I have the DMH electric cutouts on my 2007 Super Bee. They are really cool. When they are open my car sounds like Allen Johnson's Pro Stock Stratus. When the family is in the car I shut them and the exhaust sounds stock. Regarding smoke smell and blackening the bottom of my car, neither of those things happen. But on an older car with higher emissions it might. When I was a teen my first car was a 1967 Pontiac with a Thrush muffler that ended under the back seat with no pipe after it. That car did get smoky on the inside at stop lights. Sounded damn good though.........g-g-g-g-g-g-g-g-g-g-g-g-g-g-g-g-g-g-g.
