The Stovebolt Site Forums The Shop Area Engine Shop General Question Y? BTDC

Because you want the engine to fire the plugs at exactly the sweet spot for the particular engine. Influences that affect timing are camshaft profile, compression ratio, type of fuel used and so on. Ignition lead is required in most case because the firing event much occur right before as the cylinder reaches top dead center, so the energy produced during the firing event will push the piston down the cylinder with the most power. Because there is time involved for the fuel to ignite and burn it must happen in advance of the piston reaching top dead center. Some slow cranking engines like the 6 volt stovebolts could have timing specs at top dead center or after top dead center, but as soon as the engine starts mechanical advance is built in to advance the timing at idle speed.