Pass the 'db' sound level test
Dirt Bike events and Motocross events have been dogged with strict noise regulations for a long time. And, just like us Scooter Sprinters, DB and MTX riders have endeavored, and shared information on how to quieten their machines so they can compete at events.
With this in mind, I have gathered the following information from a number of different web sites and forums which suggest ways to quieten 2 stroke engines for the db, or sound meter test.
If you have any suggestions which I can add to this page, please forward them to me by filling in the contact form in the left hand column.
The "Sound Level Meter" reads the overall noise produced by your Scooter
and not as you may think, just the noise coming from the exhaust. Subsequently, there is more than just the silencer to consider when attempting to quieten your Scooter for the db test.
For example;
If you are running an open carburetor the air intake noise will be greater than if you run an air filter.
Vibrations will also add to the overall noise, check around to eliminate unnecessary vibrations.
Wrapping the expansion chamber with automotive manifold wrap smothers the high pitched ting (some Dirt Bike riders wrap the expansion chamber with an old inertube). To back up this statement, and to observe the effect wrapping the expansion would have, one forum suggested that simply resting your leg on the expansion deadened the ting.
There are a number of noise issues and in actual fact gains to be achieved when considering the silencer itself.
For example:
If the silencer packing is partially or completely blown out of the silencer, it will gradually become louder and the packing will need replacing.
When packing is missing, back pressure increases which hurts throttle response and overall horsepower.
The silencer packing may become saturated with oil and again will need to be replaced. This same oil and carbon saturation can plug the holes in the silencer core which would also lead to an increase in volume.
Re-packing your silencer can actually give you more horse power.
Packing density is a factor. Hard packing makes the bike sound peaky, whereas a medium packing makes the exhaust tone quieter and actually provides an improved mid range torque.
Lengthening the silencer by 4" will quieten the exhaust noise by 5db, a longer core will absorb low tones, and providing the outlet diameter is not reduced, performance will not be affected.
Under normal riding conditions it is recommend that you service your packing every 30 engine hours for optimal power and noise performance.
Running the exhaust as close to the track as possible and pointing the silencer exit down to the track will have a quietening effect as the exhaust resonance is decapitated by the track surface. Attaching a makeshift elbow to the silencer with a jubilee clip should suffice, and providing the outlet diameter is not reduced, performance will not be affected.
Running the engine to thoroughly warm the packing in the silencer before taking the Scooter for the db test will reduce the exhaust noise level.
Please remember, I have found all of the above statements from different websites and forums with the intention of helping riders struggling to meet the required 98db noise levels. I have not personally tried any of them.
Fortunately, my home made exhaust and 'Off the Shelf' silencer quietens my 38hp sufficiently to pass the db noise test.
Click on the images to take a closer look
Additionally, there is an attachment called the DB Snorkel used by Dirt Bike's and MTX Bikes, which is made of a tough high-temperature elastomer that absorb exhaust noise and actually reduces the exhaust noise by 8db. Here's a link the site for more information DB SNORKEL.