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).
Test report comparing the performance of various spark plugs under different conditions.
| Identifier | ExFiles\Box 33\3\ Scan095 | |
| Date | 17th January 1921 | |
| 96EC X1812a File E 17.1.21. RECEIVED JAN 17 1921 Plugs fitted to 9.EX for 1st. and 2nd. Test. CA for 3rd. Test Date. | Type. | 1st. Test over-strong mixture. | 2nd. Test breaking down insulation. | 3rd. Test Over-oiling. | 4th. Test Pre-ignition Bed. | No. fitt:d | No. failed. | Remarks 6.1.21 | Bethelem | O.K. | O.K. | O.K. | O.K. | 12 | 4 cracked porcelain. | Fitted to 49-GB and Goshawk. Good plugs but porcelain will not stand up to heat 7.1.21 | Splitdorf B.148 | O.K. | O.K. | failed | | 6 | 3 | These plugs cut right out mica insulation - gave out. 8.1.21 | Mercury GB.2 pt. | O.K. | O.K. | O.K. | O.K. | 6 | 0 | Very good plugs. One porcelain cracked before using. 9.1.21 | Mercury GB.3 pt. | O.K. | O.K. | O.K. | O.K. | 4 | 0 | Very good plugs. 10.1.21 | Heath. | failed. | | | | 6 | | Very Good. missed - slow running picking up. 12.1.21 | Cooke spec: | O.K. | O.K. | failed | 1 broke down. | 8 | 5 | Will not stand central pos: & cannot be cleaned. Plugs returned from Col. Harker, not liked. 12.1.21 | Forward. | failed. | | | | 6 | | Missed, slow running and picking up. 13.1.21 | Bosch. | O.K. | O.K. | failed | | 6 | 1 | 1 right out - cannot be cleaned. 13.1.21 | Lodge Z.19C. | O.K. | O.K. | failed | O.K. | 6 | 1 | Missed fire very bad. Contd. | ||