doug

[quote author=KATOOM link=board=7;threadid=14778;start=30#139920 date=1053368269]<br><br><br>hard to argue with these results, reported by AlpineRam:<br>&quot;The results we got from an exhaust test stand were that if we pumped an amount of air through an exhaust system we got a certain backpressure and a certain flow rate. (logical)<br>But when we introduced sound by means of a speaker battery we got sometimes significantly different results.[/quote]

doug

[quote author=KATOOM link=board=7;threadid=14778;start=30#139920 date=1053368269]<br><br>[/quote]<br>hard to argue with these results, reported by AlpineRam:<br>&quot;The results we got from an exhaust test stand were that if we pumped an amount of air through an exhaust system we got a certain backpressure and a certain flow rate. (logical)<br>But when we introduced sound by means of a speaker battery we got sometimes significantly different results.

doug

<br><br>The engine with a 4&quot; pipe is sequentially fired, and the engine with a 3&quot; pipe is sequentially fired. Both try to expel the same volume of air per crank revolution in their attempt to exchange hot air molecules for cool ones. <br><br>

doug

[quote author=AlpineRAM link=board=7;threadid=14778;start=30#139936 date=1053371163]<br>Well, OK now my &quot;shockwaves&quot; seem to give away that my mother tongue is not english. ;D<br>[/quote]<br>hey, that was a GREAT explanation. helps me understand what you were up to. I think I understand your position now, and it is very interesting. <br><br>fastinating, and great contribution! yea, the exhaust pulses would be modeled as a &quot;burst&quot; of air pressure that will contain a distrubution of sound waves at different frequencies, in addition to a pressure distribution of the air itself. <br> <br>wow, this is great info. What fascinates me here is that I wouldn't expect a sound wave to impact the pressure point at the turbine inlet, but your work sounds like it must indeed do that. You changed nothing except played some loud sounds and found that the introduced sound impacted aggregate air flow! My only thought here is that, in order for the sound wave to do that, it must be a standing wave. your speaker battery simulated that.<br> <br>hey don't appologize. you continue to make very good contributions - it may take a couple of rounds but I think you're doing an excelent job. I have great respect for those who are multi-lingual.

HOHN

Alas, some of my musings need modification. In other words, some of what I said might have been just plain wrong.<br><br>On sounds waves and flow: I understand a pulse to be a pressure wave (an area of higher pressure followed by an area of lower pressure). I also believe that all such waves travel at the speed of sound. This makes me think that waves are only in issue in the sense of how they are reflected and how those reflections behave.<br><br>Am I correct in assuming that standing waves can only form at a constant frequency? Wouldn't that have to be true, since the changing the frequency would change the phasing of the direct and reflected waves, and thus whether they are complimentary to each other (resonant) or destructive (out of phase)?<br>Furthermore, doesn't there have to be a wave reflection in order to have a standing wave?<br><br>It seems to me that if you did the math on all the parts in the stream that could reflect a wave, you find the right lengths to make parts to avoid complimentary waves. I mean, you don't want pipe lengths to be whole fractions of each other (one twice as long as the other, etc.)<br><br>It's important to remember that when a waves reflects, it doesn't do it with 100% efficiency. Part of the wave still presses on. Say a wave is heading down the exhaust and hits the resonator- part of that wave will continue through it, while another part of it will be reflected back to the turbo. True? What affects the ratio?<br><br>I also believe that timing is important, since the same waves that can cause a drone (complimentary or constructive interference) can cancel each other out (destructive interference) when the freq changes.<br><br>It makes sense that a resonator (with its expansion, and hence, higher pressure) could cause a wave reflection as the higher density increases the percentage of that wave that is reflected vs passed through that point.<br><br>All this gives me more respect for all the tech that goes into designing a muffler that isn't restrictive (Flowmaster).<br><br>HOHN

doug

[quote author=HOHN link=board=7;threadid=14778;start=30#139938 date=1053371197]<br>Doug, great lead post. I love the hard-core tech. I initiate (and add to) posts like this on TDR when they come up (see my pumps, lines, and what not post there)<br>[/quote]<br><br>Hohn, great post. Indeed, my point all along has been that aggregate air flow is the way to control EGTs, and the 4&quot; system would represent a lower restriction and better EGT control. But I did assume that the intake system was up to snuff -- I just thought that would go without saying, since the factory air box is a piece of junk. So you're certainly right that the move from 3&quot; to 4&quot; only makes sense after you loose the factory airbox. A Scotty is going on my truck long before I dink with the exhaust systems!<br><br>great rule-of-thumb numbers regarding boost pressures and pipe sizes, too. <br><br>I find AlpineRam's findings very interesting, as I too don't think sound waves would materially affect aggregate air flow since the velocity of sound is an order of magnitude higher than the air moving through the pipe. <br><br>yea, the &quot;expansion chamber&quot; represents a sudden increase in density and decrease in velocity -- so the hot air molecules ripp'n down the pipe suddenly slam into cooler, denser, slower air! <br><br>anyway, great, informative post.

KATOOM

Doug, &quot;I assume&quot; is not PROOF. And to what level of restriction was he getting or was a restriction at all? <br><br>I feel like I'm chasing my tail here. I AM only saying that some guys were compairing the total displacment of the motor as to the exhaust size being the reason 4&quot; is better and that is not right!<br><br>Please continue.<br> AGREED!

KATOOM

I must add, that you guys are great and its good that we take the time to help each other understand and see what makes these sweet trucks tick. Even if it requires a little arguing!

AlpineRAM

Doug: We did indeed &quot;play loud sounds&quot;(actually it was a bit more sophisticated, we used a kind of flow bench on a mock up cylinder head and this included a tee that came from the &quot;sound machine&quot; - If sound waves (or pulses) were negligible to mass flow because they actually do not change the mass inside the system (teenagers with thumpskates would either suffocate or get into a divers coma if it wasn't that way) we wouldn't have seen the results. <br>There was an increase in backpressure over the &quot;silent flow&quot; of up to about 15%.<br>We did not need to get the sound into any of the obvious resosnance frequencies of pieces of the exhaust to see this behaviour. I think that since most of the work had to do with the early catalytic converters on rather small high performance 4 cylinder engines the magnitude of the effects on an I6 turbodiesel with much higher displacement and relatively small valve flow area will be much less visible on our trucks than on the stuff we worked at then. <br>I agree that if we assume that mass flow is one of the keys in bringing down EGTs that the intake system is of crucia importance, and due to the turbocharger's workload being directly dependant on the restriction before the compressor wheel (of course after it too not to be neglected) the supply of air to the turbo has to be fixed first. A part that should not be forgotten is the import of heat into the cylinder by the air charge- a turbo running outside it's map can cause much sorrow. Ithink that in terms of EGT an optimal exhaust system would provide a window of minimum pressure for each exhaust pulse, therefore reducing the need to shove the gas out. Most of this work is done in the exhaust manifold, but in the frequencies of the pulses involved also the exhaust piping will have some effects.<br>Gary: If I may ask when I take your analogy of whistling into a fan then why do we need mufflers or anything more than a short piece of pipe to avoid having the exhaus gasses in our face?<br><br>AlpineRAM

doug

[quote author=HOHN link=board=7;threadid=14778;start=45#139991 date=1053378683]<br><br>On sounds waves and flow: I understand a pulse to be a pressure wave (an area of higher pressure followed by an area of lower pressure). I also believe that all such waves travel at the speed of sound. This makes me think that waves are only in issue in the sense of how they are reflected and how those reflections behave.<br><br>[/quote]<br>I'm not sure an exhaust pulse and a sound wave are necessarily the same thing. In other words, the exhaust pulse is a huge rush of air volume combined with a distribution of sound waves at various frequencies. the waves of course travel at the speed of sound and their behavior is of great interest, as you imply. the pressure &quot;pulse&quot; (not the sound wave but the air itself) travels at whatever is required to support the displacement volume per unit time (about 100 L per second at 2000 rpm, normally asperated) when it collides with the other pressure pulses from the other cylinders. <br><br>all correct. A reflection, of course, could occur (with some efficiency &lt;100% as you correctly point out) whenever there is a discontinuity in the medium (water waves reflect when they reach the shore, waves reflect and &quot;stand&quot; on a violin string that is fastened solid at each end, etc). thus a reflection could occur at a sharp bend (hence the desire for smooth corners), or an expansion chamber, resonator, muffler, cat, etc.<br><br>Yes, a steady audio source is requried to produce a standing wave. I have assumed that AlpineRam's experiment must have been with test tones or some constant frequency, for example. But given the arguably broad distribution of frequencies produced by the engine, it would not be unusual to have a standing set up at some frequency corresponding to the distance between turbine and resonator. <br><br>yes! but part of the problem is that the spectra of acoustic energy coming out of the engine is very broad. So you can't avoid any arbitrary standing wave from happening, but you can certainly avoid ones that have been shown to be problematic by experimentation. placement of the resonator would be critical in that regard. One approach that Diesel Dynamics uses, to reduce the audible sound of the exhaust system, is to put in two resonators, spaced a few feet apart. They claim that their approach has vanishing low EGT consequences, but the benefit is that area between the resonators -- dissipating acoustic energy it appears.<br><br>all correct. and what determines the ratio is how large the change in air density is. at the point where there is a discontinuity in the media (that is, the air in the pipe changes density) there is a partial reflection (back) and transmission (through) that point. For example, if the wave encounters a sudden increase in pipe diameter (say, a resonator), this represents an increase in media density, and the percentage of the wave's energy that continues through (instead of being reflected) will be high if there is very little change in air density.<br><br><br>absolutely. computer modeling would be requried to predict all the possible outcomes. and, depending on how the exhaust system responds, you might have the potential for a standing wave (or done) to exist at a certain frequency, but your engine rpm has to stay at a certain value for a certain amount of time for it to actually develop. <br><br>yes! So I think there is substantial argument for standing waves to exits between resonator and turbo, and that placement of the resonator in the pipe will have a profound effect on the sound of the system. It may even have (per AlpineRam's work) some effect upon aggregate cooling which until this discussion I did not think was possible.<br><br>no kidding

AlpineRAM

Well I know about the muffling effect of turbos. Still there is a lot of pulsation and sound left after it. It would be very interesting to measure sound output of a- a straighpiped truck with turbo and b: same truck eith just the turbine taken out of the housing. Definitely b will be loudr but how much louder ? And maybe c: Same truck with the exhaust turbine held steady instead of spinning. This would be the most interesting comparison: c-a <br><br>AlpineRAM