FMF Megabomb/Q4 Hex Muffler on KTM 500/350

What it is

• An exhaust system for the KTM 500/350 EXC that increases power and keeps the sound down.

FMF Racing”s Megabomb header and new Q4 Hex aluminum muffler show some of the science behind sound reduction and power improvements. The two-piece stainless steel header has FMF’s exclusive Megabomb chamber where a series of holes in the headpipe allows exhaust gas and sonic waves to escape  and re-enter the main line of the standard pipe. The main advantage has been a noticeable reduction in sound output, up to 1.5 dB in some cases. FMF has also seen as much as a 3 hp gain on some bikes.

The Q4 Hex is a now made from a larger aluminum canister than the previous Q4 and has machined mounts riveted into the rebuildable muffler. It uses a system of chambers and baffles as well as packing to reduce sound while still claiming to increase power over stock.  Depending on the application there is often a reduction in weight as well. The USFS-approved spark arrestor is an internal screen-type design.

How it works

• Increases torque and power throughout the RPM range. On the bottom for the 500 and from mid to top for the 350.
• Drops a pound of weight off the bike.
• Only a dB more in the stationary test and not noticeable to the ear in a pass-by.

The first thing we'll comment on is the reduction in weight with the FMF system. On both KTM 500 and 350 EXC the system dropped a pound over stock. Thought the header was slightly heavier, the muffler was 1.1-pounds lighter on each bike.

Fitting the exhaust to the bike is easy, but on the KTM requires the removal of the rear shock to get the stock header off the bike. The FMF system mounts in the stock locations and has all the required hardware if it does not use the standard parts. And in the case of the extra slip-fit junction, there is a spring to keep tension on the pipe.  The two-piece header makes it easier and removal of the FMF system can be done without removing the shock. The slip-fit junctions are tight and applying a little penetrant oil (especially on removal after some time) makes the task simple.

The Megabomb takes up more room in the right side of the cylinder and riders who regularly burn their pants on the header will now have a larger target to hit. There are some companies that make hear shields for these if you need that protection. The muffler basically tucks in just like stock and has a raised bumper to keep the side panel from touching and getting burnt. If you have a large foot and your ankle presses on the frame guard that comes standard on the KTM you may burn this piece easier than on the stock muffler.

As for the power, this is one of the best improvements attributed to an exhaust system I have felt on a fuel injected bike. It really changes the amount of power the bike puts out, right off of idle and them fills in any holes or gaps the bike has stock. This when tested on a bike that was completely 100% stock and again on a bike with an ECU that had been flashed to the Euro EXC map. As the power builds the FMF system lets the bike rev a little freer and pull longer up top with a nicer feeling taper before the bike hits the rev limiter. On the 500 the bottom power becomes smoother but there is also so much more response when you hit the throttle. It isn’t the jerky feeling of the stock bike but one that oozes with torque and makes it so much easier running a gear high any time. It makes the throttle feel so much more connected to the rear wheel it is very impressive. For the 350 the bottom end sees a very similar improvement in the smoothness and boosted feeling in torque, but that isn’t where this pipe really comes alive. It is in the mid-range and then on up where it wakes up the bike from a docile or vanilla powerband to one that is responsive and sharp. Both bikes feel like they are making more power at a lower RPM all the way through the spread and as if the torque is a much more linear curve. All good and much more than I expected.  And everyone that rode the FMF equipped bike, coming off a stocker, was quick to ask if there was anything else done to the bike besides the pipe, it was that good.

Some of these gains and feelings are typical to adding a Megabomb header. In the past on different bikes the Megabomb (and to a lesser extent the Powerbomb) gives the bike a crisper feel in the throttle and also adds a feeling of more torque at lower RPM. The Q4 muffler is very effective at letting a bike work in the upper RPM without running through the power too quick like a full-open system can.

The next issue is the sound. Increases in performance like this are easier to get if you ignore the sound issue. But the FMF pipes test at 1 dB higher at the stationary sound test, from 92 dB stock to 93 dB with our certified sound test meter. Your ear, especially when on the bike or right next to it will tell you it is a little bit louder than that. And in a real-world drive-by they were nearly identical at both mid-RPM and wide-open at about 100-feet. So they do an excellent job of being responsible and helping to keep our riding areas from getting noticed because of noise.

Typically exhaust systems on trail or dual purpose bikes accomplish two things more than anything. They are way louder and serve mostly as eye candy. These FMF systems work the eye candy part but at the same time stay sound responsible.  And most of all they really give a stronger and better spread of power.  I've put well over 500-hours on older Q4 mufflers without any degrade in the internal packing and have only had minor issues with the canister screws coming loose after a long time and never checking them, so durability has been excellent. We did have either some rocks kick up off the rear wheel or possibly some snuck in at the muffler in a tip-over and we got some small dents in the muffler canister. Any muffler would have scratched but the FMF was actually dented a bit. The finish on the header tarnished a little bit right out of the exhaust port but the coating did not ever flake or degrade.

The systems are not cheap, at close to $800.00 all said. But there isn't really an easier way to improve the power delivery on these KTM’s in a bolt-on fashion.

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