FMF Factory 4.1 Exhaust for the 2025 Kawasaki KX450 | Track Tested
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Price: $1374.99
The 2025 Kawasaki KX450 is the second generation of the latest chassis and engine design, as the bike was all new in 2024. Praised for its great handling and planted feel, the green machine was “held back” by a smooth, tractable powerband that didn’t provide as much punch and excitement as many racers longed for. The solutions? Aftermarket exhausts were the first steps chosen by most, followed up by airbox modifications that left the innards looking like a block of green Swiss cheese. And while Kawasaki should be praised for its RIDEOLOGY app that allows consumers access to their bike’s ECU mapping, most hardcore racers ent a step further by utilizing an aftermarket Vortex ignition.
As far as exhaust systems go, we utilized a FMF Factory 4.1 Slip-On on our 2024 KX450 and noticed a nice gain in low-end throttle response thanks to its freer-flowing core. The OEM muffler has an internal baffle in the muffler to dampen sound that makes the Kawasaki on of the quietest bikes in its class for the first time ever, but the toll it takes on the bike’s excitability is huge. Pleased with the FMF muffler, we waited with bated breath for the matching header pipe, anticipating even more power gains to our surprise, the header never came and it wasn’t until we had our 2025 KX450 in our possession that a complete Kawasaki system showed up
For those who own a new ‘25 KX450; try these settings! 
But what about the FMF complete system? Isn’t that what we’re here to write about? Yes! according to FMF, the biggest challenge of producing the Megabomb header was the manufacturing of its exclusive cast aluminum header flange to match with the new KX’s odd rectangular exhaust port. The finish of the header as a whole is beautiful, and the KX450-only flange tooling in itself, required a notable commitment from FMF, as the pieces it produces are applicable solely to this single machine.
How did the install go? We’ll be honest: it wasn’t smooth! But that has nothing to do with the fit and finish of the FMF system…it’s Kawasaki’s frame downtube design that blocks any and all direct access to the exhaust stud nuts. Removing and reinstalling them requires the use of either a 1/4” ball joint adapter and a ratchet extension, or a box-end wrench and some long, thin, fingers.
Beyond those challenges, the rest of system was a cinch to install, and the unit as a whole fit perfectly. From the titanium header and mid pipe to the blue anodized titanium muffler canister and carbon fiber end cap, we never get tired of the look a new FMF system gives a motocross bike.
One of the biggest surprises about the system is that it maintains the pleasant stock exhaust note and doesn’t produce an offensive amount of noise like some aftermarket systems do. Still, the tone of the bike is deeper, with a throatier bark when you crack the throttle. The power gains produced by the complete system – header and muffler – are impressive, with snappier throttle response that makes it easier to break the rear wheel loose to initiate a slide through a corner and get things going. Low-end torque is improved and this allows you to carry a tall gear through the corners with much more confidence that the KX will pull through and respond when you crack the throttle wide. The overall power delivery remains as broad and controllable as stock, with no uncontrollable spike as the rpm’s build; only with a much more substantial pull down low and an improved mid range. At the top of each gear, we were also surprised to find that the Kawasaki continues to pull a bit further than stock…usually when big gains are made somewhere in a powerband it comes at the expense of some performance elsewhere.
Test rider Kyle Puerner reports that the 2025 KX450 with the FMF system alone, out-performs the ‘24 version he raced with a different pipe, airbox mods, and lower gearing.
As a whole, the FMF Factory 4.1 Exhaust system for the 2025 Kawasaki KX450 is a very impressive addition to the green machine, and we are very impressed with every aspect of it. From the cast header flange and beautiful construction of the rest of the system, to the weight savings and power gains, this system is a home run for Kawasaki owners.
