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).
Experiments with the 20/25 ignition system, focusing on distributor spark path and safety margins.
| Identifier | ExFiles\Box 61b\2\ scan0131 | |
| Date | 9th November 1930 | |
| X5690. By.{R.W. Bailey - Chief Engineer} from EFC. c. Hs{Lord Ernest Hives - Chair}/AJL. EFC4/AM9.11.30. X.5690. 20/25 IGNITION SYSTEM. X 5610 Your Ev{Ivan Evernden - coachwork}6/C4.11.30. X. 8712 Experiments carried out some time ago indicated that there was only a moderate margin of equivalent safety gap distance in a battery ignition distributor, the effective length of safety gap actually being less than the actual length on account of the fact that when sparking took place in the distributor the sparks did not go straight across from the finger to the aluminium casing as intended, but took a shorter electrical (though longer mechanical) path in largely jumping from the rotating finger to the inside surface of the distributor head moulding and then passing along that surface to the periphery of the aluminium casing. This was proved in experiments on the bench in which a distributor head which was provided with windows was used. Experiments were also made on a 20 HP. carr running up Ticknall hill with weak mixture retarded spark and full throttle and these went to show that even in an engine of ordinary compression sparking over in those circumstances sometimes took place in the distributor. We have therefore arranged to carry out, when this is convenient, a similar test on one of the high compression chassis. Of the result of this test we will duly inform you. The conclusion to be derived from this is that the ignition coil is reasonably capable of withstanding any increased potential stress to which it may be subject as a result of the increased compression, the limiting factor being the sparking over in the distributor. It will be our intention to make a test both with sparking plug gaps of .020" and .025". EFC. | ||