April 21, 2011
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WHAT IS IT? It’s a redesigned version of FMF’s go-to motocross exhaust system.


CONTACT? or (310) 631-4363.

Here’s a list of things that stand out with the FMF Factory 4.1 exhaust system.

(1) RCT. The Factory 4.1 has an empty chamber built into the front portion of the muffler section of the pipe. This chamber mimics the one found on the production Honda CRF450, and FMF has dubbed it “Resonance Chamber Technology” (RCT). Virtually every exhaust system sold in 2010 comes with some type of resonance chamber, but FMF has led the way in testing and tweaking the dimensions, shapes and positions of these unique components. Although a resonance chamber does have some effect on the way the power is delivered, its most significant contribution is that it reduces the sound signature by one decibel.

(2) Can and core.
FMF has changed their canister shape. First, the internal volume has been increased to allow for more muffler packing. Second, the muffler insert is installed and removed with a circlip (instead of a bolt). Third, FMF refined the perf-core specifications for each application. Across the board, they have lowered sound levels while trying to keep the horsepower up. Fourth, the muffler has a built-in spark arrestor that’s not designed to be removed. Finally, the muffler mounting bracket has been beefed up (a claimed 70-percent stronger).

(3) Performance. The stock YZ250F is blessed with awesome bottom and passable midrange, but lacks top-end power. The FMF 4.1 (with MegaBomb headpipe) shifts the power to a little bit more of a traditional location, sacrificing some low-to-mid power to gain mid-and-up thrust. Every MXA test rider liked the power placement with the FMF system. The FMF-equipped bike revved slower, which allowed the YZ250F to spend more time building power, and translated into a very broad-feeling powerband that required less shifting on the track.

(4) Weight. Combined, the headpipe and muffler weigh in at 5.7 pounds. This is a significant weight savings over the stock exhaust system. The resonance chamber effectively counts towards the total “tuned length” of the exhaust system, which helps make for a more compact system. FMF markets their ability to build a shorter pipe as “Factory Forward Engineering.”

(5) Wrenching. The FMF pipe is easy to install and simple to work on. The muffler can be taken apart by removing four screws on the front end, but the new circlip insert is more difficult to deal with.

(6) Options.
The FMF 4.1 is available in natural aluminum or a blue anodized finish. The muffler is also compatible with the stock header so that it can be used as a slip-on exhaust for the budget conscious. FMF’s Quiet Core 94 dB insert is included in the package. FMF is working on some carbon fiber end caps that should look pretty sharp.
We found two drawbacks with the new design: (1) We liked the circlip idea in principle, but it was a more time consuming to deal, and (2) the spark arrestor. FMF claims there’s no loss in power with the spark arrestor, but for closed-course racing it is unnecessary.

FMF should get a pat on the back for keeping their R&D department running overtime on the Factory 4.1. For pure motocross, we don’t like every single feature, but each one serves its purpose.


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