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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).
Engine performance tests with modified pistons, concluding that no material improvement was achieved.

Identifier  ExFiles\Box 148\1\  scan0209
Date  13th November 1939 guessed
  
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plotted in RD.830 and the constant speed test in RD.829 indicate that the whole sequence of engine test results obtained falls within the range of experimental error. The major difference in engine running condition is the amount of ignition advance which the engine will stand with different compression ratios and this ignition feature is clearly defined on both the constant speed and variable speed performance curves.

As a further set of pistons was available it was decided to modify these so that the protrusion on the piston crown is tapered, as indicated on the attached sketch. The results obtained with this modification are plotted on curve RD.831 and again indicate that the performance cannot be materially improved.

Conclusions:
The conclusions reached from these tests are that no material improvement in engine performance can be obtained by using this type of piston, and that any minor gain recorded is at the expense of engine smoothness and a critical ignition characteristic.
In view of the results obtained it is recommended that no further action regarding these pistons should be taken.

Jmarkland

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