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.
Last edited by sstock; 07/16/2019 1:52 PM.
1953 Chevrolet 3100261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
In the DITY GalleryVideo of the 261 running1964 GMC 1000305 Big Block V6, sm420, the next cab off restoration
