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).
Features of a shorter duration valve lift curve camshaft for a Bentley cylinder head.
| Identifier | ExFiles\Box 85\3\ scan0048 | |
| Date | 18th December 1936 | |
| To EY. Copy to Rm.{William Robotham - Chief Engineer} Da.{Bernard Day - Chassis Design} HPS.{Horace Percy Smith - Experimental Factory Mgr} Rm{William Robotham - Chief Engineer}/Swdl.{Len H. Swindell} E/PSN.1/MN.18.12.36. 207 Bentley Shorter Duration Camshaft. Further to E's memo. E.1/HP.16.12.36, we send you herewith LeC.5426 of a shorter duration valve lift curve for trial with the new Bentley cylinder head, with larger ports. The camshaft to the valve lift curve incorporates the following features. (1) Shorter duration with the object of stepping up low speed m.e.p. at the expense of some of the gain in top power which is anticipated with the large port head. (2) The shorter duration has involved a reduction in valve overlap which should improve slow running. (3) Large valve clearances to accommodate the expansion of the KE.965 exhaust valves and still ensure that the tappet does not come down off the toe of the cam on the closing side. In this connection it should be noted that an increase in tappet clearance on the standard Bentley camshaft from .004" to .006" results in an appreciable increase in tappet noise which is put down to the fact that the tappets are closing off the end of the toe which is .010" high. Recent tests on the Phantom III have indicated that the KE.965 exhaust valves will take up as much as .008" tappet clearance. To take care of this range of tappet clearance a long camshaft toe to run with initial settings of about .012" clearance, is very desirable. (4) An opening toe of .004" less lift and 50% higher velocity than the closing toe, as tests have shown that the tappets will stand approximately .004" more clearance before becoming noisy on the opening side than they will on the closing side due to the compression in the valve gear. The reason for the higher velocity is that the present Bentley cam is quiet on the opening side with this velocity. The object of the finessing with the camshaft toes is to obtain a cam form which will enable the tappets to be set with a clearance which will enable them to pick up the cam close to the end of both the opening and closing toes as this condition is the best for slow running. | ||