Muffler for 4 stroke engine.

I don't know about Lou, but I don't want mine to sound like a motocross bike. Even a stock one. I want mine to sound like a new Toyota Prius, sitting at a light.
Just an idea for making your own muffler, put a thin walled pipe with lots of holes drilled in it inside of a larger pipe and fill the space between with fiberglass or steel wool to soak up the sound. If you have access to welding equipment, or a high school shop class that needs a project, you are in business.

To make it even quieter, place some restrictions in the inside pipe to force the exhaust to go through the wool between pipes.

Simple really if you are ambitious enough.

Also, you could check fleet/farm type stores for small tractor mufflers.
Believe it or not the muffler on a dirt bike cuts the sound down a lot, you wouldn't think so by how loud they are but its true. I definitely don't want my MB to sound like one either and it wouldn't with the plan I had mentioned:
1. Keep the stock exhaust pipe and muffler in place but add on...
2. A larger pipe and motorcycle muffler as well (2 mufflers total).
3. This shouldn't add much more restriction. It couldn't possibly make your exhaust louder and by all means should make it quieter. It will not come out sounding like a CR500, that I guarantee.
The typical dirt bike muffler is basically a tube within a bigger tube. The inner tube is a through pipe (no real restriction) but it has hundreds of holes or is made out of mesh. The space between the two tubes is filled with fiberglass packing. Sound waves reverberate between the tubes, running into each other, cancelling each other out, being absorbed by the tubes and the packing. They do absorb sound but the with the higher priority being to not restrict flow.
While we are on the subject, does anybody have any experience with this product? "Benz silent rider" or stealth muffler. This is where I came up with the idea above. I'd love to cut one in half and find out just what is going on in there. Or if we could find out the patent number I could do a patent search and get the drawings for it. Kind of pricey at $150-200.
Anyway, it will be a few months before I report back on actual results.
KeepOn, thanks for the link. That's just the sort of thing I'm looking for.
Quote from Kolpin:
"The ATV Stealth Exhaust System fits on any ATV with a four-stroke engine to quiet ATV noise by 50% or more. It can be easily installed according to user preference and rear-of-machine configuration.Engineered with an innovative high-flow, low-restriction spark-arresting muffler, the Stealth Exhaust System quiets ATV noise by 50% without sacrificing engine performance. The patented design is used on military ATVs by both U.S. and British Special Forces, is also a USDA Forest Service Approved Spark Arrester.

The ATV Stealth Exhaust System includes an efficient, aluminized steel muffler measuring 22 inches in length and 4½ inches in diameter. Fully welded, the muffler is proven to work well on all sizes of ATVs from 250s to 700s. The system comes complete with a universal kit including mounting hardware and detailed instructions for quick, easy installation; a universal adaptor elbow for high mount installations; a zinc-plated two piece wrap-around clamp hanger assembly; and galvanized flex tubing that turns the exhaust and noise closer to the ground.

The ATV Stealth Exhaust System reduces noise levels for decreased disturbance of other public land user groups, and provides added protection to the environment through the use of a second spark arrester. Its revolutionary design results in less disruption of wildlife for the benefit of outdoor enthusiasts, as well as livestock for farmers and ranchers that use ATVs. In addition to significant noise reduction, the ATV Stealth Exhaust System actually delivers an increase in mid-range power and overall performance on some models, according to extensive dynamometer tests.

How Does it Work?

THE MUFFLER. We start with an extremely efficient, super quiet, high flow, USDA Approved, spark arresting muffler that is manufactured from high quality, aluminized steel to our specifications, exclusively for our patented ATV Sealth Exhaust System. The muffler is 22 inches long, including the outlet, and 4½ inches in diameter. It is not just a straight through silencer and it has no packing material that requires maintenance. It is a fully welded, USDA Approved, spark arresting muffler designed with a series of tubes and chambers. It has been tested and proven to work well on all sizes of ATVs from 250s to 700s."
It's got me thinking.
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Car muffler would be the ultimate for quiet and since they are often designed for engines
many times our engines size they would offer practically no restriction to exhaust flow....tricky part tho is making a pipe to join the exhaust outlet of our engines to the inlet of the car muffler....oh...of course the car muffler is a little bigger than we are used to on our bikes.
I always wondered why they couldn't quiet gas operated weed eaters, lawnmowers, leaf blowers. Why when you operated one the whole neighborhood should have to hear it. I guess there must be more to it than what I think. But since we're discussing it on a bicycle, if someone sold a good muffler you can add on to a 4 stroke motor that would muffle the sound, you would think they can make some good money selling that

1) Muffler performance is a function of construction, back pressure, and size - and you can usually get two out of three. In the case of small power tools, the designer usually figures something like "as small as possible without excessive noise(?) or back pressure" - and what determines excessive noise is usually (half-baked guess) a legal issue. (This varies, most likely, depending on where the equipment is sold.)

2) I myself have wondered about noise issues (as in I do not want undesired attention when using an engine) - and I wish to minimize back pressure (in hopes of less fuel consumption). I suspect that means a *large* muffler - like this:


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Right now I'm leaning towards taking a small car muffler, cutting it open, stuffing the central chamber with glass or steel wool, and welding it back together. As Andy points out, it is designed for an engine many times larger, the additional back-pressure created by the dampening material shouldn't effect performance much.