DIYEFI.org Forum

This explains the benefit for Classic Racers (Bikes) most Norton's have an offset spark plug, so putting one on the other side balancing things up a treat.

http://www.mez.co.uk/mezporting/page2.h ... conversion

We are convert a Commando shortly, so will see what improvements we can measure...yards/horsepower ect

Nissan/datsun NAP-Z engines (twin plug) fire both coils simultaneously up until heavy acceleration or engine loads.

NAP stands for National Air Pollution something or other, and these started coming out in the mid 80's.

http://www.honda.co.nz/technology/engine/idsi/

Some info from Honda about their i-DSI dual plug setup. There's also animation on right hand side of page, click play and wait for a while to see it in action.