Exhaust Wraps versus Performance

[RideFree]

I started asking for information on this in the Twin Cam thread as it relates to cam overlap and scavenging but thought I would bring it out on its own in the General Forum. What I am thinking and looking for evidence of, such as dyno results, is proof that exhaust wraps do not increase performance but rather decreases it by 1 to 2%. Or evidence to the contrary.

Empirical evidence suggests wraps are nothing more than leg coolers. I have not seen a wholesale use in racing circles and if it increased performance at all they would be the ones to all be using it. It's very cheap and easy to install and looks cool so why not if it increases performance, as we all are always looking to get a little more out of our bikes within our own monetary capabilities. However, I do not see that.

More to physical evidence is the volumetric decrease that occurs as a gas cools. I have heard the analogy for wraps to fluids for keeping the exhaust gases hot so they keep flowing better. However, I do not think comparing how fluids flow with temperature changes applies to gases. For gases I think the volumetric decrease as a gas cools overshadows any resistance to flow. Look at tuned pipes and their step increases in diameter to get the exhaust gasses to expand thus creating lower pressure areas that help continue to draw the exhaust away from the cylinder. Add cooling to that i.e., no wraps, and it should lower the pressure and draw even more which could help scavenge even more during overlap.

I have only read one dyno operator say they have seen that wraps decrease performance, any others or to the contrary? Note that was a car dyno.

[Matt C]

Those steps are more for sonic wave action than actual gasflow. When the sound wave hits that expansion, it reflects a negative
pulse back at the ex port, so what you get is a series of negative pulses while the valve is open, and one big pulse when it hits the
end of the pipe (hopefully right before the valve closes during overlap). All this helps scanenge the cylinder and pull in fresh charge.

But wrapping the pipe definately keeps the velocity high. That's not even up for debate, it's scientific fact.

P.S. Forget fluid, that's a whole different thing. (fluid is not compressable)

[Matt C]

If you've ever been to the Harley drags, you'll see lots of wrapped pipes.

[Admiral Akbar]

Those steps are more for sonic wave action than actual gasflow. When the sound wave hits that expansion, it reflects a negative
pulse back at the ex port, so what you get is a series of negative pulses while the valve is open, and one big pulse when it hits the
end of the pipe (hopefully right before the valve closes during overlap). All this helps scanenge the cylinder and pull in fresh charge.

But wrapping the pipe definately keeps the velocity high. That's not even up for debate, it's scientific fact.

P.S. Forget fluid, that's a whole different thing. (fluid is not compressable)

Coupla things..

It's not sonic waves that are tuned in an exhaust system, it's gas flow pulses..   They are different.. Sonic waves really don't have any energy..

How much higher does it keep the velocity?  If it's a scientific fact, it has to be documented somewhere..

I would expect that wrapping the pipe does keep the volume slightly higher.. If it keeps the volume higher all it really does is make the exhaust system seem a little smaller...  If the system is oversized, it may help.. If the exhaust system is undersized, wrapping might hurt..

Max

[Hillside Motorcycle]

.................and in automotive application the wrap helps reduce under-hood temps...........helping to keep the engine's incoming air temps lower.

[Matt C]

Those steps are more for sonic wave action than actual gasflow. When the sound wave hits that expansion, it reflects a negative
pulse back at the ex port, so what you get is a series of negative pulses while the valve is open, and one big pulse when it hits the
end of the pipe (hopefully right before the valve closes during overlap). All this helps scanenge the cylinder and pull in fresh charge.

But wrapping the pipe definately keeps the velocity high. That's not even up for debate, it's scientific fact.

P.S. Forget fluid, that's a whole different thing. (fluid is not compressable)

Coupla things..

It's not sonic waves that are tuned in an exhaust system, it's gas flow pulses..   They are different.. Sonic waves really don't have any energy..

How much higher does it keep the velocity?  If it's a scientific fact, it has to be documented somewhere..

I would expect that wrapping the pipe does keep the volume slightly higher.. If it keeps the volume higher all it really does is make the exhaust system seem a little smaller...  If the system is oversized, it may help.. If the exhaust system is undersized, wrapping might hurt..

Max

Its BOTH! Why do you think header primary tubes are a "tuned" certain length? They're cut to the
length that makes the sound wave arrive back at the port at just the right time. (Usually just before
the ex. valve closes)

There are TWO things at work in an exhaust system. Gas flow and Sonic waves. It's a very complex
interaction but you cannot condiser one without the other if you want to maximize the power produced.

P.S. Sonic waves have a considerable amount of energy.

[Matt C]

Coupla things..

It's not sonic waves that are tuned in an exhaust system, it's gas flow pulses..   They are different.. Sonic waves really don't have any energy..

How much higher does it keep the velocity?  If it's a scientific fact, it has to be documented somewhere..

Its documented in physics books, or look up universal gas law, boyle's law, charles' law. You can transpose
these equasions to solve for temperature pressure or whatever.

I think some people are calling with sonic pulses "gas flow pulses". Sonic pulses definately play a huge role
in pipe tuning. Thats why a header pipe is tuned to a certain length. It's tuned for sound, not some arbitrary
gasflow number.

Lowering the temperature of a moving gas will: 1. make it more dense 2. remove energy 3. slow it down
Look, I'm not here to argue. If you don't want to believe what I'm saying, I really don't care.

Do your own research, I'm not going to debate you on something I know to be true. Go grab some physics
books and look it up

[rbabos]

A wrap will trash the pipe quicker.
Ron

[Admiral Akbar]

Its BOTH! Why do you think header primary tubes are a "tuned" certain length? They're cut to the
length that makes the sound wave arrive back at the port at just the right time. (Usually just before
the ex. valve closes)

There are TWO things at work in an exhaust system. Gas flow and Sonic waves. It's a very complex
interaction but you cannot consider one without the other if you want to maximize the power produced.

P.S. Sonic waves have a considerable amount of energy.

Lets say the sound pressure at the exhaust opening is 140 decibels..  (stick your ear right next to the end of the pipe)

That would make the sound pressure pulse about 200 pascals or 0.029 psi   

When dealing with a range of 14.7 psi.. It's only about 0.2% of ambient..  Doesn't seem like much energy to me.. Anyone ever actually measure the sound energy?

Max

[Admiral Akbar]

A wrap will trash the pipe quicker.
Ron

I would agree.
While some like to use a special ceramic coating on the inside of the pipe and I suspect that it might help a little.. Dragos?   I prefer a special carbonic coating that requires a bit of time to apply..

Max
 

[rbabos]

A wrap will trash the pipe quicker.
Ron

I would agree.
While some like to use a special ceramic coating on the inside of the pipe and I suspect that it might help a little.. Dragos?   I prefer a special carbonic coating that requires a bit of time to apply..

Max

Took me most of the season to do my pipes. Can't tell the difference other than sound maybe
Ron

[Matt C]

Its BOTH! Why do you think header primary tubes are a "tuned" certain length? They're cut to the
length that makes the sound wave arrive back at the port at just the right time. (Usually just before
the ex. valve closes)

There are TWO things at work in an exhaust system. Gas flow and Sonic waves. It's a very complex
interaction but you cannot consider one without the other if you want to maximize the power produced.

P.S. Sonic waves have a considerable amount of energy.

Lets say the sound pressure at the exhaust opening is 140 decibels..  (stick your ear right next to the end of the pipe)

That would make the sound pressure pulse about 200 pascals or 0.029 psi   

When dealing with a range of 14.7 psi.. It's only about 0.2% of ambient..  Doesn't seem like much energy to me.. Anyone ever actually measure the sound energy?

Max

Im quite sure you can find LOTS of published papers on this. Those sound waves are not insignificant. Ever looked at
a two-stroke expansion chamber? Those divergent and convergant cones you see are strictly designed to harness the
sonic energy from the sound wave that occurrs when the EX port opens. The only difference is, the sound wave is reflected
as a positive pulse as opposed to a negative one.

An open megaphone reflects a negative wave, a megaphone with a convergent cone attached reflects a positive signed wave.
That positive signed wave in the two-stroke pushes unburned gasses back into the cylinder just before the piston
closes the port. That pressure wave is quite strong. Don't ask me to calculate it, I've forgotten most of that stuff.

[Matt C]

A wrap will trash the pipe quicker.
Ron

You got that right! Ceramic coatings do the same thing (keep the heat IN the pipe) and
they look much nicer. You can coat the outside and get the same affect.

[joe_lyons]

I thought the steps had something to do with the heat expansion and the velocity of it and the wrap or ceramic coating helps keep the heat in so the velocity stays high.

[14Frisco]

Lowering the temperature of a moving gas will: 1. make it more dense 2. remove energy 3. slow it down

What is the temperature difference of the exhaust gas (the gas, not the pipe) when the pipe is wrapped vs. when it is not wrapped?

[rbabos]

Lowering the temperature of a moving gas will: 1. make it more dense 2. remove energy 3. slow it down

What is the temperature difference of the exhaust gas (the gas, not the pipe) when the pipe is wrapped vs. when it is not wrapped?

Roughly 10%. The way it works is the hotter gas has more pressure in the pipe than a cooler gas so the tranfer to atmosphere out the end is faster.
Ron

[14Frisco]

Lowering the temperature of a moving gas will: 1. make it more dense 2. remove energy 3. slow it down

What is the temperature difference of the exhaust gas (the gas, not the pipe) when the pipe is wrapped vs. when it is not wrapped?

Roughly 10%. The way it works is the hotter gas has more pressure in the pipe than a cooler gas so the tranfer to atmosphere out the end is faster.
Ron

Did you measure the 10% temperature difference on a Harley?

[rbabos]

Lowering the temperature of a moving gas will: 1. make it more dense 2. remove energy 3. slow it down

What is the temperature difference of the exhaust gas (the gas, not the pipe) when the pipe is wrapped vs. when it is not wrapped?

Roughly 10%. The way it works is the hotter gas has more pressure in the pipe than a cooler gas so the tranfer to atmosphere out the end is faster.
Ron

Did you measure the 10% temperature difference on a Harley?

No, just a ballpark hotrod figure. I find the whole exercise pointless but that's just me.
Ron

[Cracked Head]

Its BOTH! Why do you think header primary tubes are a "tuned" certain length? They're cut to the
length that makes the sound wave arrive back at the port at just the right time. (Usually just before
the ex. valve closes)

There are TWO things at work in an exhaust system. Gas flow and Sonic waves. It's a very complex
interaction but you cannot consider one without the other if you want to maximize the power produced.

P.S. Sonic waves have a considerable amount of energy.

Lets say the sound pressure at the exhaust opening is 140 decibels..  (stick your ear right next to the end of the pipe)

That would make the sound pressure pulse about 200 pascals or 0.029 psi   

When dealing with a range of 14.7 psi.. It's only about 0.2% of ambient..  Doesn't seem like much energy to me.. Anyone ever actually measure the sound energy?

Max

The work that Gordon P Blair provided in his book of modeling the 4 stroke engine,will verify what you ask.Most engine simulations are from the result of Blair and others work on the theory and proof what what they labeled as finite amplitude waves.The pressure ratios are far above the loudest acoustic pressure  ratios.Modern pressures transducers pretty much verify the chapter in Blairs SAE book of unsteady gas flow dynamics.This book will give you enough formula and experimentation results to make your own simulation models if you are in love with the math.Go over to speed talk and check out Vannicks program and find out what it was based on.

[Matt C]

Lowering the temperature of a moving gas will: 1. make it more dense 2. remove energy 3. slow it down

What is the temperature difference of the exhaust gas (the gas, not the pipe) when the pipe is wrapped vs. when it is not wrapped?

Hell I don't know. Why don't you get some temperature transducers and measure it? Than get back to us.

[Cracked Head]

Professor Blairs bio
http://www.profblairandassociates.com/GPB_Tribute.html

[Admiral Akbar]

The work that Gordon P Blair provided in his book of modeling the 4 stroke engine,will verify what you ask.Most engine simulations are from the result of Blair and others work on the theory and proof what what they labeled as finite amplitude waves.The pressure ratios are far above the loudest acoustic pressure  ratios.Modern pressures transducers pretty much verify the chapter in Blairs SAE book of unsteady gas flow dynamics.This book will give you enough formula and experimentation results to make your own simulation models if you are in love with the math.Go over to speed talk and check out Vannicks program and find out what it was based on.

Blair mentions acoustic waves and spends about a page on it (Fred says "boo") then promptly moves on to finite analysis after stating that it's power would have little effect.. He mentions a rifle shot of 200 decibels only multiplies the pressure by 1.02 or say 2%

Smith's 1968 book dedicates a chapter on sound but moves on to pressure waves..  I think Smith wasn't sure it had any value except reference to exhaust quietness. He goes on to develop a pressure sensing device and measures the corresponding pressure waves..

Max

[Admiral Akbar]

Im quite sure you can find LOTS of published papers on this. Those sound waves are not insignificant. Ever looked at
a two-stroke expansion chamber? Those divergent and convergant cones you see are strictly designed to harness the
sonic energy from the sound wave that occurrs when the EX port opens. The only difference is, the sound wave is reflected
as a positive pulse as opposed to a negative one.

An open megaphone reflects a negative wave, a megaphone with a convergent cone attached reflects a positive signed wave.
That positive signed wave in the two-stroke pushes unburned gasses back into the cylinder just before the piston
closes the port. That pressure wave is quite strong. Don't ask me to calculate it, I've forgotten most of that stuff.

Absolutely.. Have Jennings book sitting on the shelf right in front of me.. It's not sound but pressure waves that are reflected..

Blair and Atherton say no.. BTW Athertons book Desktop dynos is a pretty easy read though I think he screws up on the discussion of the Kadenacy theory. 

Max

[Matt C]

I think we're arguing semantics. The sound wave and pressure wave are one in the same (it travels at the speed
of sound in a hot gas).

On a side note: How do you get into in this forum? I got here by accident. I don't see any header in the main page
that gets back. What the hell?

[Matt C]

Never mind, I figured it out by disecting the URL and working backwards. LOL what a dummy