I wish I had the time and money to test more of this stuff... I sure many of you do to. Last time I went from singles to dual plug it was also from worn out head to fresh seat and guides. Not much of a test. It is so very hard to truly do one thing at a time.
I have a hypothesis that dual plugging is more of an advantage at lower RPMs when there is less turbulence to spread the flame front then at high RPM where turbulence assist with rapid combustion. Couple this with the idea that spark advance is stopped at higher RPMs because turbulence takes over and spreads the flamefront more quickly for us.
I wish I had more to add than that, but that is what has been rolling around in my head as I think about buildin my next race engine.
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