For Fat Boy 4 Instructions Click Here


DRI offers a LIFETIME WARRANTY against defects in materials and workmanship on all Fat Boy 4 systems sold. Warranty is for original purchaser only and is non-transferable. Each system is supplied with warranty registration card that must be read, signed and returned to DRI.

Optional Accessories
#FB4-4A250 Bolt-On Turndown (required for spark arrestor usage).
#06-554 Spark Arrestor Screen.
#FB4-RP-110 Repacking Kit.
For additional parts see DRI price list for full listing.

Other Required modifications:
Pro Flow Air Cleaner with K&N Air Filter, Carburetor Re-Jetting.On DRI Retail direct sales jets for stock OEM carburetors are supplied at no charge.
Larger C/S sprocket advisable in hard pack terrain or on with 18" tires.

Performance: 10+ HP Gain when used with a Fat Boy 4, Pro Flow and Vortex ECU.
Click here for Dyno results

Fat Boy 4 4-Stroke Power Exhausts are a proprietary product of Duncan Racing International, Inc. DRI is the sole manufacture and developer of this product. DRI has designed this product to be the top ATV 4-Stroke exhaust on the market.


Fat Boy 4 Features

-CNC Bent 304 stainless steel headers, mid-pipes and tail sections. Standard finish; brite finish.
-CNC machined stainless steel header flanges.
-Custom designed .100" wall aluminum extrusion muffler body. Standard finish satin. Polished by special order.
-Front and Rear End Caps CNC machined from 6061 billet aluminum.
-Muffler mounts CNC machined from single piece of 6061 billet aluminum. Attached to muffler body by heli-arc weld.
-O-Ring Seal at tail pipe to muffler joint.
-All mufflers completely rebuild able and repack able.
-Stainless muffler cores.
-Optional (at additional cost) Bolt On Turndown; redirects exhaust fumes and noise toward the ground. Also required for use with spark arrestor. Attaches with 4 stainless screws (supplied) and has O-Ring seal.
-All systems are individually marked with their own serial number.
-Complete DRI Technical phone support for installation and carburetor jetting questions.