Change all of them. They all are subjected to the same environment. Two plugs are required for better combustion starting from two fronts. Airplanes have two plugs per cyl and you definately notice a power loss when you turn one set off.

Also be very happy you are not changing plugs on Howard Hugh's Spruce Goose. 8 engines, 28 cylinders each, 2 plugs per cyl and about $30.00 per plug.

$13,440



John Roncallo