[longhornchris04]

Short answer - under 10:1 and <98mm bore you should be OK. A mild compression 2.7 should be fine. High compression not so much, same for a big-bore motor.

Of course, if you have the money you could always spring for the machine work for the twin-plug and just put old plugs in until you can splurge for your choice of spark options. If you are in the upper range of safe single plug compression (running conservative timing) you will benefit from twin plugging (and more aggressive timing).

Explanation follows:

Best info I can give you is out of Wayne's book + some general physics. The hemi-style combustion chamber (which Porsche's have, so do Harleys, it ain't just Dodge) with its high dome and off-center spark plug isn't great for flame front propagation, at least when compared to the more modern 4-valve center-plug design.

For a low compression engine, this is less of a problem. In order to increase compression, the piston dome is raised... to the point where it can actually prevent the flame front from crossing to the far side of the cylinder if the spark-advance isn't high enough. This of course raises the risk of spark knock. So, if you aren't running high compression (under 10 is usually the "magic number" but its actually dynamic compression which is also tied to cams, and the ability to keep the chamber temps in check) then the flame should easily cross the chamber.

With the large chambers (>98mm), the key problem is the distance between the spark plug and the far side of the chamber. Again, the options are either a controlled second flame front or more advance to get the flame to cross the chamber.