FMF 4.1 RCT System For KX450F

What it is

• A performance exhaust system for your Kawasaski KX450F.

FMF’s Factory 4.1 RCT uses Resonance Chamber Technology to build a performance exhaust that is claimed to increase power, massively. The header utilizes FMF’s patented and often imitated Megabomb header to additionally decrease sound levels. The system is designed for a shorter overall length, which is less susceptible to crash damage as well as locates the muffler closer to the engine to centralize mass. The Factory 4.1 is manufactured 100% in FMF’s Southern California factory and constructed from aerospace grade materials. We tested the (more economical system, yet same performance) natural aluminum muffler with a carbon fiber end cap, stainless header and mid-pipe. Note that slightly lighter full titanium systems are available also. The Rapid Tuning System allows the simple installation and removal of a quieter core and a spark arrestor with just a spiral clip. Huge weight savings are also claimed, even with this price point materials makeup.

How it works

• You can't deny the weight drop even in this price-point system.
• It brings back the snap and hit of the old KX if you want it.
• Can enhance the new-style KX power delivery and cut sound at the same time.

The first thing we did was to weight the system and compare the numbers to the stock exhaust on the 2016 Kawasaki KX450F. The FMF system dropped 1.3 pounds compared to stock. It bolted on as easily as stock with some supplied hardware, which differs in length slightly. Our muffler was delivered open with the additional quite core and the clip not installed yet.
The first thing you will notice is that the noise of the bike, from stock to the open FMF system, increases a bit, especially when you crack the throttle open. On the track the sound is a little louder at lower RPMs, yet it is comparable to the stock pipe when the bike is on the gas. Kawasaki has done an excellent job of quieting the KX down (even though in the past it was the loudest bike on the track yet could fool a sound meter during a test) and the FMF muffler lets a little bit of that noise back out. It isn’t overly offensive but it is not quiet either.
This also helps bring out the biggest change this exhaust can deliver. A lot of comments about the new bike center on the fact that Kawasaki tamed or mellowed the hit off the bottom and into the mid.. This pipe brings it right back. If you missed your 2015 KX450F powerband, this setup can fix that. But it isn’t flat or just noise, this pipe gives the bike true snap and tug right as you hit the gas, you will not need the clutch to boost the power. Larger and heavier riders or experts who know how to use this boost will really appreciate this. Those that like the smoother and friendlier KX will have another option.
Run with the muffler open, the KX continues to build the strong and capable power a little faster than stock and definitely has a little more steam on top but it also signs off harder when it finally gets near the rev cut. Simply put, the power is more aggressive and definitely faster feeling if not outright faster. It spins the rear tire more, spins it up easier and will allow riders to go to a lower RPM and just gas it out.
Those that like the smoother KX will really appreciate the FMF system with the quiet insert/SA installed. It slides in and the spiral ring secures it in place, just make sure to leave the bent end of the clip facing outward or you may never get it back out. With this change the bike instantly becomes even quieter that stock by about the same amount as it rose when run open. Mostly at idle and at that first crack of the throttle. Through the RPMs it is stays quieter on the track but it is very noticeable to the rider’s ears more than anything. Here the powerband is almost a mirror of the stock power in delivery and in the amount of power and if anything almost feels a little slower. But that is the noise playing with your head. We knew there was more power based on how far the bike would fly on long jumps with short approaches and the gear the bike would pull out of some uphill turns. It was just a little bit better than stock and for sure some of that was from better traction. Additionally some liked the way the insert slowed the overall speed with which the KX revved, especially on the very top part of the power curve which made for less shifting.
However, no matter how the muffler was configured, the KX wanted a richer fuel mixture, especially in the lower range, this also improved the power delivery as well. Additionally, the Megabomb’s characteristic of increasing the throttle response and crispness is not as noticeable as it has been in the past and on other bikes. It is definitely improved over stock, most noticeable in quick on/off/on throttle usage, just not to the level we have seen before. We’re pretty sure (and we’d bet a six pack FMF isn’t too happy about this) that since the KX now has a chamber on the stock header, which tries to do the same things as the Megabomb, and probably succeeds at the task
This exhaust is built with high-quality materials with attachments and fitment that were spot on. The craftsmanship is excellent and the carbon end cap on this mid-priced system gives it a very factory appearance. With the switching of the muffler insert this system delivers two completely different characteristics for one price. It looks great, weighs less and especially for those wanting their old KX back, just lighter with better handling, this system will accomplish that task.

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