Why does a car or cycle with no muffler make noise?

Late night thought hearing a Harley go by. What is the mechanical explanation?

  1. The natural result of gas/air explosions in the engine.
  2. An attempt to increase HP through the reduction of exhaust back pressure.
  3. A physical action by resulting from the “loose nut on top of the bike” who doesn’t give a hoot that you FINALLY got your babies to sleep.

I wonder if they call it a muffler because it muffles the sound ?


In early years it was called a silencer. I think the Brits still call it that.

As Beancounter posted… the engine runs on explosions…explosions are loud and that escapes out the exhaust. Mufflers suppress that sound.

Harley people argue that they want to be heard for safety. I think they want to be heard so everyone around them knows a Harley rider is nearby.


Is it the sound of the cylinders that do not have the exhaust valve open firing?

No, it is the finish of the bang exiting the open exhaust valve.


That seems like a waste of power.

There isn’t much power in noise… Sort of like firing a bullet from a gun. A pistol firing bullet makes more noise than the very same bullet fired from a rifle. Plus the rifle bullet comes out a bit faster. But not a lot faster. The trade-off for this small increase in power is the rifle is heavier and less maneuverable.

A longer exhaust stroke would extract a little more of the power. That is known as an Atkinson cycle engine. Mechanically it is very hard to have a longer exhaust stroke than intake stroke. Not worth the extra mechanical bits, mass and drag to accomplish it. You can approximate it with variable valve timing but that captures even less. The Prius and some other hybrids use that principle to squeeze a little more out.

Watch a WWII movie, night take offs you see the flames from the exhaust on the aircraft.

1 Like

It’s the best kept industry secret. The muffler isn’t needed. You remove it and the noise goes down. The question does not appear well thought out…

1 Like

I hope that is meant in jest.

When the fuel is burned in the engine a quite high pressure is developed to push the engine around its cycle. At the end of the effective power stroke the exhaust valve or port is opened and there is still a quite high pressure inside the cylinder. The pop you hear when there is no muffler is the pressure burst as the remainder pressure is released out the exhaust pipe. another related issue is the speed of sound is slower in lower temperature gasses than at higher temperature gasses. As the exhaust pulse travels down the exhaust pipe the wave front shocks up as the faster hot gasses push up against the slower wave front. That is the crackle that the Harley boys love from their straight pipe Harley exhaust pipes.


And I’ve heard that Harley has a patent on its muffler sounds! If you can wrap your head around that.

One of my neighbors has a Harley he rides often. He is VERY considerate about driving it slowly and very quietly coming and going through the neighborhood. He waits to let it rip and roar on the highway. Whenever his buddies with motorcycles of any type drop by to visit, they are equally considerate.

On rare occasions we get to hear it being revved off and on for a few hours on a weekend afternoon when he needs to work on it some.

On the other hand, another neighbor who thankfully moved took particular delight in speeding through the subdivision at high speed as loudly as possible, especially at two and three a.m. in the deep night.

1 Like

That is hard to wrap my head around, did not sound the same with hooker headers on my triumph cycle, but it was subdued unless I really opened the throttle up.

When I took my Mo Ped out in the country, I’d remove the insert in the muffler just to make it sound neat, like a real motor bike. Then I’d put it back in again before returning to town. I don’t know if it was that, the loud music, the gun shots, rattling cans, or what but I can’t hear worth a dang yet.


That patent attempt was denied, actually. The Potato-Potato sound can be copied by anyone. Why, they’d WANT to is beyond me.

WOW! I am glad to hear that!

I used to work for Kodak in Rochester and they sued a small photo print making company for having the roofs of their kiosks painted the same yellow! Big yellow won, as I recall so the small company changed the color just enough to avoid the Trademark? infringement. Not totally sure this is correct but same sort of stuff.

Addendum: One of the kiosks was right outside some 15 yards from the front door of Kodak’s employee’s center.

Years ago we looked at the huge Kodak copier in yellow. We didn’t buy that one but years later I saw what appeared to be the same unit painted in Japanese gray. Don’t remember whose name was on it-Ricoh, Kyocera , or something. At any rate where the paint was scratched you could see the yellow underneath. I knew it was Kodak but no one else did. Good machine too bad.