I have some CMB 90's that have both the single and dual plug setups. I have tried both and have not found a substantial difference in performance between them in the gold head EVO. The purple head EVO2 seemed a slight better with the single plug. The head clearance was about .002 in difference and that may have made the difference. I did not have a set of shims at the time that could get me to the same clearance to truly compare.
What Chuck said is true having an extra plug. You can also burn 2X the plugs if you lean on it excessively or have too tight a head clearance.
One thought for the engine guru's. Anyone found an optimum location for the twin plug setup? Such as the plugs at the 12 and 6 o'clock positions? Or the 3 and 9 o'clock positions? Think about the fuel charge and discharge and how to optimize the firing of it. This may be part of the equation of the benefit of a dual plug head vs a single.
