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Doc, back in the Shovel days we always twin-plugged our heads and a lot of us did the same on Evos too. No-one seems to do it now. Why is this? The question came up over a few beers with some mates. We presumed that better combustion chamber design makes it no longer helpful but after wandering around the web a bit tonight I thought to myself, why not go to the source of all things Harley-Davidson?

– Rick

Rick, I ran your question past Mark ‘Hoody’ Hood of Hood Motor Service, who has dual- plugged many a head back in the day. He said, “Dual-plugging cylinder heads is an application specific to a need. Dual-plugging in motorcycle street engines is more of an advantage if the combustion chamber is not ‘turbulent’ as in a Shovelhead, or in a hemispherical chamber sometimes used by performance shops to get away from any prospect of the chamber shrouding the valve’s flow characteristics.

This design chamber also offers the capability of using the two largest valve diameters for a given bore diameter. Dual plugging a performance-modified Shovel engine, installing a decent electronic ignition and tuning it properly will pay dividends.

On the other hand, Evo and Twin Cam engines have turbulent chambers with squish shelves incorporated in the combustion chamber and also retain the two-valve diameter optimisation of a hemi chamber.

Setting squish clearance will give optimal fuel charge burn characteristics here. When you look at older car engine designs, guys talk about ‘quench’ chamber heads, while performance engine guys talk about ‘squish’. On a drawing, they look essentially the same but the car engine manufacturer was looking to make that part of the chamber cooler, hence the term ‘quench’, so that engine’s cylinder used a smaller, more efficient combustion chamber, while ‘squish’ which is ‘squeeze’, creates the turbulence required for more complete combustion.

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