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).
The trial of 25/30 rockers, and modifications to bottom tappets and valve gear to reduce noise.
| Identifier | ExFiles\Box 6\6\ 06-page257 | |
| Date | 19th August 1937 | |
| -2- We have been able to mock-up 25/30. rockers on Wraiths for trial regarding power, but these are not suitable in making comparisons for noise etc. As shown on attached curve (S.22), the 25/30 rockers (.340 lift) lose 6 B.H.P. or 5%, at the power peak of 3500-3750 R.P.M. when running open exhaust, but only 2.B.H.P. or 2% when the exhaust system is fitted, whilst at low speeds gain 2 - 3 lbs/sq.in. M.E.P. or 2%. It will be seen, therefore, that if any material advantage is obtained by the lower lift regarding the valve gear operation, this might be worth the sacrifice of a slight drop in performance. On the road, however, the loss in performance with the low-lift rockers has so far been more than the test bed figures indicate. Apart from the trial of the mock-up unit, the position is as follows:- BOTTOM TAPPETS. The 'scuffing' of the bottom tappet face experienced on the original design has up to the present been overcome by collection and adoption of American principles including:- (a) Conical tappet face with tappet bore inclined at an angle of 45' to obtain positive rotation. (b) Push rod housed in the tappet base to cut out possibility of side thrust. (c) Tappet in I/C with chilled face. This type of bottom tappet is shown on attached drawing EX.27960.A. and as fitted to the 15,000 miles car 36.G.VI. We hope to make a success of this in a simplified form as shown on EX.28101.A., whereby the housing for the push rod is cheaper for Production by being a plain conical shape instead of a spherical housing, also the external spiral oil groove is deleted. VALVE GEAR. The original unit which had atrocious valve noise was somewhat improved after the 'scuffing' of the bottom tappets had been cured, but were still far too noisy at clearances in excess of .003. Theoretically, disregarding 'squash out' of the mechanism, the valve gear should be quiet up to a clearance of .013. By increasing the duration of the low velocity foot of the cam to give theoretical quietness up to .020 clearances, quiet operation was obtained up to clearances of .008 - .010. It will be continued | ||