Gnome 100 hp Engine - Valve Gear Case

The exhaust timing is based on the 9 Cam gears, one cam per cylinder, mounted onto the Cam Shaft in the Valve Gear Case (Cam box). The cam shaft is driven by two planet gears which are mounted onto the (phantom animated) front cover of the Valve Gear case. These planet gears drive the cam shaft, causing it to rotate at half the engine speed, thus providing the correct timing for a 4 stroke 9 cylinder engine.

The gear ratio for the Main drive Gear and Magneto gears is two and a quarter. The contact breakers give 2 sparks per revolution and run at two and a quarter times the engine speed, each giving nine sparks every two revolutions of the engine.
The firing order therefore is 1, 3, 5, 7, 9, 2, 4, 6, 8.