Use J6C as the others said. The J8C is a hotter plug. Heat range is all about the temperature of the firing-tip of the spark plug to keep it from fouling. If you get that too hot, you can get early detonation (also known as knock or ping) which is like hitting the piston with a hammer, and can eventually cause damage. It can also shorten spark plug life. Heat range has nothing to do with how hot the motor runs or how fast it goes.
Dave
