If you want to choose an FMF exhaust, start with the engine type. On a 2-stroke you mainly set the character with the expansion chamber (Fatty, Factory Fatty or Gnarly) and finish it off with a silencer such as the PowerCore 2.1 or TurbineCore 2.1. On a 4-stroke you choose between a slip-on silencer (Factory 4.1 RCT, PowerCore 4, Q4) and a full system with a MegaBomb or PowerBomb header.
Which parts make up an FMF exhaust?
An exhaust roughly consists of three parts. What you replace determines how much changes.
- Expansion chamber (2-stroke): replaces the stock 2-stroke pipe and connects to the silencer. This part has the most influence on where the power is delivered.
- Header (4-stroke): replaces the front section of the stock exhaust and connects to the silencer.
- Silencer (slip-on): replaces the stock silencer and is fitted to the existing exhaust.
- Full exhaust system: replaces the complete stock exhaust, from header to silencer.
You will find all lines and versions by make and year in the FMF collection.
2-stroke: which FMF expansion chamber suits your riding style?
FMF makes several pipes for 2-strokes, each with its own character. In the range you will find:
- Fatty (Gold Series): according to FMF, the Fatty makes the power curve broader and smoother, with more bottom end, more mid-range and more over-rev. An all-rounder for motocross and enduro, from minis up to 250/300cc.
- Factory Fatty: inspired by the "works" pipes that factory teams used to run. Available as a pipe and as a separate header section for specific models.
- Gnarly (Gold Series): focused on horsepower and torque low in the rev range, to pull you through tough terrain. FMF cites its 18-gauge steel construction and targets heavy off-road work on machines from 200 to 500cc.
In short: if you mainly ride motocross or want gains across the whole rev range, look at the Fatty. If you ride technical enduro or hard enduro and want pulling power from down low, the Gnarly makes more sense.
Which silencer should you choose for your 2-stroke?
After the pipe, you choose the silencer. According to the manufacturer, many FMF silencers fit the stock pipe or an FMF one, but the effect is greatest when pipe and silencer are matched.
- PowerCore 2 and PowerCore 2 Shorty: the Shorty has a shorter inner core and an aluminium silencer body, and according to FMF it is tuned for bottom-end and mid-range power. FMF calls it the most sought-after silencer for supercross.
- PowerCore 2.1: the newer generation 2-stroke silencer, for recent models from brands including Beta, GASGAS, Husqvarna and Sherco.
- TurbineCore 2 and 2.1: silencers with the Turbine Dampening System and a spark arrestor, intended for riders who also ride in areas where a spark arrestor is required.
4-stroke: slip-on or full system?
On a 4-stroke this is the most important choice. A slip-on is easier to fit and builds on the stock header. A full system changes the entire exhaust route and therefore more of the engine characteristics.
| Line | Type | Feature according to the catalogue |
|---|---|---|
| Factory 4.1 RCT | Slip-on or full system | Resonance Chamber Technology, removable spark arrestor, on some versions a QuietCore insert (94 dB, manufacturer's figure) |
| PowerCore 4 / PowerCore 4 HEX | Slip-on | Power gains across the whole curve; on some versions an optional Quiet Core insert |
| Q4 / Q4 HEX | Slip-on | Chambers, baffles and multi-stage packing for a lower noise level |
| MegaBomb | Header | Patented chamber at the start of the header; stainless steel or titanium |
| PowerBomb | Header | Stainless steel; for certain Honda models in an SX and MX version |
A note on the headers: FMF states that most PowerBomb headers are not designed for use with the stock silencer, because the performance gain is then lost. So ideally combine an FMF header with an FMF silencer. The Q4 is the choice if a lower noise level matters a lot, for example for enduro.
Which materials does FMF use?
These materials appear in the catalogue:
- Pipes and headers: steel (2-stroke pipes, often nickel-plated), stainless steel and titanium.
- Silencer bodies: mostly aluminium, as well as titanium and in some cases stainless steel.
- End caps: stainless steel, carbon or titanium, depending on line and version.
Titanium is mainly chosen for extra weight savings; stainless steel stands for strength at a low weight.
Noise, FIM rules and maintenance
For a large share of FMF exhausts the product information notes that they do not comply with the 2025 FIM noise regulations. According to FMF, a few Q4 HEX silencers comply with the FIM World Enduro regulations. If you race, check what is stated for each product and what your rulebook requires. Also always check whether the exhaust is road legal for your country and bike; race versions are for closed-circuit use only.
For maintenance, the FMF exhaust parts include silencer packing (including the Premier version), end caps, spark arrestor screen inserts, exhaust springs with O-rings and wash plugs. If your silencer sounds louder than before, worn packing is often the cause.
How to choose in 3 steps
- Determine your engine type and goal: 2-stroke or 4-stroke, and mainly motocross, enduro or recreational riding.
- Choose how much you replace: on a 2-stroke the pipe first (Fatty for broad power, Gnarly for bottom-end torque), then the silencer. On a 4-stroke a slip-on or a full system with a MegaBomb or PowerBomb header.
- Check fit and rules: search by make, model and year, and check the noise notes and the type (with or without spark arrestor) on the product page.
Frequently asked questions
What is the difference between an FMF slip-on and a full exhaust system?
A slip-on silencer replaces only the stock silencer and is fitted to the existing header. A full exhaust system replaces the entire exhaust, from header to silencer. At FMF, the Factory 4.1 RCT is available in both versions.
On a 2-stroke, should I replace the pipe or the silencer first?
The expansion chamber has the most influence on the character of a 2-stroke, because its shape determines where the power is delivered. If you want bottom-end torque, a Gnarly suits; for gains across the whole powerband, a Fatty. The silencer, such as a PowerCore 2 Shorty or TurbineCore 2.1, completes the package.
When should I replace the packing in my FMF silencer?
Replace the packing as soon as the silencer starts to sound noticeably louder or harsher, because worn packing dampens less and can affect performance. How quickly that happens depends on use and engine type. FMF supplies separate silencer packing, including the Premier version, and on some silencers the body can be taken apart to replace the packing.
Is an FMF exhaust suitable for races with noise testing?
That varies by product. For a large share of FMF exhausts the product information states that they do not comply with the 2025 FIM noise regulations. According to the manufacturer, a few Q4 HEX silencers comply with the FIM World Enduro noise regulations. So always check the product page and the rules of your event.
Is a titanium exhaust better than stainless steel?
Titanium is mainly chosen to save weight; stainless steel offers strength at a low weight. For the MegaBomb headers, FMF states that they are available in stainless steel and in titanium. On the Factory 4.1 RCT you can also choose between end caps in carbon and titanium, among others, depending on the version.
Can I ride on public roads with an FMF exhaust?
Always check whether the exhaust is road legal for your country and bike. Race versions are intended for use on a closed circuit or off-road terrain only. In the catalogue, FMF exhausts are largely designated for off-road use.
Browse all exhausts, silencers, headers and parts in the FMF collection; everything is shipped from the Netherlands. Not sure which version fits your bike? Send us a message with make, model and year, and we'll look it up for you.