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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).
Spark plug and magneto performance related to engine slow running, overheating, and oiling issues.

Identifier  ExFiles\Box 33\3\  Scan134
Date  10th May 1921 guessed
  
Contd -2-

inclined to suffer from overheating of the points, but more inclined to oil and soot up.

5. It is certainly a fact that slow running on the intermediate position of plug can be obtained with certain magnetos. We even find this is the case with some of the present standard Watford magnetos. They certainly are exceptional and we have tried production cars on which the magneto ignition with slow running has been very good, and by changing over the magneto from this car to other cars we find the slow running has been consistent on each engines to which the magneto is applied, the plug gaps in each case being set as per our present standard, i.e. .020.

On investigating the characteristics of such a magneto, the bench test did not reveal anything very striking such as length of gap obtained without missing fire at low speeds, etc., although one had the impression that the spark given off was much more fiery and caused more of a blue flame.

We are, however, very emphatic that the difference in strength of the spark given off by such a magneto as compared with a poor one, was very definitely illustrated by the shock which one could feel when touching the plug with the engine running under slow running throttled down conditions.

Even with the magneto, longer ignition plug spark gaps are an advantage for slow running. The length of gap is controlled by the missing fire which occurs when picking up under full throttle conditions.

Our present recommendations, in order to make the best of the knowledge and material we have at our disposal on this matter, are that Mercury plugs should be used for the intermediate position and Lodge plugs for the pocket position. With this scheme we find we are able to encourage ample lubrication, which is a good thing at all times for the engine, and at the same time would not have any serious trouble of missing fire due to plugs overheating or overoiling.

EP.{G. Eric Platford - Chief Quality Engineer}
  
  


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