During magneto timing, the piston in cylinder 1 should be in which position for ignition to occur?

During magneto timing, the piston in cylinder 1 should be in which position for ignition to occur?

• A. At top center on the exhaust stroke
• B. About 30 degrees before top center on the compression stroke
• C. 30 degrees after top center on the intake stroke
• D. At bottom dead center

Answer: (B)

Ignition must occur a short time before the piston reaches top dead center on the compression stroke. Framing the spark a bit BTC allows the flame front to start and propagate so that peak cylinder pressure occurs as the piston is near or just past TDC, converting more of the combustion energy into useful work. The typical target is about 25–30 degrees BTC, which is why the best answer is roughly 30 degrees before top center on the compression stroke. Igniting later, such as after TDC on the compression stroke or on the intake stroke, would push the combustion peak past the optimal point, reducing power and efficiency. Firing at bottom dead center is far too late to harness the pressure rise effectively, and firing on the exhaust stroke wouldn’t use the compression energy properly.

Ignition must occur a short time before the piston reaches top dead center on the compression stroke. Framing the spark a bit BTC allows the flame front to start and propagate so that peak cylinder pressure occurs as the piston is near or just past TDC, converting more of the combustion energy into useful work. The typical target is about 25–30 degrees BTC, which is why the best answer is roughly 30 degrees before top center on the compression stroke.

Igniting later, such as after TDC on the compression stroke or on the intake stroke, would push the combustion peak past the optimal point, reducing power and efficiency. Firing at bottom dead center is far too late to harness the pressure rise effectively, and firing on the exhaust stroke wouldn’t use the compression energy properly.