2023 KTM 350 SX-F FMF Racing Factory 4.1 Slip-On | Track Tested

FMF Factory 4.1 Slip-On Muffler

Price: $799.99

fmfracing.com

The KTM 350 SX-F is all-new for 2023 with the chassis and suspension that was introduced on the 2022 Factory Edition 250 and 450 SX-F bikes, as well as a completely redesigned powerplant. Simply put, the 2023 is the best 350 SX-F KTM has ever produced and the bike is an absolute weapon. While past 350 SX-F bikes have felt like super-beefy 250s, the new mid-sized machine feels more like a manageable 450 that is easy to ride in a tall gear, or revved out. In stock condition, the new 350 SX-F boasts great power throughout the rpm range with a versatile personality thanks to the dual map switch and traction control option. To be honest, we couldn’t think of a flaw in the bike’s powerband when questioned about it. “You’ll have a hard time finding an aftermarket exhaust system that outperforms the stock stuff in 2023,” KTM’s Tom Moen told us. We wanted to see if this was true!

In the past, we’ve had great luck with slip-on mufflers when it comes to bikes that have excellent stock exhaust systems. Leaving the OEM header pipe intact, an aftermarket slip-on muffler like the FMF Factory 4.1 can yield some weight loss, better looks, a spark arrestor option, and power gains. That said, we phoned up our pal Jason “Hapa” McCune at Insideline Connect, who handles much of FMF Racing’s press relations, and requested a slip-on for our new test bike. 

FMF Racing sent up the Factory 4.1 in blue-anodized titanium with a carbon fiber end cap. While many bikes require extra-long mufflers to keep the sound at an acceptable level, the FMF system benefits from the FMF Resonance Chamber design, which adds a sound-dampening cone at the front of the muffler, where the mid-pipe connects. Furthermore, some internal baffling inside the muffler also helps keep the sound levels down. 

The part was a breeze to install and fits perfectly; all we had to do was remove two 6mm bolts that hold the muffler to the frame, and reinstall the stock rubber grommets in the FMF mounts. All told, it may have taken five minutes.

In addition to looking far cooler than the stock muffler with its blue-anodized finish and carbon end cap, the FMF system tucked in nicely and was no longer than the stock unit. The Factory 4.1 muffler comes equipped with a spark arrestor installed, and we took to the track with it still in place to see what the bike felt like. 

To be honest, we didn’t like the milder personality the 350 SX-F had with the spark arrestor installed. While the bike had more torque down low and was smoother and easier to ride, we felt that the stock system yielded far more overrev in each gear, which made the bike not only more thrilling to ride but easier to go fast on as well. We tried the spark arrestor in both map settings and with the traction control both on and off (we don’t love the feature, to be honest), and found that the map 2 setting was most appealing, as it felt close to the map 1 setting with the stock muffler intact. The verdict? If you ride in an area where a spark arrestor is required, the Factory 4.1 system is a great option, but don’t expect any power gains, so to speak; only compliance with the requirements of your riding area.

We returned to the pits to remove the spark arrestor and were at first a little puzzled as there were no bolts, either on the outside of the muffler canister or inside the core. That’s when we noticed the small circlip-style fastener inside of the end cap. Using a small flat-blade screwdriver, we coaxed the tail of the clip out of the end cap and pulled the rest of it out before using a pair of pliers to remove the spark arrestor insert. 

With the spark arrestor removed, the Factory 4.1 muffler gained a mean, authoritative bark. While some may dislike the louder exhaust note, we love it! The bike seems to rev more freely with the FMF slip-on installed and though the low-end, roll-on gains that the system yielded with the spark arrestor installed are no longer there, the way the bike revs out further in each gear is a more valuable plus! By maintaining the bike’s OEM header pipe, this set-up ensures great roll-on power and healthy mid-range, and much more pull up top thanks to the FMF muffler. 

To be fair, we tested the muffler at Cahuilla Creek which is at 3500 feet above sea level, and even bigger gains may be detectable down the hill. We plan to spin laps this week at Glen Helen Raceway both for practice and racing at this weekend’s AMSOIL/Dirt Bike Kidz Swapmoto Race Series event on Sunday, and are looking forward to seeing what the Factory 4.1 slip-on does for the 350 SX-F there!

As it stands, the FMF Factory 4.1 slip-on muffler yields the versatility of a spark arrestor, better looks, lighter weight, and increased top-end performance. Two thumbs up from the SML team!

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