Big bike exhaust silencer - why this ?

[JackisBack]

I'm not expert and I have never owned a large motor bike, but the thing I'm curious about is this

I all ways thought that the larger the exhaust out let pipe size the better fuel economy and faster performance , so why am I seeing numerous larger motor bikes with tiny small exhaust out let pipes , so why not have an exhaust silencer but with a big out let pipe ?

[OmegaRacer]

I'm not 100% sure, but that looks more like a small to medium sized bike, maybe in the 250-300 range. They don't need a big exhaust outlet and have big muffler more for esthetic reasons. Many bikes now have those and big fairings to make them appear bigger than they are in reality.

[dave_boo]

In addition there is a bit of Bernoulli's principle in play which helps with scavenging exhaust.

[lust]

Ya looks like 250cc with OEM exhaust. The reason why its large is for noise canceling and to appear aggressive.

[Hili]

Noise regulations, bigger exhausts can absorb more noise and a tiny outlet wont generate much noise

[garryjohns]

More to comply with stringent low db levels required now than much else.

[ll2]

It can be for creating back pressure or for noise regulations.

[Garry]

The bigger the end can, the easier it muffles the engine noise and the less chance of it robbing any significant horsepower from the engine,

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[garryjohns]

It can be for creating back pressure or for noise regulations.

Yes but in conforming to strict db requirements there is mostly too much back pressure created.

I believe that for the average off the assembly line bike that conforming to db regs is paramount to the manufacturers,1st and foremost.

Maximum mid-high RPM power [where we need it most] is sacrificed by this, as the lowest possible backpressure is desirable to achieve full power potential.

Its a trade off.

That's why we fit aftermarket mufflers and often full exhaust ,to get full back the power that is available.

[AllanB]

I read somewhere that a high gas velocity is important, the smaller the pipe bore the higher the velocity for a given flow-rate. Will have to read up on it as my own silencer is far from ideal, but it sounds complicated.

[ll2]

It can be for creating back pressure or for noise regulations.

Yes but in conforming to strict db requirements there is mostly too much back pressure created.

I believe that for the average off the assembly line bike that conforming to db regs is paramount to the manufacturers,1st and foremost.

Maximum mid-high RPM power [where we need it most] is sacrificed by this, as the lowest possible backpressure is desirable to achieve full power potential.

Its a trade off.

That's why we fit aftermarket mufflers and often full exhaust ,to get full back the power that is available.

Yep. Exactly. But for low cc bikes with minimal amount of exhaust gases, you do not need a very freer flowing pipe.

[dave_boo]

I read somewhere that a high gas velocity is important, the smaller the pipe bore the higher the velocity for a given flow-rate. Will have to read up on it as my own silencer is far from ideal, but it sounds complicated.

There's a whole bunch of figuring and real life experiments that need to go on to get a good exhaust.

Tuning for one region will make another suffer. Remember the power valves? They were designed to help both lower RPM and higher RPM performance. It was even more important for 2t due to the lack of valves; proper scavenging of the exhausts throughout the RPM range prevented back flow from choking off power.

[seedy]

And proper exhaust design in modern engines can achieve more than 100% volumetric efficiency in normally aspirated engines.

Ain't technology wonderful !

[papa al]

I've always admired volumetric efficiency and perpetual motion devices.

[cornishcarlos]

Just make em loud...

[ktm jeff]

Im not sure if a naturaly asspirated engine can achieve more than 100% volumetric efficiency , even with the use of negative pressure pulses / sound waves that do help with charge re-fill and cylinder scavanging. You would need a cylinder pressure , at BDC , higher than ATM .This is why 2-strokes , especialy diesel , respond so well to pressure charging , either it the form of a turbo or a supercharger it the case of 2-strokes.

[papa al]

I read somewhere that a high gas velocity is important, the smaller the pipe bore the higher the velocity for a given flow-rate. Will have to read up on it as my own silencer is far from ideal, but it sounds complicated.

There's a whole bunch of figuring and real life experiments that need to go on to get a good exhaust.

Tuning for one region will make another suffer. Remember the power valves? They were designed to help both lower RPM and higher RPM performance. It was even more important for 2t due to the lack of valves; proper scavenging of the exhausts throughout the RPM range prevented back flow from choking off power.

And back-flow at the proper timing in the cycle prevented unburnt mixture from passing through. Right?

[Bung]

The large size of exhausts is because they house catalytic converters as well as the muffler.

[dave_boo]

And back-flow at the proper timing in the cycle prevented unburnt mixture from passing through. Right?

Yes.

As the wave propagates back to the origin it collides with the closed valve/port. Bouncing off it then creates a low pressure pocket in its wake that is utilised by the opening valve to pull the gasses out of the chamber.

[seedy]

Im not sure if a naturaly asspirated engine can achieve more than 100% volumetric efficiency

https://www.physicsforums.com/threads/explain-to-me-how-it-is-possible-to-go-above-100-volumetric-efficiency-on-an-ice.152096/

Reply #2

[dave_boo]

Im not sure if a naturaly asspirated engine can achieve more than 100% volumetric efficiency

https://www.physicsforums.com/threads/explain-to-me-how-it-is-possible-to-go-above-100-volumetric-efficiency-on-an-ice.152096/

Reply #2

Technically it can be done. But as you start aiming for greater than 100% efficiency, you start accelerating the charge so much it can actually break the sonic barrier. Not good when there are spl limits.

[ktm jeff]

Thanks dave_boo and seedy , some interesting technical information being posted.