The large Shovelhead hemi chamber does not give efficient combustion, and this is further exasperated by the domed pistons, particularly with the higher compression versions. The addition of an extra spark plug in each head helps redress this issue, and whilst power isn't always noticeably increased on a stock motor, this modification gives easier starting, sweeter running, and better driveability.
For higher compression builds and the use of cams with early closing intakes (which give higher cylinder pressures and better low rpm throttle response) this conversion allows more appropriate ignition timing to maximise the performance gains.
Please note that twin plugging Shovelheads also requires the use of a modern single fire ignition system and duel coils of the correct resistance. We also supply these, along with the correct spark plugs, and can install and correctly tune if required.
We have designed and manufactured a machining fixture that enables us to machine the additional plug holes in a position that allows the use of 14mm spark plugs with a full size 13/16 hexagon. A 3/8 drive plug socket can be used to install and remove the new plugs without having to remove the standard pushrod tubes.