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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).
Analysis of power loss in various exhaust system components, comparing standard, Vokes, and large resonant silencer setups.

Identifier  ExFiles\Box 14\6\  Scan297
Date  1st March 1934 guessed
  
-2-

An analysis of this system is shown on (Hs.{Lord Ernest Hives - Chair}1311) giving the power loss to all of the component parts, accompanied by a sketch under which condition the complete system was tested. Taking representative figures at 2750 R.P.M. we find that of the 23 B.H.P. lost as a complete system, the swan neck Hs.{Lord Ernest Hives - Chair}789 is responsible for 7 B.H.P. and the outlet nozzle in the silencer top hat for 5 B.H.P. Possibly without affecting silence, the dimensions of these could be increased with a material reduction in power loss.

Both the end baffle and the third series of holes 80.5/16 dia. each cause a 2 B.H.P. loss and the downtake pipe and front expansion box for 4 B.H.P. Finally, there is a loss of 2 B.H.P. in the manifold itself, as compared with short open pipes.

There is no loss of power due to the silencer length of main pipe, which is 3 3/4" diameter.

The expansion box and downtake pipe used for the large resonant silencer differed slightly from the standard one used in conjunction with the remaining exhaust systems, the latter being the better of the two by 2 B.H.P. at 2750 R.P.M. the pressure in the manifold being reduced from 2.6 Ins.Hg. to 1.8 Ins. The only difference between them is the way in which the downtake pipe enters the expansion box; in the standard, the downtake pipe is radiused and enters almost on the top centre line of the expansion box, whereas in the other, the downtake pipe is continued straight down, the exhaust impingeing upon the engine side of the box.

As each exhaust system had its own swan-neck, a figure was taken in each case with this removed, and resulted as follows, at 2750 R.P.M. -

Power loss to Standard Swan Neck - 8.5 B.H.P.
" " " Vokes - 11.0 "
" " " "Large Resonant " - 7.0 "

In the standard silencer the outlet nozzle is the same diameter of 1 3/4" as the internal diameter of the swan neck pipe.

The greater power loss in the Vokes arrangement can be attributed to the fact that the silencer nozzle is 2.1/16" diameter and the swan neck which other than the special flange is of standard dimensions and 1 3/4" internal diameter.
  
  


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