Late model wide case exhausts

[machten]

Thanks Harvey and Bob.

This prompted me to do a bit more reading on the subject to improve my ongoing education!

Much appreciated.

Kev

[machten]

This was an interesting graph on one of the sites I was reading...

exhaustbacpressure.jpg

Kev

[Harvey]

Kev would have a link to the site the diagram can from. I would like to look at it, as it seems to what I have been talking about.

[machten]

I cant seem to link directly to the article. Follow this link, select Articles Index, then look for the article "Advanced Exhaust Tech II - Backrressure and Area". You dont see the option from a mobile phone. Need to use a pc or ipad.

There's some good articles on that site.

http://www.team-integra.net/forum/blogs/michaeldelaney/103-advanced-exhaust-tech-ii-backpressure-area.html

Kev

[DewCatTea-Bob]

[quote= machten ...
" Thanks Harvey and Bob. "

____ Sorry I stupidly failed to notice that your new-post got placed on a new-page yesterday !
__ ( That oversight has happened to me a few times before, so I sort-of wish our w.site-forum's thread-pages exhibited different color-tints once they've become filled-up [to the default 10-post limit]. _ [In-fact, it sorta seems like I recall from back in 2009, that these pages used-to be rather shaded a light-pink color instead of the current light-blue shade,, anyone-else remember so ?] )



" This prompted me to do a bit more reading on the subject to improve my ongoing education! "

____ I-myself have only ever read no more than two mag.articles concerning the related subject, published within cycle-mags.
The first article was in a late-60s cycle-mag covering 'ex.pipe-tuning', and the other was in an early-70s bike-mag covering the subject of how megaphones are able to widen the 'power-band'.
__ So by the time you've read & absorbed whatever you've bothered to find on the w.net concerning these related tech.matters, you then ought know-of considerably more than I-myself ever have.
So it certainly would be of interest to get your summery of whatever you've learned, at-least concerning some of that which has already been brought-up thus-far !
__ Anyhow,, after you've learned exactly how the megaphone-shape affects the power-band's rev.range, you then ought well comprehend how my mentioned "fluted" straight-pipe exhaust-extension somewhat mimics the same desired effect of a megaphone.
Also, it would be of worthy interest if you'd relate whatever info you find that describes how a megaphone effects exhaust-system efficiency and/or eng.power-production.


Enlightening-Cheers,
-Bob

[Harvey]

I cant seem to link directly to the article. Follow this link, select Articles Index, then look for the article "Advanced Exhaust Tech II - Backrressure and Area". You dont see the option from a mobile phone. Need to use a pc or ipad.

There's some good articles on that site.

http://www.team-integra.net/forum/blogs/michaeldelaney/103-advanced-exhaust-tech-ii-backpressure-area.html

Kev

Thanks Kev, found and read the article, a bit disappointing, they just don't have the whole picture. The read on the Yamaha EXUp. valve is unbelievable. They are stating that the valve closes and opens during A cylinders valve overlap, have a look at the system, not a chance.

I will write some on the subject, just take time.

[machten]

The read on the Yamaha EXUp. valve is unbelievable. They are stating that the valve closes and opens during A cylinders valve overlap, have a look at the system, not a chance.

Clearly! My interest in reading has been in the physics of the rarefraction and scavenging and reverse scavenging aspects, pipe lengths etc. I'm coming from some way behind you on understanding and I needed to understand the pressure wave motion (in both directions) a little better. One of my degrees is in applied mathematics, I just needed to get a base line into the theory. I now feel I understand the theoretical physics of the effect of the straight pipe and the megaphone, but I'm sure you can explain it far better than I!!! There's at least as much art as there is science in this. I used to mess with expansion chambers on my H1 Kawasaki 500 (3 of them). Never really got it right.

Kev

[Harvey]

They describe the Inertia action ok, but get a bit lost after that.
This is just my opinion of the action, the way I see it.

As I said, there are two events that start when the exhaust valve opens. The first is the release of the gas pressure of about 65/70 psi into the pipe, this pressure forms a gas front a 'plug' that pushes the stationary gas in front of it, down the pipe. The Inertia of that gas front moving down the pipe pulls a depression in the pipe behind it, that removes the residual gas, and reduces the pressure in the cylinder. This reduces the pressure that the piston has to push against as it rises on the exhaust stroke, saving power.

In the early days the pipe was long enough to have this depression last till the inlet valve opened on valve over-lap, to induce the inlet gas to start flowing into the cylinder. So the pipes were much longer that we use now, but as the rpm that the torque is developed at rises, the Inertia speed of the gas is too slow, as it will only travel at a speed of about 300ft/sec. regardless of the engine speed, and the engine will have moved past the point of that depression being useful. So we only now use that event to reduce the pumping losses that occur about 70/80* ABDC.

The second event to start with the exhaust valve opening, is the release of a positive sound wave that travels down the pipe till it reaches a point of expansion. The high pressure wave is replaced by a low pressure wave that travels back up the pipe to the cylinder. We time this wave return by the length of the pipe from the valve to the expansion point, so that the low pressure returns during valve over-lap to start the inlet flow and the start of the sound wave in the inlet tract. This action can be use as the rpm rises as the sound wave travels at about 1600ft/sec. so it can accommodate the very high speeds that the torque is desired in modern engines.

Both these actions happen at the same time, the sound wave travels through the exhaust gas just changing the pressure as it goes, it is not moving gas at all, just changing the pressure of the gas as it moves through it. So the exhaust gas can be moving in an opposite direction to the sound wave, without interference, just changing the pressure as it goes.

We can't use both inertia. and sound wave action together, as the pipe lengths can't suit both actions, the Inertia has to have a long pipe to last for over-lap, and the sound wave will use a shorter length due to the different speeds of the actions, so we use the Inertia to reduce the residual gas and to reduce the pumping losses, and use the sound wave to start the inlet flow.

If you want to learn more about this subject, I would suggest you read " The Scientific Design of Exhaust and Inlet Systems" by Philip H Smith. It contains the research that was done by Dr. John C Morrison, that took the theory of Walter Kaaden and turned it into a form that we could see and understand.

[DewCatTea-Bob]

[quote= Harvey ...
" This is just my opinion of the action, the way I see it. "

____ Firstly I must acknowledge that Harvey's post is certainly appreciated ! _ However I'm having a little trouble with some parts of it that I'd like to be cleared-up. ...



" there are two events that start when the exhaust valve opens. The first is the release of the gas pressure
into the pipe, this pressure forms a gas front a 'plug' "

____ I was unsure of exactly what you had previously meant by "plug", so I hoped you'd also use a substitute-word as well. _ But now I think I properly gather what you actually mean by that term,, and-so I'd not refer the expelled charge of ex.gas as something that tends to get stuck, but rather more-so as a 'bullet' of an expanding-wad of ex.gas-charge. _ So I-myself prefer to refer-to the expelled charge of ex.gas as a 'bullet-charge' or "bullet" (which gets shot-out through the ex.pipe).



" The Inertia of that gas front moving down the pipe pulls a depression in the pipe behind it, that removes the residual gas, and reduces the pressure in the cylinder. "

____ Clearly I interpret this to mean that once the expelled bullet of ex.gas is being shot-out through the ex.pipe, it's momentum must leave-behind a relative-vacuum (to do pretty-much as stated).



" In the early days the pipe was long enough to have this depression last till the inlet valve opened on valve over-lap, to induce the inlet gas to start flowing into the cylinder. "

____ I'm pretty-sure that Harvey doesn't mean that the vacuum-depression effect is no-longer effective at-all these-days,, as it must still have some of it's beneficial effect during low-revs, since the bullet-length is much longer during lower-RPMs (and extended ex.pipe length only helps to keep the entire bullet-length more intact for a longer time so that the momentum of it's fuller mass remains robust enough to help vacuumize the cyl.chamber during lower eng.revs).



" but as the rpm that the torque is developed at rises, the Inertia speed of the gas is too slow, as it will only travel at a speed of about 300ft/sec. regardless of the engine speed, and the engine will have moved past the point of that depression being useful. "

____ This is news to me,, but I take your word for it, as it stands to good-reason.




" The second event to start with the exhaust valve opening, is the release of a positive sound wave that travels down the pipe till it reaches a point of expansion. "

____ This is-not news to me (anymore),, as I think this was the same aspect which you had once-before corrected me on, previously (in an older thread where this same particular subject came-up once before).
However I still have some trouble sensibly comprehending the importance of it's desired (or undesired) function. ...
For one-thing, does the creation of this sound-wave begin as soon as the ex.valve lifts .001" off it's seat ?
And-also, I'm left unsure whether your "positive sound wave" is somehow supposed to be considered as the-actual sole cause of the particular 'pressure-wave' that travels ahead of the 'bullet' (and [perhaps?] independently directly leads-to the creation of the desired neg.pressure returning sound-wave),, or rather, (except for having the same origin at the ex.valve), is actually another separately independent wave-front effect that's fairly unrelated in relevant functionality. - (You can probably tell I'm still pretty-unclear on this particular aspect which you've raised [again].)



" The high pressure wave is replaced by a low pressure wave that travels back up the pipe to the cylinder. "

____ It seems we yet need to clearly distinguish between 'sound-waves' & 'pressure-waves'.
__ In the past, it was my understanding that when the positive-pressure of the pressure-wave (of an ex.bullet) reached the ex.pipe-outlet (or any other point within the ex.system where the containment of the bullet became allowed to expand any at-all, [such as with the widening confinement within a megaphone]), that then a created negative pressure-wave would travel back up the ex.pipe at the speed of sound towards the ex.valve (and preferably reach the valve during 'overlap').
So-then,, only the returning neg.pressure-wave travels at the speed-of-sound, and the ex.pressure-wave (at the front of the bullet) rather travels at a greatly reduced speed (which you claim is limited to a mere 300ft/sec).



" We time this wave return by the length of the pipe from the valve to the expansion point, so that the low pressure returns during valve over-lap to start the inlet flow "

____ Indeed so,, (as the expansion-point is marked at any spot where the bullet's pressure is allowed to expand & drop it's established pressure,, either suddenly [as at the tip-end of a straight-pipe], or rather more constantly [as within the progressively expanding-confines of a megaphone]) !



" This action can be use as the rpm rises as the sound wave travels at about 1600ft/sec. "

____ Now I've finally reached the point within your post that I have the most trouble completely accepting. ...
__ I had always been taught that the speed-of-sound (through normal sea-level air) is 1126ft/sec.,, but I do realize that actually, the speed of sound can vary quite significantly ! _ And while I could possibly accept the news that sound-waves manage to travel faster through exhaust-gas, I guess that I'm left to assume that you just happen to know that sound-waves then travel near the particular higher speed which you've stated.

If the maximum torque is wanted to appear at say 6500 rpm, then a pipe length from the valve to the expansion point would be about 41.5".

____ I suppose that if the speed of sound could actually be firmly pinned-down (to some factual speed-figure, such as either the 13500 inches-per-second speed that I've known-of, or rather the 19200in/sec which Harvey has indicated), then I could accept the possibility that ex.pipe-lengths can actually be figured down-to such finely-incremented .5-inch result-figures (for calculated tuned-lengths matching whatever chosen RPM.ranges).
However, while your stated pipe-length seems reasonably within the expected ballpark for "6500 rpm",, your presented theory which indicates that the neg.sound-wave (that's sent back through the ex.pipe to the overlap-period), is actually generated as a consequential-result of a positive pressure-wave that travels (from the opening ex.valve down-to the first ex.expansion-point) also at the speed-of-sound, really doesn't 'add-up' for me ! _ Cuz even at the slowest possible speed for sound -(down near 13,000in/sec), that 41.5" pipe-length is way-much too-short to properly account for both those two sound/pressure-waves to complete the round-trip (slowly enough) to be properly timed for 6500-RPM, if both travel at sound-speed in both directions (through that length of pipe) !
__ So I'm left wondering how you-yourself,Harvey, calculate the desired ex.pipe-length to properly/timely 'tune' the exhaust for any particular eng.RPM, (such as your "41.5" inch length for 6500-RPM example) !?
I-myself don't have any established formula for that, but I've done the math-figuring (which you may yet possibly prove wrong), which has indicated that the tuned ex.pipe-length (for 6500RPM) needs to be over 10-feet long (if those two pressure-waves both travel at the speed-of-sound).

So that is why the divergent cone on the end, is so effective, as the 41.5" can be in the middle of the cone, and the sound wave will expand all along the length of the cone, covering all the rpm lengths from the start of the cone to the outlet. Of course the intensity of the returned pressure is lower, but it is still effective, covering a wider rpm range.

____ Of-course that concerns the proven theory of how the megaphone-shape does what it does,, however while the cone/megaphone-effect certainly indeed provides an improvement in power-band width, I-myself am not really absolutely certain that there's actually any definite overall-increase in averaged power-production (compared to that of a straight-pipe extension). _ Cuz while the megaphone-effect more evenly spreads-out the tuned-band's power-peak (resulting rather with a W I D E N E D & diminished power-band peak), it's max.peak thus consequently can no-longer be as intensely peaked-up quite as high as that of the top-output provided by a straight-pipe's direct ex.outlet, (being afforded due-to it's more concentrated & intense resulted-neg.pressure-wave).
So overall power-wise,, both resulted 'power-bands' plus their-own surrounding rev.ranges (of the megaphone and straight-pipe ex.systems), may each pretty-much balance-out to end-up equaling the same averaged power-output overall.
__ However I suppose that the straight-pipe's particular working-advantage could possibly exceed an efficiency-level where it's totaled-strength may surpass a possible saturation-point, where-after it's increased intensity only yields 'diminished-returns' that provide no worthwhile added effect.
So assuming that possibility, that's what had inspired the 'fluted' extension-pipe (to compensate for any such possible effect-deficiency with respect to the megaphone-effect).



" the sound wave travels through the exhaust gas just changing the pressure as it goes, it is not moving gas at all, just changing the pressure of the gas as it moves through it. "

____ Right, (just the same as when we're able to hear things that vibrate the air) !


" So the exhaust gas can be moving in an opposite direction to the sound wave, without interference, just changing the pressure as it goes. "

____ Except the resulting sound-speed traveling against the direction of the ex.gas-flow, certainly of-course must be relatively slowed-down by the same amount as the speed of the expelling ex.gas.


Enlightening-Cheers,
-Bob

[Harvey]

[quote= Harvey ...
" This is just my opinion of the action, the way I see it. "

____ Firstly I must acknowledge that Harvey's post is certainly appreciated ! _ However I'm having a little trouble with some parts of it that I'd like to be cleared-up. ...



" there are two events that start when the exhaust valve opens. The first is the release of the gas pressure
into the pipe, this pressure forms a gas front a 'plug' "

____ I was unsure of exactly what you had previously meant by "plug", so I hoped you'd also use a substitute-word as well. _ But now I think I properly gather what you actually mean by that term,, and-so I'd not refer the expelled charge of ex.gas as something that tends to get stuck, but rather more-so as a 'bullet' of an expanding-wad of ex.gas-charge. _ So I-myself prefer to refer-to the expelled charge of ex.gas as a 'bullet-charge' or "bullet" (which gets shot-out through the ex.pipe).

The usual term used for this action is " gas front" or "gas plug", this is because the high exhaust gas pressure that is released does not have a free path in front of it, the pipe is filled with stationary gas that has to be pushed down the pipe by this gas pressure. This formes the flow into a wall or a front, of high pressure, or a Plug of gas.

" The Inertia of that gas front moving down the pipe pulls a depression in the pipe behind it, that removes the residual gas, and reduces the pressure in the cylinder. "

____ Clearly I interpret this to mean that once the expelled bullet of ex.gas is being shot-out through the ex.pipe, it's momentum must leave-behind a relative-vacuum (to do pretty-much as stated).

Yes if you want to call it that way.

" In the early days the pipe was long enough to have this depression last till the inlet valve opened on valve over-lap, to induce the inlet gas to start flowing into the cylinder. "

____ I'm pretty-sure that Harvey doesn't mean that the vacuum-depression effect is no-longer effective at-all these-days,, as it must still have some of it's beneficial effect during low-revs, since the bullet-length is much longer during lower-RPMs (and extended ex.pipe length only helps to keep the entire bullet-length more intact for a longer time so that the momentum of it's fuller mass remains robust enough to help vacuumize the cyl.chamber during lower eng.revs).

If we used a pipe that was long enough for the exhaust gas "plug" to develop a depression at low rpm, it would be too long for the sound wave to work at the desired rpm. So unfortunately it is one or the other.

" but as the rpm that the torque is developed at rises, the Inertia speed of the gas is too slow, as it will only travel at a speed of about 300ft/sec. regardless of the engine speed, and the engine will have moved past the point of that depression being useful. "

____ This is news to me,, but I take your word for it, as it stands to good-reason.




" The second event to start with the exhaust valve opening, is the release of a positive sound wave that travels down the pipe till it reaches a point of expansion. "

____ This is-not news to me (anymore),, as I think this was the same aspect which you had once-before corrected me on, previously (in an older thread where this same particular subject came-up once before).
However I still have some trouble sensibly comprehending the importance of it's desired (or undesired) function. ...
For one-thing, does the creation of this sound-wave begin as soon as the ex.valve lifts .001" off it's seat ?
And-also, I'm left unsure whether your "positive sound wave" is somehow supposed to be considered as the-actual sole cause of the particular 'pressure-wave' that travels ahead of the 'bullet' (and [perhaps?] independently directly leads-to the creation of the desired neg.pressure returning sound-wave),, or rather, (except for having the same origin at the ex.valve), is actually another separately independent wave-front effect that's fairly unrelated in relevant functionality. - (You can probably tell I'm still pretty-unclear on this particular aspect which you've raised [again].)



" The high pressure wave is replaced by a low pressure wave that travels back up the pipe to the cylinder. "

____ It seems we yet need to clearly distinguish between 'sound-waves' & 'pressure-waves'.
__ In the past, it was my understanding that when the positive-pressure of the pressure-wave (of an ex.bullet) reached the ex.pipe-outlet (or any other point within the ex.system where the containment of the bullet became allowed to expand any at-all, [such as with the widening confinement within a megaphone]), that then a created negative pressure-wave would travel back up the ex.pipe at the speed of sound towards the ex.valve (and preferably reach the valve during 'overlap').
So-then,, only the returning neg.pressure-wave travels at the speed-of-sound, and the ex.pressure-wave (at the front of the bullet) rather travels at a greatly reduced speed (which you claim is limited to a mere 300ft/sec).

OK, the two events are unrelated, both start with the opening of the exhaust valve. The gas plug travels slowly down the pipe pulling its depression behind it, to reach the open end, its action is now finished.
The positive sound wave starts at the same time. It has to travel to the open end, expand to change signs to a negative wave, and travel back up to the cylinder arriving at over-lap to induce the inlet flow.
(without wanting to confuse the issue, but as both valves are open at over-lap, and the piston is at TDC, it continues to travel up the inlet tract to start the inlet wave action.)

" We time this wave return by the length of the pipe from the valve to the expansion point, so that the low pressure returns during valve over-lap to start the inlet flow "

____ Indeed so,, (as the expansion-point is marked at any spot where the bullet's pressure is allowed to expand & drop it's established pressure,, either suddenly [as at the tip-end of a straight-pipe], or rather more constantly [as within the progressively expanding-confines of a megaphone]) !



" This action can be use as the rpm rises as the sound wave travels at about 1600ft/sec. "

____ Now I've finally reached the point within your post that I have the most trouble completely accepting. ...
__ I had always been taught that the speed-of-sound (through normal sea-level air) is 1126ft/sec.,, but I do realize that actually, the speed of sound can vary quite significantly ! _ And while I could possibly accept the news that sound-waves manage to travel faster through exhaust-gas, I guess that I'm left to assume that you just happen to know that sound-waves then travel near the particular higher speed which you've stated.

If the maximum torque is wanted to appear at say 6500 rpm, then a pipe length from the valve to the expansion point would be about 41.5".

____ I suppose that if the speed of sound could actually be firmly pinned-down (to some factual speed-figure, such as either the 13500 inches-per-second speed that I've known-of, or rather the 19200in/sec which Harvey has indicated), then I could accept the possibility that ex.pipe-lengths can actually be figured down-to such finely-incremented .5-inch result-figures (for calculated tuned-lengths matching whatever chosen RPM.ranges).
However, while your stated pipe-length seems reasonably within the expected ballpark for "6500 rpm",, your presented theory which indicates that the neg.sound-wave (that's sent back through the ex.pipe to the overlap-period), is actually generated as a consequential-result of a positive pressure-wave that travels (from the opening ex.valve down-to the first ex.expansion-point) also at the speed-of-sound, really doesn't 'add-up' for me ! _ Cuz even at the slowest possible speed for sound -(down near 13,000in/sec), that 41.5" pipe-length is way-much too-short to properly account for both those two sound/pressure-waves to complete the round-trip (slowly enough) to be properly timed for 6500-RPM, if both travel at sound-speed in both directions (through that length of pipe) !
__ So I'm left wondering how you-yourself,Harvey, calculate the desired ex.pipe-length to properly/timely 'tune' the exhaust for any particular eng.RPM, (such as your "41.5" inch length for 6500-RPM example) !?
I-myself don't have any established formula for that, but I've done the math-figuring (which you may yet possibly prove wrong), which has indicated that the tuned ex.pipe-length (for 6500RPM) needs to be over 10-feet long (if those two pressure-waves both travel at the speed-of-sound).

OK I think we can cover this section, by saying that the speed of sound depends on temperature of the gas it is moving through. 1226ft/sec is at ambient temp and sea level, as the temp changes the speed does too. In the inlet track it is about 1100ft/sec, in the average road exhaust about 1500ft-sec, an all out race engine with a fairing over the pipes, at race conditions, about 1700ft/sec. So for a sports road engine about 1600ft/sec.
We work the length out with the formula: 180 X 1600 / 6500 = pipe length in ".
The 180 is the number of engine degrees for the sound wave to do the return run down and back to the cylinder, the 1600 is for the speed of sound in that gas temp. The 6500 is the engine rpm that we want the maximum torque to occur.

There are a few variables here, if we accept the speed of sound in the pipe to be 1600ft/sec, we then have to settle on how many degrees for the trip.
We say 180* as that is approximately the number of degrees from the exhaust valve opening, to over-lap. So if the effective exhaust timing is open at 70* BBDC and the inlet opens at 30* BTDC, it is about 180*. This is about right for a single inlet valve engine.
If it is has two inlet valves , it changes due to the shorter over-lap that they use, so it may be 190*/200* to arrive at right time. This is where we build the model, and test to see where it turned up at, and then modify it, and test again.

So that is why the divergent cone on the end, is so effective, as the 41.5" can be in the middle of the cone, and the sound wave will expand all along the length of the cone, covering all the rpm lengths from the start of the cone to the outlet. Of course the intensity of the returned pressure is lower, but it is still effective, covering a wider rpm range.

In the example of 41.5" for 6500rpm being the length to the middle of the cone. The Conti is about 22" long, so there is 11" each side of the middle. The longest end would be 52" to resonate at 5540 rpm, and the shortest end of 29.5 would resonate at 9600rpm. I don't know what the pipe length is on the single, but that is the range that the Conti cone would cover.

____ Of-course that concerns the proven theory of how the megaphone-shape does what it does,, however while the cone/megaphone-effect certainly indeed provides an improvement in power-band width, I-myself am not really absolutely certain that there's actually any definite overall-increase in averaged power-production (compared to that of a straight-pipe extension). _ Cuz while the megaphone-effect more evenly spreads-out the tuned-band's power-peak (resulting rather with a W I D E N E D & diminished power-band peak), it's max.peak thus consequently can no-longer be as intensely peaked-up quite as high as that of the top-output provided by a straight-pipe's direct ex.outlet, (being afforded due-to it's more concentrated & intense resulted-neg.pressure-wave).
So overall power-wise,, both resulted 'power-bands' plus their-own surrounding rev.ranges (of the megaphone and straight-pipe ex.systems), may each pretty-much balance-out to end-up equaling the same averaged power-output overall.
__ However I suppose that the straight-pipe's particular working-advantage could possibly exceed an efficiency-level where it's totaled-strength may surpass a possible saturation-point, where-after it's increased intensity only yields 'diminished-returns' that provide no worthwhile added effect.
So assuming that possibility, that's what had inspired the 'fluted' extension-pipe (to compensate for any such possible effect-deficiency with respect to the megaphone-effect).[quote/]







" the sound wave travels through the exhaust gas just changing the pressure as it goes, it is not moving gas at all, just changing the pressure of the gas as it moves through it. "

____ Right, (just the same as when we're able to hear things that vibrate the air) !


" So the exhaust gas can be moving in an opposite direction to the sound wave, without interference, just changing the pressure as it goes. "

____ Except the resulting sound-speed traveling against the direction of the ex.gas-flow, certainly of-course must be relatively slowed-down by the same amount as the speed of the expelling ex.gas.


Enlightening-Cheers,
-Bob

[/quote]