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).
Valve gear design, comparing Cadillac and Halford schemes, and considering the number of valves per cylinder.
| Identifier | ExFiles\Box 17\6\ Scan294 | |
| Date | 27th July 1934 | |
| -2- (3) Cont'd.{John DeLooze - Company Secretary} We are not yet sure that we can make the ball valve work as well as the more complicated spring loaded poppet valve that Cadillacs use. For the Phantom we therefore think that we shall start the 10,000 miles with the poppet valve. On the other hand, the Halford tappet if it can be made sufficiently accurately, works well if the cams have a toe of the same order as that of the present 20/25 camshaft. The set running 10,000 miles on 21-G-IV at present have a little too much clearance but even so they are an improvement on the standard valve gear. Even with the short toe Bentley camshaft they are a big improvement; on the latter car, however, they reduce the valve bounce speed from 4900 to 4300, which means considerable lightening of the valve, push rod etc, to compensate. Briefly, therefore, since the Cadillac scheme can be most difficult to apply to many types of valve gear, whereas the Halford scheme is more easily adaptable, we are running both schemes continuously and our progress is such that we shortly anticipate that we shall be in a position to offer for standardization the Cadillac scheme for the 40/50 and the Halford scheme for the 20/25 and the Bentley. (4) Number of Valves per Cylinder. On a cylinder of the Bentley size we can see serious difficulties in getting a silent push rod operated valve gear to run above 4,500 r.p.m. particularly if we want a high standard of slow running and good low speed pulling, both of which necessitate a short duration camshaft. Therefore, apart from the advantages accruing from improved volumetric efficiency, we think that for silence and life we may be forced to 4 valves per cylinder, or at least 2 inlet and one exhaust. It may be argued that such an arrangement is not used in America, but there they are not hampered by British taxation which favours large cylinders with a long stroke. | ||