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).
Axle ratios and performance calculations for the P.2. engine in the J.3. chassis.
| Identifier | ExFiles\Box 123\3\ scan0009 | |
| Date | 20th June 1933 | |
| To Ds. from Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer} c. to E.{Mr Elliott - Chief Engineer} c. to By.{R.W. Bailey - Chief Engineer} Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer}1/KT.20.6.33. P.2. ENGINE IN J.3. CHASSIS. Referring to DA.{Bernard Day - Chassis Design}1/G.2.5.33., we think the axle ratio of 12/41, that you suggest for P.2.J.3. is too low. We think the maximum M.P.H. figure you quote for P.2.J.3. with 11/41 axle should be 84 M.P.H. instead of 86.8 M.P.H. Actually however, it is the Continental Phantom that in practice laps at 85.5 M.P.H. with a 12/41 axle and this is 3150 R.P.M. The 8 litre engine so far peaks slightly lower than the normal Phantom as will be seen from the attached power curve and, even with open exhaust, the power falls away rapidly after 3250 R.P.M. If we want 95 M.P.H. lap speed out of P.2.J.3. at 3150 R.P.M. the axle ratio will have to be 2.73 with the 18" x 6" tyres at present fitted which have 92" effective diameter. Both from windage, weight, and appearance we do not want to use larger wheels than necessary. However, it may be necessary to consider the 95 M.P.H. car as the exception, and be content with 90 M.P.H. and the improved acceleration from a lower axle ratio. This requires a 2.88 axle ratio, or as a round figure 3/1. We can then fit 6" x 20" wheels which have 98.5" circumference, to bring us back to a 95 M.P.H. car. Ev.{Ivan Evernden - coachwork} states that he does not need a hypoid on this car, as the passengers sit forward of the axle casing, therefore we could use ordinary gears for our "mock up" car if it was considered simpler to alter the casing rather than go through the somewhat lengthy process of communicating with America. The figure of merit of P.2.J.3. becomes with the suggested 3/1 ratio :- | ||