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 various Watford magnetos for regular firing on 0.5mm gaps in air.
| Identifier | ExFiles\Box 4\5\ 05-page150 | |
| Date | 22th March 1921 | |
| G.A.P. X430 22.3.21. X.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.C.6a = 32°). | Magneto. | Type & NO. | X. No. | Lowest speeds. | Remarks. | |---|---|---|---|---| | | | | Adv. | Centr. | Retd. | | | Watford | E.6 / 45638. | 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. | | Watford. | E.6 / 45168. | 2375 | 156 | 226 | 320. | Advanced position 3° earlier than maker's. (Gaps had to be reduced to 4.5 mm. in fixing range by high speed sparking). Retarded position 1° later than maker's. | | Watford. | E.C.6a / 46993. | 4131a. | 160 | 140 | 136 | Increased advance or retard on maker's range unobtainable. Hence working range 32° as maker's. | | Watford. | E.C.6a / 48919. | 4131b. | 170 | 155 | 150. | Retarded position same as maker's, this setting chosen owing to slower speed for regular sparking obtained on retard. Advanced position 29° later than maker's. | | | | | (Weighted average of two sets of figures). | | | | | ||