Hello,
With airplanes it is recomended to keep the radio equipment 10-12 inches away from the engine ignition. Don't use any metal control rods or wiring between the engine and the radio. Keep the antenna away also. Use a resister spark plug gapped at between 20-25 thousands. I don't know how the car and boat people accomplish this, maybe they use sheilding of some sort. Try the car forum. Good luck.

Dave