FMF Factory 4.1 RCT Full Muffler System Product Review

It’s hard sometimes to get more power out of newer four-stroke models nowadays with aftermarket mufflers systems. To be honest with you most stock systems are pretty damn good and aftermarket muffler companies like FMF work really hard to try and give you that little extra bit out of a system. I can sit here and tell you that FMF is one company (of only a couple in existence) that will take the time to listen to the proper feedback (either from me or other media testing outlets) and make the necessary changes to alter a new four-stroke machines power in the right direction. The key word here is “alter” and maybe not “gain”. Yes, in this article you will hear me talk about “gaining” a little pulling power, but to me the configuration of the 2018 Kawasaki KX450F FMF 4.1 RCT Muffler System power delivery is more “altered” from stock than anything. 



The stock power of the Kawasaki is unchanged for 2018, but that doesn't mean it’s not a great bike. The engine has a snappy throttle response with a free feeling, on de-cel, without much engine braking. Once rolling on the throttle, the power is not as strong as the Honda or Yamaha, but still creates a lightweight feeling on the track. I didn't necessarily need more rpm response with the KX450F, but would like more meat through the mid to top end range. Bolting on the KX450F FMF Factory 4.1 system is bar far the easiest system I have bolted on for 2018. It took me literally two full songs on the Metallica Pandora station to change the muffler. The Kawasaki is a light bike stock, but I chose to go with the full titanium system that let me drop 1.8 pounds from the stock system! Visually the titanium FMF system is one of the most stunning aftermarket mufflers you can put on a bike. I love the blue anodized finish as it looks good on almost every make and model machine. 



On the track the FMF 4.1 on the KX450F makes as much bottom end power delivery as stock and keeps that peppy/exciting RPM response. Where the FMF 4.1 system really impressed me was mid range pulling power when exiting corners. The stock system feels empty in this area, but the FMF fills the power-feeling in through the mid-range. What I mean by “fill the power in” is that the FMF system actually feels like it pulls as soon as you shift into third gear unlike the Kawasaki’s power where it feels like it lacks some pull through the mid-range. There is a dip in the Kawasaki’s mid to top end power and the FMF brings back some meat through that area, which in this case “fills in” that area. Top end is also increased slightly and you get increased pulling power near the end of each straight with the FMF. 


I did go back and forth with the couplers on the KX450F and came to the conclusion that although the lean coupler is better for bottom-mid range power, the standard coupler is slightly better from mid to top end. I will have to say when you do go to the lean coupler that the KX450F will pop more on de-cel with the FMF system than the stock muffler. With the standard coupler de-cel pop is minimal and not as noticeable with either system. 



At the end of the day the FMF 4.1 RCT full muffler system is better than stock and that is not easy to do these days. It takes off some weight and looks better than the stock monstrosity. To me there is no negative with the FMF system and that is rare in an aftermarket muffler. I have tried other FMF mufflers, on some other models with some negatives, but the KX450F muffler is a good buy. Please note that re-packing of the FMF muffler is important for the can’s lifespan. The muffler portion (or can) will start to leave hot spots/scarring when the packing starts to burn out inside. Once you start seeing these “hot spot scars or markings” on the muffler, remove and re-pack is recommended. FMF sells re-packing kits and taking the can apart to re-pack is not that hard to do. I get around 10 hard engine hours (these are pro type hours) before I see these markings and have to re-pack. If you run it past the recommended time, you run the chance of hurting performance and damaging the core (or perf) inside the muffler. Kudos to FMF for building a good KX450F system and moving the power around slightly and gaining a little as well. If you have any questions about this test please email me at 


Bonus: If you listen to my Keefer Tested podcast you know that I rate some parts and motorcycles by a numeral system. I wanted to break down the FMF system to the stock system for you all below. If you don’t know what these numbers mean, please go listen to show #3 “How To Test” and you will understand better what these numbers mean. 



Bottom End: 3/3

Mid-Range:  3/3.5

Top End:      3/3+

Over-Rev:    3/3+

Bottom End RPM Response:  3/3

Mid RPM Response:               3/3.5

Top End RPM Response:       3/3+