[Rotary Evolution]

but in n/a applications the timing is more advanced, so the plug firing is closer to the leading apex tip at 35* of advance, the trailing in the 3 plug configuration is probably just before the combustion event closes right before the trailing apex tip passes over the plug port(because of its location it doesn't have much alternative unless they somehow worked out a leading>trailing>center firing configuration for optimal performance).

in 13B engines the trailing fires the closest to the center of the rotor.