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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).
Test report and blueprint for a Bentley No. 3a cylinder head, with comparison to a No. 5 head.

Identifier  ExFiles\Box 134\1\  scan0116
Date  23th December 1938
  
-4-
Rm{William Robotham - Chief Engineer}/Swdl.{Len H. Swindell}1/MH.{M. Huckerby}23.12.38.

No 3a
BENTLEY HEAD 6.4:1 RATIO
1.4 INLET & 1.3 EXHAUST

A further Bentley cylinder head was tried having 1.5 inlet and 1.4 exhaust with a compression ratio of 6.6 : 1 and by juggling with the valve timing to IO 10° BTDC EO 55° BBDC IC 30° PBDC EC 7° PTDC the power with this head (Curve 2a) is at the middle speeds 5 MEP better than the (No.5) head. The latter head, however, is still appreciably better at high speeds.
  
  


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