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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).
Letter discussing a comparison of car performance metrics for 1925 models, introducing a 'corrected displacement per ton foot' figure.

Identifier  ExFiles\Box 160\2\  scan0084
Date  8th January 1926
  
ROLLS-ROYCE
OF AMERICA, INC.
SPRINGFIELD, MASS.
X4578

Oy3-E-1826

January 8th, 1926.

Mr. E.{Mr Elliott - Chief Engineer} W. Hives,
Rolls-Royce, Ltd.,
Derby, England.

Dear Hives:

Re Car Performance

Many thanks for your various reports on car performance which are of the greatest interest.

Perhaps you will be interested in the attached report and lists giving for various makes of cars (1925 models) the displacement per ton foot of standard touring cars of each make, and also (for cars where compression ratio is quoted) a figure which I have called "corrected displacement per ton foot" which is really the displacement of an engine of 4 to 1 compression ratio that might be expected to give the same performance as the actual engine used on the car.

The correction for compression ratio is made on the slightly inaccurate assumption that the brake horse-power varies in the same ratio as the indicated hp.

or varies as r/(r-1) x (r^.3 - 1)

Since all American cars have close to 4 to 1 ratio, this error is small.

The "historical" interest in this list is the tendency in all the newer American models to advance this figure, either by increased gear ratio, smaller wheels, larger engines, or higher compression, from around 40 c.ins. to around 45 c.ins.

It will be seen how the R.R. at 38.6 for the Silver Ghost and
  
  


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