True. The other point to be made here is that there is no localization of the pressure pulse along the vector of the arc formed by the spark discharge. The fuel ignition is a free radical triggered reaction and diffusion limited. The flame front moves at supersonic (averaged) velocities traversing the chamber in a few hundred nanoseconds to microseconds depending on chamber geometry. The heat released by combustion then heats the surrounding gas by conduction (process too fast for convective effects) and the details of the pressure rise now depend on internal chamber mixing. This includes swirl, squish and valve overlap as well as the detailed shape of the piston surface and the stroke (piston dwell time).

The primary impact of a centrally located plug is the need to diffusively trigger the reaction as opposed to radiatively preheating areas of the chamber to trigger a metastable mixture.

Even surface gap-plugs are not capable of a centro-symmetric arc.

frank Grunthaner