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).
The testing and performance of high compression cylinder heads for Bentley engines.
| Identifier | ExFiles\Box 86\1\ scan0062 | |
| Date | 15th May 1934 | |
| He/Rm.{William Robotham - Chief Engineer}3/MA.15.5.34. HIGH COMPRESSION CYLINDER HEADS, BENTLEY. It is of interest to note that the 6.86:1 cast iron cylinder head to LeC.3662 has now completed 10,000 miles on the streamlined car, which has included a certain amount of track work and therefore may be considered available if at any time it is thought desirable to give a particular customer increased performance. We have also tested the 7.38:1 compression cylinder head to LeC.3645. We have made one of these in cast iron and one in aluminium. These heads had 1.450" inlet valves against the 1.40" standard inlet. It will be observed that they give about 120 HP. In this respect they are no better than the 6.86 compression head. LeC.3662 had the advantage of 1.5" diameter inlet valve. Actually of course there is no reason why the larger inlet valve should not be fitted to the cylinder head to LeC.3645. However, from road tests we have come to the conclusion that anything above 7:1 ratio is quite impossible for a closed car, as above this the engine becomes intolerably rough. 7.38 heads are therefore only of interest in case they should be required for competition work at any time. To cover this point we are proposing to run a 20 hrs. endurance test at full throttle with a 1.5 diameter inlet on a good fuel, to ensure that the engine is reliable but otherwise do nothing with them. During the tests there were indications that the aluminium head could stand a higher output than the cast iron head for the same degree of detonation. We are now checking this point carefully. Actually, however, the present position is that roughness is the limiting factor and not audible detonation. He/Rm.{William Robotham - Chief Engineer} | ||