distance separating the bores is 1.150, or 38% of the bore. On the SS.{S. Smith} the corresponding dimension is 1.250, or 29% of the bore. The increased capacity if we adopted 3.2 bore would be 47% of the SS.{S. Smith}, and the R.A.C. rating would be 24.6 HP., as against 21.6 HP. for the present 'twenty'. The inlet valves and ports on the new head are nearly 13% larger in dia., so we should be able to reap the full benefit of a larger bore. We realise that the somewhat heavier pistons would bring down the crank period slightly, but not enough to worry us. The psychological effect on customers of increasing the engine size might be very favourable. (1) | ||