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).
Increasing the compression ratio for the 20/25 engine model.
| Identifier | ExFiles\Box 27a\1\ Scan181 | |
| Date | 8th December 1934 | |
| 26013 To Sg.{Arthur F. Sidgreaves - MD} from Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer} c. to Wor.{Arthur Wormald - General Works Manager} Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer}5/KW. 8.12.34. Higher Compression 20/25. Replying to Sg.{Arthur F. Sidgreaves - MD}2/E.26.11.34. The present standard 20/25 compression ratio is about 5.7 to 1. The limit of increase which we should recommend would be to 6.25 to 1. To get this increase in compression ratio it is necessary to machine .050" off the face of the cylinder head and use a thin gasket. On selected cylinder heads it is possible to remove this .050" and still retain ample strength. We have done this and carried out a test. With the increase in compression ratio it is necessary to have a different sparking plug, also a different characteristic for the automatic ignition advance. All the stressed parts other than the cylinder head are interchangeable between the 20/25 and the Bentley, and therefore have been run many thousands of miles with a compression ratio of 6.4. Fortunately there are certain fuels on the market to-day which permit of higher ratios than in the past. The improved performance is of course best demonstrated by running two cars side by side. Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer} | ||