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2024 Kawasaki KX450 FMF Racing Factory 4.1 Slip-On | Track Tested


2024 Kawasaki KX450 FMF Racing Factory 4.1 RCT Slip-On | Track Tested


The Kawasaki KX450 is all-new for 2024 and although it has been praised for its excellent new chassis and handling, most testers have been underwhelmed by the power output of the green machine. The mellow power output is easy to ride and not as intimidating and hard-hitting as other bikes in the class, but when it comes down to it most riders and especially racers want to be inspired when they crash the throttle of their brand-new motocross bikes, While much of the problem lies in the severely choked-up new airbox and lots of performance can be gained by either drilling vent holes in the left side panel or trimming the interior edges of the airbox itself, a quick peek inside the stock muffler is all it takes to see that the bike is also held back by a set of sound-deadening (and power-robbing!) baffles inside the muffler core. 

FMF Racing is always aggressive when it comes time to design a new performance exhaust system for an all-new machine, as being first to market is a huge advantage when it comes to sales. But rushing to produce the first available product doesn’t always equal the best performance. With so much power left on the table by the new KX450 the FMF team decided to take its time with the exhaust system so that the most effective power gains could be the ultimate result. 

We got our hands on a Factory 4.1 RCT slip-on for our KX450 several weeks ago, and we have been very impressed with the power gains yielded by the anodized titanium muffler. Not only is the muffler a beautiful addition to the machine with its carbon fiber end-cap and anodized blue finish; it is significantly lighter than the all-aluminum part it replaces. 

One of the small details that we like about the newest Factory 4.1 mufflers is the raised knobs on the side of the can where the rider’s boot can come into contact with it. Whether the knobs are for added traction or greater strength to resist denting from boots, we think they give the bike an added element of cool! The unit itself installs easily and requires only a few minutes. We like that the stock mid-pipe/header-pipe junction maintains the stock exhaust clamp, as it reduces the chances of an air leak and the annoying detonation they cause. 

Photo by Ayden Greene

The Factory 4,1 Slip-On comes with a USFS spark arrestor installed, so we rode our first few motos with it in place. We were pleased to learn that the FMF muffler produces a deeper and more-authoritative exhaust note. Although louder than the stock muffler, it is by no means offensive. On the track, the bike’s throttle response is improved greatly and the bike seems to gain rpm quicker. The snappier low-end makes it easier to break the rear wheel loose in corners to initiate a directional change if steering with the rear end is your style, and the bike also pulls a taller gear with greater ease. Mid-range power is also notably improved, and the bike pulls further in each gear before flattening out. Remove the spark arrestor, though and things begin to get exciting! While the roll-on power isn’t as abundant down low, the bike hits much harder as the RPMs climb and the engine pulls much, much further on top. Truth be told, this is one of the most drastic power and performance gains we’ve ever experienced with a slip-on muffler. FMF Racing has yet to send us the matching header pipe, but we can’t wait to see what type of power gains can be had with a full system! For now, the $899.99 Factory 4.1 Slip-On is likely the best single bolt-on part you can get for the 2024 Kawasaki KX450.


Donn Maeda

Donn Maeda is a 30-year veteran in moto-journalism, having worked at Cycle News and Dirt Rider before launching MXracer Magazine and TransWorld Motocross Magazine. Maeda is the Editor-In-Chief at Swapmoto Live and you can catch him on a dirt bike or in the saddle of a mountain bike on most days.

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