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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).
Preliminary valve temperature tests for the 25/30 units, comparing them to the 4¼ litre Bentley.

Identifier  ExFiles\Box 106\5\  scan0165
Date  31th July 1936
  
To E.{Mr Elliott - Chief Engineer}
c. to Wor.{Arthur Wormald - General Works Manager}
c. to By.{R.W. Bailey - Chief Engineer}
c. to MX.{John H Maddocks - Chief Proving Officer}

x534

Valve Temperature Tests.

Preliminary tests have been taken on recording the exhaust valve temperature by means of the Cambridge Pyrometer Optical Indicator.

Further experiments are delayed by the urgency of carrying out oil consumption tests on the 25/30 units, however some interesting figures have been obtained up to the present.

The temperature of the 25/30 valves running at 4,000 r.p.m. full throttle under the weakest maintained mixture strength is approximately 875°C. In the case of the standard 4¼ litre Bentley, the valve temperatures are 750-775°C. The range of temperature between Nos. 1, 2, 3, 4, 5 or 6 valves on either unit does not vary by more than 25°C.

Variation in the water temperature results in the same alteration to the valve temperature, also no difference was noticed by blanking the front 8 water holes and making the water enter the head from the rear end.

Effect of tappet clearance from 0 - .020" results in no practical difference to the valve temperature. The weakest maintained mixture strength produces the maximum valve temperature, the effect of which is shown on the attached curve (SP.{Mr Spinney}57).

Indicated on this curve is the effect of .250" longer valve guides, which lowers the maximum temperature by approximately 30°C.

This was carried out as the first step in bringing down the temperature on the 25/30 and was to be followed by fitting larger diameter exhaust valves, the trial of which has been held up by the oil consumption tests.

Other than the induction system, the main difference between the 25/30 and the Bentley appears to be in the diameters of the exhaust valves, as comparing the heads SP.{Mr Spinney}55 and SP.{Mr Spinney}56 there is little difference with regard to the exposure of the valves or the position of water spacing relative to the guides which would account for the higher temperature in the case of the 25/30 valve.
  
  


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