Auto/Motor Exhaust baffle modification

Looking to learn the scientific way to quieten a motorcycle exhaust baffle
Hi everyone.

I dont have much knowledge in physics but I am trying to take a scientific approach to silencing my motorcycle exhaust.

The muffler is a basically a perforated straight-through tube with fiberglass packing (too loud).

The baffle (only knocks one decibel off) came as just an insert that makes the gas flow around it, never through it, and has two small bulges which i assume are to speed up/slow down the flow to disturb the sound. Not very effective.

I have drilled some holes in preparation for filling the baffle with fiberglass. The plan was to drill along the entire length but for performance reasons id now like to improve the flow a bit too if possible.

Is it possible to modify this baffle to increase its effectiveness without using fiberglass (just holes to manipulate the gas flow)

Im keen the learn the physics of how all this works as i also have an issue with the pulses not being timed correctly to enhance efficiency but thats a job for another day!

Any pointers/advice is much appreciated


jim hardy

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You;ll need to study up on the basics of how mufflers work.

You can just deaden the sound by restricting flow at expense of power loss from backpressure,
You can use cancellation by reflecting sound waves together, that's 'destructive interference'.
probably your muffler was designed to reduce only high frequencies giving a throaty roar..

Here are two articles to get you started

In my day I had wide open Dunstall megaphones
they used 'constructive interference' to increase sound level like a trumpet
The town policeman told me :
"Jim - i'd just pulled a guy over for a teeny bit noisy muffler. Then you went by and i was doggone embarrassed",
as he wrote the ticket .

old jim


Science Advisor
Back in the day, I once bought a used Honda 350. The previous owner had installed a pair of cheap mufflers that did not do much muffling. So I welded 9/16" ID washers over the outlets, which got the noise level down to about that of the original factory mufflers.
Very interesting reads. The complexity of sound deadening 'baffles' me (hehe). I now have some more ideas I'd like to start prototyping. This has gone from a curious project to something that will consume me!


Gold Member
I don't know if this helps, but rather than drilling, you might consider rolled perforated metal, or something like this. I'd probably choose something like ceramic fiber insulation to fill the void if needed. It may take some fine stainless screen to prevent erosion through the perforated surface.

Most motorcycle engines have the exhaust designed for sound reduction, but more important consideration is given to torque, throttle response, and operating behavior of the engine, so you can make it quieter, but it's operation may change.
I've built several mufflers back when I was still riding/tinkering with bikes, although "quiet" was never the goal!

I started by drilling tube, very tedious, then discovered expanded mesh, roll into tube shape, weld ends together, but the perf tube above would be much better/easier.

I just used stainless steel wool as the packing material. The first ones were too quiet, so I started taking off length and re welding till I got it so it wasn't so loud that the police would instantly pull me over, but still sounded awesome, and more importantly flowed well (no welding washers over the exhaust!!!) and no need to really look at the rev counter for shifting.

jim hardy

Science Advisor
Gold Member
2018 Award
Dearly Missed
wide open Dunstalls, ca 1965

that's a '56 Indian.
just today noticed a '57 at a shop in New Cumberland Pennsylvania..
You have straight-through muffler which is loudest design. Cut open some OEM mufflers and you'll see that end of central tube is actually closed and exhaust goes through multiple chambers along with reversal of flow in order to reach outlet. This lengthens exhaust flow-path by 3x along with lots of expansion. Result is quieter exhaust with minimal restriction. Modern OEM mufflers are miracles of design requiring no packing material (which needs to be replenished regularly in old days).
Here's cut-away of factory muffler. This one has 4 different chambers that gases go through to reach outlet.


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