My Shelby picked up a misfire on CYC 4 - Easy fix - New spark plug got me back on the road. Will change them all this winter. Strangely, the car only has 18,500 miles. I am using MSD red coil packs. Motorcraft SP-547 Gap: .054; w/ Black Coil Boots; Spark plugs are torqued to the proper specification. Spark plugs- SP-546 and SP-547 require properly torqued to 30-38Nm (22 to 28 Ft. lbs.) with a dedicated 9/16 spark plug socket.