2019 FMF 4.1 RCT Kawasaki KX450 Full System Test

Company: FMF

Price: $899.99

  • Almost 3 pounds lighter than stock
  • Looks way better than stock and more sleek
  • Increases power throughout the power curve
  • Tunable with RCT stuffer/spark arrestor
  • $899.99 price tag
  • Louder than stock, especially with the RCT stuffer removed
  • Wish it came from FMF with some sort of heat shield over Megabomb header


  • Writer: Scott Hoffman
  • Photographer: Scott Hoffman

What it is

  • Full Titanium System with Carbon Fiber End Cap
  • Nearly 3 pounds lighter than stock
  • A lot sleeker looking over stock unit

We tested the latest and greatest full system titanium FMF 4.1 RCT with carbon end cap for the 2019 KX450. This system features a shorter header than stock to decrease damage in a possible crash and the new MegaBomb header has a square back section to further tuck into the bike for less possible leg or pant contact. The muffler is more sleek than stock and tucks into the side panel better but is still very visible due to the fact the KX side panel does not wrap around. The complete unit is nearly 3 pounds lighter than stock. The system is also tunable by running or removing the RCT end cap stuffer/screen. The screen is also technically an approved spark arrestor and keeps the exhaust note quieter than if ran without. When bolted up, our bike looked faster and more bad-ass before we even hit the magic button.

"When bolted up, our bike looked faster and more bad-ass before we even hit the magic button "

How it works

  • from the push of the button we could hear the difference.

The 2019 Kawasaki KX450 is fresh in dealers but FMF already has an exhaust system available. The stock KX450 is a solid motocrosser with lots of power and very tractable and usable delivery stock. The available coupler clips do help alter the power delivery but not as much as the Kawasaki Power Tuner; however, very few people own the handheld $700 dollar-range FI and ignition tuner but it works with the stock or aftermarket exhaust.

So why the FMF aftermarket exhaust? First off, the stock unit is not the best looking and is large—yet quiet. The FMF is more sleek looking and offers two tuning options: with or without the RCT exhaust tip stuffer/screen. And mentioned above, the unit is nearly 3 pounds lighter. We tested the FMF with the optional couplers but most of the testing was with the stock coupler and this is how FMF did most of their development.

Several test riders like the easy-to-ride stock power curve because it is so connected to the ground and progressive as you further twist the throttle. Yet some riders were seeking a little more pep in the KX’s step, or a power curve that offered more romp in the mid-range. We had some success with the Kawasaki power tuner but some riders just want a simple way to get more without having to be a computer tuner, not to mention a sleeker and lighter overall system.

We first bolted on the FMF 4.1 without the stuffer and right from the push of the button we could hear the difference. Without the stuffer the exhaust note is louder than stock but not totally obnoxious but close. On the track the exhaust turns the KX into a fire breather and adds power and spunk throughout the power curve. It’s kind of that WOW factor, but for some it’s almost too much. For the average rider it will probably be too much. Yes, it’s kind of fun to ride, but hold on. Very aggressive riders and/or seasoned pros might like it; or if you are riding in deep sand or big hills, it’s good there are options to tune the system. Coming out of corners on on slicker surfaces its hard to control wheel spin or causes the rear to squat and whats to wheelie out of corners with mortal humans aboard, superhumans like Eli Tomac might think different.

We installed the RCT stuffer, which is a quick process. The unit is inserted into the exhaust tip and held in place with a simple spiral clip. It’s a two-minute process, if that. With the stuffer, the note is more mellow to the ear and FMF claims it will pass a 94 AMA decibel test. We did not have the proper sound meter during our two days of testing. It still sounds meaner and slightly louder than stock but it’s a nice note and noticeably quieter than without the stuffer on the FMF.

On the track, the FMF with the stuffer offered the best all-around performance over stock. It added tractable bottom power but it ramps up faster with more notable hit in the mid but not overbearing. For some riders, it gives the KX that added boost on jump faces to stretch the distance that extra foot compared to the stock exhaust. Without the stuffer it would cause too much rear wheel spin and the power was too hard to control for the average-skilled rider unless the dirt was tacky or with some loam.

Down low it mimics the stock curve but comes on a little stronger, and in the mid it just builds a little faster but without revving too fast into the top RPM range. If you are smooth with the throttle the FMF gives you the option to feed more or less power when needed. A few riders felt that in some slick conditions the stock exhaust felt a little more connected to the ground with the same throttle positions. With the FMF you have to learn to feed the throttle a little slower when conditions offer less traction to the ground. Yet when you want the engine to be more aggressive, you can feel the difference if you feed the KX a handful of throttle with the FMF over stock. Test riders also noted that with the FMF it pulls third gear in corners better than stock. Even in pretty tight corner you could just leave it in third and it pulls without hesitation and keeps pulling as the revs build. Some felt they wanted to grab some clutch in those conditions but learned fast it can pull it. It does pull third with the stock exhaust but with the FMF it does a better job. However, hitting corners in second, you have to be smooth cracking the throttle. If you grab a handful, the FMF comes on faster and harder than stock, and can cause some wheel spin and upset the chassis and in ruts wheelie if you are too aggressive with the throttle. If you learn to be less aggressive with the throttle in corners depending on traction, you can learn to utilize the added pickup of the FMF system and use it to your advantage when needed. The couplers do make slight changes, even with the FMF.

If you are looking for a more bottom and further potent mid-range surge/hit on your 2019 KX450, the FMF is a very solid option. And without the stuffer, the FMF makes the KX a full fire-breathing wheel-standing beast. The FMF 4.1 RCT is sleeker looking over stock, meets sound with the stuffer, and is nearly 3 pounds lighter over stock. If $899 is a hard pill to swallow, FMF does offer other options like a slip on, aluminum can option or stainless header, which can help the budget but still gain the same performance.


"On the track the exhaust turns the KX into a fire breather and adds power and spunk throughout the power curve"

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