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 Wraith car's cooling system, including performance comparisons with a Vauxhall 25.HP car and notes on thermometer placement.
| Identifier | ExFiles\Box 131\5\ scan0031 | |
| Date | 24th November 1937 | |
| -4- Rm{William Robotham - Chief Engineer}/Std.3/AP.24.11.37. In this connection it may be mentioned that we have road tested the Vauxhall 25.HP. car in a similar manner (approx. same average speed) and the critical air temp. under full speed full throttle conditions is approximately 43°C., while the critical air temp. under dead slow idling conditions after prolonged flat out running on the road is something less than 12°C. (5) In view of the excellent standard of the cooling system and the fact that we have now greatly reduced water loss with the Wraith III. radiator, it is doubtful whether it is necessary or worthwhile fitting a steam valve to Wraith III. However, we are going into the cost of same in order to decide whether it outweighs the saving which would result from reduction in matrix size. (6) Throughout our tests it was observed that there was a considerable difference between the reading of our test thermometer in the top tank and the dash thermometer whose bulb is located in the cylinder head - the cylinder head temperature reading on an average 7°C below the top tank temperature at maximum speeds, yet reading from 0°C. to 3°C. higher than the top temperature under idling conditions - this state of affairs is undesirable since it makes the normal running temperature as controlled by the radiator shutters appear too low, yet accentuates idling overheating and means that the main purpose of a thermometer (i.e. to tell the driver if and when the cooling water is boiling) is not fulfilled. This defect is due to the position of the thermometer bulb which is at the rear of the head and is subjected to the impact of the cool water coming into the head from the water pump - the bulb should be at the front of the head as near as possible to, if not actually in, the outlet from the head. SUMMARY The cooling system of the Wraith car as being built for the first six production cars is satisfactory and slightly superior to that of the standard 25/30.HP. car - a further improvement can be made by means of a larger water pump - we shall carry out further tests on this subject. It may be mentioned that the respective matrix weights of the standard Bentley, standard 25/30.HP. and Wraith matrices are 32½ lbs. 39 lbs. and 24 lbs. respectively, while the complete radiator weights (full of water) are 88 lbs, 104 lbs. and 89 lbs. respectively (Wraith weight includes headlamp supports tubes and brackets) The position of the dashboard thermometer bulb should be altered as mentioned above immediately. From a cooling standpoint, a steam valve is not necessary for Wraith but after further tests etc. we shall shortly be able to say whether this feature is worth fitting. Rm{William Robotham - Chief Engineer}/Std. | ||