FMF 4.1 Slip-On Muffler For KX450

What it is

• Stainless Steel Slip-On Muffler
• Removable Spark Arrestor Insert Included.

Since 1973, FMF Racing has been an industry leader in aftermarket exhaust systems. Sponsoring some of the top teams and riders in the sport, they are one of the elite’s building systems for nearly every make and model on the market. The Factory 4.1 RCT Stainless SL (Slip-On) w/ Carbon End Cap is FMF’s budget friendly ($200+ cheaper than the Ti version) high-performance slip-on muffler designed to enhance the power and sound of the new KX450 and KX450X. Featuring a stainless steel construction for added durability and a carbon fiber end cap for a sleek, lightweight finish, this exhaust is engineered with FMF’s Resonance Chamber Technology (RCT) to deliver maximum horsepower gains across the RPM range while reducing noise output. As a slip-on system, it replaces only the stock muffler, making installation relatively straightforward and offering a significant upgrade in both performance and aesthetics compared to the original equipment. The Stainless muffler retails for $674.99 and their Titanium version retails for $899.99. A removable spark arrestor insert is included as well.

How it works

• Increased power across the range.
• Maintain's a smooth power delivery.
• Most testers preferred the spark arrestor insert IN.

Kawasaki is the first of the major OEMs to play into the upcoming sound regulations, and with that comes a very restrictive muffler. The stock muffler is quiet (something we don’t mind), but it limits power output and in some cases, leaves us wanting more out of the bike. Here the FMF 4.1 Slip-On muffler enters the chat. Installation is very simple as it bolts directly onto the stock header, only requiring the stock rubber grommets and collars to be swapped from the stock muffler to the FMF system. The fit and finish is as good as it gets, something we’ve come to expect over the last several decades with FMF exhausts.

On the trail, the gains are drastic. From top to bottom, there’s increased pulling power across the rev range. From the bottom, there’s added torque to maintain pulling power at low RPMs and into the mid-range. The top end on the KX was solid beforehand, and with the FMF muffler installed, you do notice small gains up top but it isn’t as drastic as the bottom to mid-range increase. Even with the added power and the crisper throttle response that makes the bike feel lighter and more playful, it avoids the jerky throttle response at low RPM, low throttle opening situations that can be upsetting in corners or technical riding. 

We felt we couldn’t be as aggressive with the throttle as we could with the stock muffler due to the added power, but this came as a positive to most of our testers as we could be “smoother” or more “lazy” under acceleration and not have to ride the bike so hard.

Something that we did like with the stock muffler in an off-road setting is how smooth and linear the power delivery is, and we feel a similar characteristic with the FMF system, albeit with more of it. This delivery lends itself to true off-road riding and racing in maintaining the rideability of the KX450 platform while still providing enough power to be competitive and fun. We prefer running the quiet insert/spark arrestor screen in our four-stroke mufflers for increased back pressure, more bottom end power, and smoother delivery off the bottom and this muffler was no different. 

Durability wise, the 4.1 Slip-On has been rock solid. Some other aftermarket mufflers need to be repacked more often than most would like, but we're on ~30 hours on the muffler with the original packing and it sounds very strong and healthy still. The sticker on the can needed to be replaced after ~20-25 hours, and there are some scratches as can be expected on any exhaust on an off-road bike.

Modern mufflers and exhaust systems have a lot of R&D sunk into them and it isn’t always better going to an aftermarket system, but with Kawasaki abiding by the upcoming sound regulations, there is a lot of room for improvement around the muffler on this new bike and FMF did the job. There are noticeable gains, weight savings (~1.2 lbs), and for those that like bling and care for the appearance of your bike, the Factory 4.1 Slip-On answers the call.