Ns 4 Ss/Al Slip On Yfz450Every DR. D exhaust is built, developed, and tested by Doug Dubach. DR. D pours more time into track testing, with more experience, than any other company. A former Factory Supercross rider and career test rider, Dubach is widely considered *the greatest test rider in the sport*. DR. D exhaust performance is unequalled; while others simply search for peak horsepower on a dyno, Dubach spends hours riding stock bikes to understand where each needs