Just some guy
Join Date: Jun 2001
Location: SC foothills, USA
It's just a pollution thing. The guys pumping up their SVO's have found the "8 plug" heads to be fairly useless for a couple of design reasons. AND the extra plugs are useless for power purposes. Nissan did the same thing on 80's Pickups and 200SX engines. No appreciable power difference between 4 plug and 8 plug there either.
For many, many years Harleys have run a "waste spark" too. Nothing to do with pollution, it was just easier and cheaper to build them that way. The waste spark has no purpose nor effect on the running of the engines. For a while the big thing was to modify the heads for dual plugs. The plugs fired together and the waste spark was still produced. This is almost unheard of now since it's only REALLY useful for dragracers running nitromethane or alcohol. There are now some kits that eliminate the waste spark and allow individual cylinder tuning. The value of these kits has yet to be really proven beneficial, at least to me.
The only street engine I know of that really benefits from dual plugs is the Mazda RX7 type rotary engine, especially if they're turbocharged.
.... it wouldn't be the first time reality took theory and conjecture out behind the woodshed and gave them a good thrashing.