FMF Racing needs no introduction when it comes to dirtbike performance.
FMF Racing founder Don Emler Sr. is one of the pioneers of aftermarket dirtbike exhaust systems, starting in the two-stroke era of the 1970s. Under his guidance, FMF has always been one of the premier pipe and muffler builders in the industry, and that never changed even after the sport evolved to hyper-performance four-stroke motocross and off-road machines. The company is relentless in its pursuit to pull more horsepower and torque out of every stock machine no matter how well-developed the stock engine/pipe combination. Slip an FMF Racing pipe on your stock machine, and it’s a safe bet that you’re going to go faster.
So when FMF Racing promised massive horsepower gains throughout the rev range with its Factory 4.1 RCT muffler, we wanted to know just how much constituted massive. After all, if you’re shelling out nearly $400 for a muffler, you want to get what you’re paying for.
To the uninitiated, the RCT in the Factory 4.1 RCT muffler’s nomenclature stands for Resonance Chamber Technology, and FMF trademark design that incorporates a muffler core shape that alters the exhaust pressure wave for maximum power output while minimizing exhaust noise as much as possible. The RCT also helps FMF to create mufflers that are shorter in overall length, which helps to tuck them in toward the front of the motorcycle. Mass centralization is a bonus, but the key benefit, according to FMF, is that the muffler will be less susceptible to crash damage FMF Racing Factory 4.1 RCT mufflers are constructed from only aerospace grade materials that meet the company’s strict specifications, and they are all manufactured at FMF’s Southern California factory.
While our test bike was fitted with a lot of trick-looking accessories, the engine was stock except for a minor ECU mod that is identical to Euro-spec Husqvarnas. We selected Mickey Cohen Motorsports of Placentia, California, to handle the dyno testing of the Factory 4.1 RCT muffler for us. In addition to running a complete motorcycle repair shop, owner Mickey Cohen is also the manager of Triumph Motorcycles’ west coast motorcycle fleet and a former AMA Pro Thunder Championship-winning road racing tuner, so he knows what to look for when it comes time to squeeze more power out of a four-stroke engine.
Right off the bat, our baseline test showed that the FE 501 is no slouch in stock form—even if it is Euro stock. Our test machine churned out a solid 39.3 rear wheel horsepower @ 8100 rpm with a peak torque figure of 28.4 lb.-ft. coming in at 6300 rpm. What’s impressive is that the Husky motor makes over 20 lb.-ft. from 4400 rpm all the way to 9100 rpm before only gently dipping below that as the engine reaches its 9700 rev limit. Those our stout numbers that only reaffirm our belief that a 500cc is just about the ideal displacement for a four-stroke Single.
Still, we had no doubt that the FMF Factory 4.1 RCT muffler would enhance the FE 501’s power output. We simply had no idea by how much.
Swapping the Factory 4.1 RCT muffler onto the stock Husqvarna header was easy, as FMF supplies a slip-on collar that matches the muffler’s larger i.d. with the header’s smaller o.d. The Factory 4.1 RCT is then installed using the grommets, spacers and bolts from the stock Husqvarna muffler. The whole process took only a few minutes once the stock muffler was cool enough to handle. Another benefit of the Factory 4.1 RCT is that it is substantially lighter than the stock muffler. FMF supplied us with its stainless slip-on model, but an even lighter titanium version is also available. We were glad to see that our test sample came with the carbon fiber end cap, which adds to the muffler’s already trick appearance. FMF also ships all of its Factory 4.1 RCT models with a USFS-approved spark arrestor for off-road use (in search of a max power gain we opted not to install it). FMF claims to use the highest-quality proprietary blend stainless steel in the Factory 4.1 RCT.
That’s all well and good, but what kind of power does it make? The answer is… a lot. Once fitted with the Factory 4.1 RCT, the Husqvarna’s exhaust burble sounded a whole lot healthier at idle, and it delivered a radical though not overly piercing roar as Cohen hammered the throttle to record the dyno run. The FE 501 responded to the Factory 4.1 RCT by pumping out 43.6 rear wheel horsepower at the same 8100 rpm as the stock muffler while peaking with 30.9 lb.-ft. @ 6100 rpm—200 rpm earlier than the stock muffler. That’s 4.2 additional horsepower and 2.5 lb.-ft. of additional torque at the peaks, but taking a look at the dyno graphs, you can also see that the Factory 4.1 RCT basically mimics the stock horsepower and torque curves. There’s simply more power everywhere!
Going through the gears, there’s simply no mistaking a stock Husky FE 501 when compared to an FMF Racing Factory 4.1 RCT-equipped version of the same machine, all other things being equal. The slip-on makes a huge difference in both grunt and steam throughout the rev range, delivering as much as 10.6 percent more power along the way. Even the usually skeptical Cohen agreed that the FMF Factory 4.1 RCT delivered an impressive power gain.
The FMF Factory 4.1 RCT muffler in stainless steel (as tested) for the Husqvarna FE 501 retails for $399.99 while the Ti version retails for $499.99. That may sound like a lot of money, but considering that this, like other FMF products, delivers on its promises, we’d call it a hell of a bargain.
FMF Racing Factory 4.1 RCT Muffler, $399.99 (stainless w/carbon fiber endcap)/$499.99 (titanium w/carbon fiber endcap)
FMF Racing
18033 South Santa Fe Ave.
Rancho Dominguez, CA 90221-5514
Phone: 310-631-4363
Website: www.fmfracing.com
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