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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).
Calculation for Bentley (85 m.p.h. Full Throttle) using Marston Mark VI 52 m.m. Matrix.

Identifier  ExFiles\Box 87\1\  scan0175
Date  7th November 1935
  
(Appendix - Sheet 4) Hm{Capt. W. Hallam - Head Repairs}/Std.14/KW.7.11.35.

Calculation for Bentley (85 m.p.h. Full Throttle)
using Marston Mark VI 52 m.m. Matrix.

From test figures (sheet ) heat dissipation
at 85 m.p.h. = H / Δt = 90 B.T.U's/min/°C.
temp. difference between
inlet air and water.

Assuming same reduction of cooling due to under
bonnet pressure as in case of SpectreCodename for Phantom III, actual
heat dissipation = .936 x 90 = 84.3 B.T.U's/min/°C.
temp. difference.

H = 3600 B.T.U's
∴ Δ = 3600 / 84.3 = 42.7°C.

∴ Critical air temperature = 100 - 42.7°C.
= 57.3°C.

NOTE: Brooklands tests gave critical air temperature
figure of 52°C.

Similarly, using Mark V 58 m.m. matrix (as
standard at present) under same conditions,
critical air temp. = 53.2°C.
Note: Brooklands tests gave figure of 48°C.
  
  


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