
Splitfires work mainly by providing a more-or-less unshrouded spark to the
combustion chamber. If an engine's cylinder head design can benefit from this,
then the splitfires will yield a slight performance increase, most noticeably
in lower rpm range torque. Splitfires didn't do diddly-squat for my 1992 GMC
pickup (4.3l V6) but do give a noticeable performance boost in my 1991 Harley
Sportster 1200 and my best friend's 1986 Sportster 883. Folks I know who've
tried them in 1340 Evo motors can't tell any performance boost over plain
plugs (which is interesting since the XLH and big twin EVO combustion chambers
are pretty much the same shape, just different sizes). Two of my friends who
have shovelhead Harleys swear by the splitfires but if I had a shovelhead,
I'd dual-plug it instead since they respond well enough to dual plugs to make
the machine work and extra ignition system worth the expense (plus they look
really cool with a spark plug on each side of each head)