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The Kawasaki KX450 was the most improved 450 machine of 2019 and with Kawasaki not changing anything on the 2020 KX450, I wanted to test some new aftermarket parts to see if I could improve on what I already knew from last year’s machine. I knew FMF had two headpipes available for the KX450, but until recently I wasn’t able to try it. The SX Style headpipe comes down like a “lowboy” headpipe and increases the length in order to shift the power character around. So what exactly did the “SX Style” headpipe do to the KX450 and how is it compared to the MX Style headpipe? First let us tell you about the stock KX450’s power character.
The stock power curve of the 2020 KX450 engine has a snappy throttle response from 0-10% throttle opening (with a free feel to it) and at times can almost be too touchy through corners with the standard green coupler installed. Once rolling on the throttle and passed the corner, the power is not quite as strong as the Honda or Yamaha, but still creates an easy to ride power feel on the track. I didn’t necessarily need more rpm response with the 2019 KX450, but I would be lying if I said I couldn’t use more bottom and mid range puling power when coming out of corners. If you go back and read my review of the MX Style FMF Megabomb headpipe with 4.1 muffler from 2019 (https://www.keeferinctesting.com/offroad-testing/2018/8/14/fmf-41-rct-titanium-muffler-system-2019-kawasaki-kx450) you will read that the MX Style Megabomb smoothed out the low end power delivery more than the stock muffler system did, but had more mid range meat, which was noticeable immediately. In a perfect world, I would love to get more of that newfound mid-range “meat” (that the MX Style and FMF 4.1muffler system brings) and shift some of it towards the bottom end. This is why my curiosity lead me to this FMF “SX Style” headpipe.
As far as bolting on the complete system, it really is a painless install. The 2020 Kawasaki is the lightest Japanese bike in stock form, but when FMF sent me the full titanium system, I was surprised it dropped the Kawasaki’s weight by 2.4 pounds! Also this is just my opinion, but the titanium FMF 4.1 system is one of the most stunning aftermarket mufflers you can put on any bike. I love the blue anodized finish against the green plastic and it makes the whole bike pop more!
Comparing The FMF SX Style Headpipe To The MX Style FMF Megabomb Headpipe:
So how do the two headpipes with the FMF 4.1 RCT muffler compare to each other on the track? The SX Style headpipe, along with the 4.1 RCT muffler made the KX450 feel more playful out of corners than the MX style megabomb that I tried on the 2019. The low end RPM response (0-10% throttle opening) was slightly less with the SX Style headpipe (without the muffler insert), but now the KX450 had slightly more bottom end pulling power without sacrificing that extra mid-range that I liked so much from the MX Style Megabomb headpipe/4.1 muffler system. You will get slightly less pulling power on top end/length with the SX Style headpipe, but to me it was a minimal loss. Some less sensitive riders may not feel the loss of top end pull with the SX Style headpipe, but it’s something that should be noted. I prefer the SX Style headpipe on most motocross tracks simply because the KX450 pulls harder out of corners and I can shift to third gear sooner with the SX Style headpipe.
Insert in…
You want to know a tip? Don’t throw out that insert that comes with your FMF 4.1 muffler system! Why? Because I actually liked the SX Style headpipe/4.1 muffler system better with the insert in. Why? Because it gave the muffler some added back pressure and gave me some added RPM response without sacrificing a lot of mid range pulling power. With the insert in the muffler, the KX450 now has increased (0-10% throttle opening) low RPM throttle response that I was missing through mid corner and had better corner exiting power. With the insert in you also will get more connection to the rear wheel and more traction when the track gets firmer. The sound of the 4.1 muffler with the insert in is also more pleasant and isn’t as loud as the larger opening of the FMF 4.1. NOTE: Not all bikes respond to inserts/back pressure the same so just know that each bike is different. If you feel like you may need a little more low end throttle response from your FMF 4.1 muffler system, try the insert.
It’s tough to get more horsepower, added connectivity, as well as an overall better engine character than what a stock exhaust can give, but FMF did a good job with the 2020 Kawasaki KX450 SX Style headpipe/4.1 RCT muffler. It retained the stock system’s low end RPM response, while getting some added bottom end pulling power, as well as mid range meat coming out of corners. In order to get the SX Style headpipe when you order, you must order the SX Style headpipe separate and use the 4.1 RCT slip on. If you order the full factory 4.1 RCT system you will be getting the MX Style headpipe. The SX Style headpipe costs $349.99 and the 4.1 RCT slip on will run you $549.99. The FMF 4.1 RCT muffler also comes with a mounting bracket for the subframe tab to ensure that the tab will not break or crack. Overall, I think this is a good upgrade to your 2019-2020 K450.
If you have any questions about this test please feel free to contact me at kris@keeferinctesting.com. I can guide you through any questions you may have.