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).
Letter to Humber Ltd. discussing engine idling issues related to valve overlap and tappet clearance.
| Identifier | ExFiles\Box 126\2\ scan0035 | |
| Date | 13th July 1936 | |
| X1069 Exptl. Dept. Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer}14/KW. 13th July, 1936. A.{Mr Adams} Winter, Esq., Messrs. Humber Ltd., COVENTRY. Dear Mr. Winter, Thank you for your letter of July 11th. We are having a look at the valve spring data you sent us and will write you in due course. With reference to the idling problem, it is exceedingly difficult to be dogmatic on the minimum amount of overlap required to give good idling. In the first place it is necessary to find how the overlap is measured. We quote all our timings at .020" tappet clearance, this eliminates the silencing ramp on the camshaft. The silencing ramp, however, does have an effect upon idling and therefore this should also be specified when quoting the overlap in relation to slow running efficiency. With a ramp that was very satisfactory for silence we have had excellent slow running on one particular induction system with 6° overlap, measured at .020" tappet clearance. As soon as the induction system was opened up and improved generally for power output, however, the overlap had to be reduced to get the same degree of tick-over. The 16° negative overlap quoted is rather an exceptional case, because this is the first instance where we have used hydraulic tappet adjusting mechanism. With this mechanism, when idling the valve runs with an actual clearance of .001" and this increases the effective overlap with any given camshaft appreciably. With ordinary valve gear we should not expect to have to go to more than 5° negative overlap to get good idling unless the induction system was fundamentally poor for small throttle opening distribution. | ||