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).
Table of lowest speeds in RPM for regular firing of magnetos with 0.5mm gaps in air.
| Identifier | ExFiles\Box 48\3\ Scan094 | |
| Date | 22th March 1921 | |
| 22.3.21. K.430. MAGNETOS. TABLE OF LOWEST SPEEDS IN R.P.M. FOR REGULAR FIRING ON 0.5mm. GAPS IN AIR. N.B. Test range between advance and retard 34° in all cases (except Watford E.O.6a - 32°). Magneto. Type & No. A. {Mr Adams} No. Lowest speeds. Remarks. Adv. Cent. Retd. Watford E.6 2375. 150 250 340. Advanced position same as maker's. (Gaps had to be reduced to 4.5 mm. in fixing range by high speed sparking). Retarded position 4° later than maker's. 45638. Watford. E.6 2375 156 226 320. Advanced position 3° earlier than maker (Gaps had to be reduced to 4.5 mm. in fixing range by high speed sparking). Retarded position 1° later than maker's. 45158. Watford. E.O.6a 4181a. 160 140 136 Increased advance or retard on maker's range unobtainable. Hence working range 32° as maker's. 46993. Watford. E.O.6b 4181b. 170 155 150. Retarded position same as maker's. This setting chosen owing to slower speed for regular sparking obtained on retard. Advanced position 2° later than maker's. 48919. (Weighted average of two sets of figures). N.E. 338A (100) T. 798. (E.S.) (101) 10-12-20(C) 1947 | ||