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).
Technical analysis comparing a high-power cylinder head design with the standard Bentley head, focusing on power output and valve timing.
| Identifier | ExFiles\Box 134\2\ scan0016 | |
| Date | 27th May 1935 | |
| - 3 - The high power type cylinder and heads are entirely different in design from the standard Bentley (as will be seen on LeC's. 4075, 4123 and 4157) in that the combustion chamber is situated at the side of the cylinder and the piston coming to within the gasket thickness of the cylinder head. The inlet valve (or valves) is fitted in the cylinder head and the exhaust valve is in the cylinder block at the base of the combustion chamber. In the original design the exhaust valve was 1.400" diameter but owing to the poor power output this was increased to 1.500" diameter early in the development period and gave greatly improved results. POWER OUTPUT. A comparison between the high power heads and the standard Bentley head is shown on sheet 1 of the appended curves. The twin inlet head with sodium cooled exhaust valve and 6.50 compression ratio, shows the most promise; with an improvement over standard Bentley of 11.0 lbs/sq. inch, M.E.P. at 1000 R.P.M., 9.0 lbs/sq.inch at 2000 R.P.M., 15.0 lbs/sq.inch at 3000 R.P.M. and 28.0 lbs/sq.inch at 4000 R.P.M. This 6.5 compression ratio was obtained by the use of a cylinder head gasket which is too thin for normal use but the head can be machined back in order to permit a thicker gasket. This has been done and a power curve taken with a compression ratio of 6.8, in order to ascertain the margin on detonation with the salt cooled valve. This is shown in comparison with the 6.13 compression ratio curve on sheet 1.a. of the curves. It will be noted that the main gain is at the lower engine speeds. The power output with the single inlet valve head (normal exhaust valve) falls short of the twin at almost all engine speeds. This head is not recommended, however, owing to the large size of the inlet valve causing trouble with bouncing at the higher engine speeds. The curves on sheets 2 and 3 show the effect of variation in carburetter choke size on the twin and single inlet heads respectively. Sheet 4 gives the power output of the single inlet high power head when using the normal Bentley valve timing. It also shows the value of the 1.500 exhaust valve when using this timing. From these curves it will be seen that under these conditions of valve timing the standard Bentley head is actually well above the high power head in performance below 3100 R.P.M. | ||