The DuraMax Exhaust Brake is now Available! BD Engine Brakes enable your Diesel to navigate downhill grades and mountainous terrain easier and safer than using the vehicle's conventional braking system. This will increase the life of the stock foundation brakes and also prevent brake fade under severe grade driving. The factory-approved backpressure provides the retarding horsepower to slow down the vehicle on even the steepest descents or just slowing down for a highway off ramp. The exhaust flow is not restricted when the stainless butterfly is open in the large 4” casting that bolts into the exhaust system. The kit is complete with a lighted toggle switch for mounting conveniently on the dash. The brake releases as soon as the throttle is applied or the switch is flipped off. As in other BD designs, the backpressure is self-regulating. Once the pressure reaches the DuraMax specification, the butterfly position changes to maintain the highest pressure possible. This variable orifice design offers far superior retarding performance throughout the engine RPM range compared to a fixed orifice design that only offers peak retarding performance at peak rpm. The new Allison equipped DuraMax is designed to downshift once the tow/haul button is selected, which results in a Silent Retarding Force at all vehicle speeds.