Motocross exhausts: full systems, db killers and noise limits

Changing the exhaust is the most common modification on a motocross bike and the most misunderstood. You buy sound and weight, you think you are buying power, and you forget the pipe is also the part that decides whether the local track is still open next year.

01What you actually gain

• Power: 1-3 hp, mostly at the top
• Weight: 500 g to 1.5 kg less
• Sound: more aggressive, and that is the catch

The claimed gain deserves perspective. One or two horsepower on a 250 four-stroke is about 5%, and it arrives high in the range where an amateur rider spends the least time. The weight saving is felt everywhere: a kilo removed high and to the rear changes how the bike handles more surely than two horsepower at the top end.

On a two-stroke the logic is different. The pipe body is part of the tuned circuit: its shape decides the usable range. A race pipe moves power up and hollows out the bottom, which makes the bike faster on a flowing track and more tiring in tight enduro.

02Reference brands

Silencer only or full system: the silencer brings most of the weight saving and part of the sound for half the price. The full system, header included, brings the rest, but it usually needs a mapping or jetting change to be a genuine gain.

03Noise limits

• FFM/FIM: 112 dB maximum at 2 metres at the measured engine speed
• Some tracks: 96-100 dB, considerably stricter
• db killer: a removable insert cutting 3-6 dB. Mandatory at many venues.

Testing is done with the engine running, at a set microphone distance and angle, at a speed derived from capacity. A pipe that passes cold can fail hot: the measurement is more sensitive than people assume.

The issue goes well beyond an individual penalty. Track closures almost always follow noise complaints from neighbours, never dust or traffic. An over-loud pipe on a tolerated site costs everyone the site.

04Repacking the silencer

This is the part almost nobody keeps up with. The packing settles and burns away: after twenty to forty hours on a two-stroke, ten to fifteen in hard use, it filters nothing. The pipe gets louder and people think it is opening up when in fact it is emptying out.

Repacking costs fifteen to thirty euros and takes twenty minutes: drill out the rivets or undo the screws, pull the old packing, wrap the new one tightly, reassemble. A machine with fresh packing gains decibels in the right direction and sometimes a little efficiency.

05What a pipe does not do

It does not compensate for a clogged air filter, does not fix poor jetting, and changes nothing about mechanical wear. For a given budget, tyres and suspension almost always beat an exhaust.

06Effect on servicing

An aftermarket pipe does not change service intervals. But an engine that breathes better can run slightly hotter in extreme conditions, so keep an eye on the coolant.

And like any part with its own service life, silencer packing is worth logging with the engine hours next to it. It avoids the unanswerable question two years later: has this pipe been repacked once, or three times?