You mean on the car? I think she starts better, idles better, gas is a bit better, plugs run cleaner. When I had to meet NYS emissions an MSD often meant the difference between passing and failing. Other guys swear they are useless but me and my friends all like them. And I once got a zap from my hand, up my arm, across my chest, down my other arm, to my other hand. NOT FUNNY!! Be carefull.