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).
Comparison of high compression aluminium and cast iron cylinder heads.
| Identifier | ExFiles\Box 86\1\ scan0054 | |
| Date | 22th March 1934 | |
| 22.3.34. Comparison of High Compression Aluminium and Cast Iron Cylinder Heads. These experiments were carried out with the intention of determining whether it was possible to run with a higher M.E.P. with an aluminium Cylinder Head than with a Cast Iron Head. The two heads compared were identical in design, the compression ratio of each head was 7.38 : 1, the inlet valves were 1.450 dia., and the exhaust valves 1.200 dia. Our first test was a comparison of the two heads for power and as shown on curve B.V.22 attached, the aluminium head was considerably better than the cast iron head at low speeds, our reasons for this are as follows :- (1) When carrying out tests on the aluminium head we found that we had to use steel push rods because the duralumin rods intended for use with this head reduced the tappet clearance too much, in fact to such an extent that when the engine was thoroughly hot the exhaust valve clearance was nil after being set at .004 with the engine cold. By using steel push rods this difficulty was overcome but of course in this case the tappet clearance increased and reduced the amount of overlap so that the low speed M.E.P. was increased over the cast iron head by 4.0 to 5.0 M.E.P. By running the cast iron head with increased clearances corresponding to those of the aluminium head with steel push rods, the M.E.P. was increased by approximately 3.0 lbs/sq.in. over the original figure. (2) By checking over the compression pressures we found also that the aluminium head gave a higher compression pressure at the low speeds than the cast iron head, this amounted to 2.0 lbs/dq.in. and we assume this to be due to the fact that a greater charge is drawn into the cylinder due to the greater heat dissipation of the aluminium head. These two reasons we think account for the losses of the cast iron head. | ||