Duncan Racing New National Fat Boy 4 Exhaust System

Get big horsepower gains with a lifetime warranty.

Duncan Racing’s complete National Fat Boy 4 exhaust system for the 2009-2021 Yamaha YFZ450R.Duncan Racing

Despite the lack of new sport quads from the major powersports manufacturers, our friends over at Duncan Racing say business is moving at wide-open throttle with no end in sight. Shop manager Leonard Duncan tells us that the lack of new machines is probably a good thing for them as they just don’t have the time to develop new tooling. That said, Duncan Racing has been turning out new products for two sport quads that continue to crank out of the factory as fast as Yamaha can build them: the Raptor 700 and the YFZ450R.

The header and mid-pipe are constructed from 304 stainless steel with stainless steel header flanges.Duncan Racing

One of Duncan Racing’s latest creations is the National Fat Boy 4 complete exhaust system for the YFZ450R. Duncan claims that this exhaust system along with a Pro Flow air cleaner and Vortex ECU produces more than 10 additional horsepower. The combination is designed to produce crisp throttle response with strong midrange power gains and increased top speed. A larger countershaft sprocket is also recommended to take advantage of the added performance.

The header and mid-pipe are made from 304 stainless steel and CNC bent for a precise fit. Stainless steel header flanges ensure a solid mount to the exhaust port. The muffler is constructed with a 0.100-inch-thick aluminum wall that comes in a satin finish or can be custom ordered with a polished finish. An O-ring seal between the mid-pipe and muffler assures a leak-resistant seal. The muffler is completely rebuildable and repackable with front and rear billet-aluminum end caps.

The muffler has a 0.100-inch-thick aluminum wall with CNC-machined billet-aluminum end caps.Duncan Racing

The complete exhaust system retails for $749. A turndown end cap can be ordered for an additional $39.95 for quieter operation and spark arrestor use. Duncan Racing also sells replacement packing and rebuild kits for the Fat Boy 4 exhaust system.

For more information, visit Duncan Racing online at duncanracing.com.