[quote author=AlpineRAM link=board=7;threadid=14778;start=15#139029 date=1053122296]<br>Doug: I did not mean standing waves- I thought about reverberations from the open end of the exhaust going to the turbo and meeting a pressure spike there. <br>[/quote]<br>ok, I follow you. The open end of the pipe is a discontinuity and this will cause a reflection of sound waves back into the system. You're saying that this reflection could meet a pressure spike near the manifold. Thats all certainly true, but (1) if the wave reflection is not a &quot;standing wave&quot;, then it occurs randomly. It may cause some instantaneous pressure change at one moment in time, but on the average the air flow will be unaffected by such behavior and cooling will occur according to aggregate air molecule movements. (2) this would only occur in a straight pipe system with no muffler or resonator. I would expect the resonator to so a pretty good job of breaking up reflections and containing them within certainly pipe segments. (3) the end of the pipe is a low pressure discontinuity so it cannot cause a high pressure point (upon reflection) at the turbo UNLESS the reflection does not result in a standing wave, and in that case the wave behavior is random.<br><br>now this is interesting. Just to be precise, here, I think you're using the word, &quot;shockwave&quot; to refer to a sound wave, (standing or not), not a buildup of sound waves that eventually result in a big boom -- sound waves don't bunch up or build up unless they are forced to travel faster than the speed of sound. I think &quot;shockwave&quot; here refers to the presence of a sound wave, and if that is the case, the soundwave is either a standing wave or a random one. If it is random, then it won't affect average air flow.<br><br>Now then, a short wavelength standing wave could certainly exist between two discontinuities, and the high pressure end points of a 1/2 wave, for example, could occur at the muffler entrance. I'd like to hear more about your experiments, how you characterized these &quot;shockwaves&quot;, etc, and how you were able to measure the effect on average flow. this sounds fascinating. <br><br>yea I think you're right, here. Its just that a sound wave cannot reflect from the end of the tailpipe and travel through the resonator all the way up to the turbo (unless we have a straight pipe system). But that aside, we can analyze the two pipe sections (separated by a resonator) as cavities that do certainly affect each other from a sound perspective at least. the two sections are separate chambers that have frequency cutoff points bandwidths and Q's. That certainly affects the way it sounds, but I'm not sure how that would affect the average air flow for the purpose of efficient cooling and moving the air out the tailpipe. <br><br>I think you are right that a reflected sound wave coming back from the resonator could match up with one coming from the manifold. But here again I don't think that this would affect cooling because the speed of sound is much faster than the air coming out of the manifold and even a standing wave could be supported without affecting average air flow. but you are right in the sense that the wave would be a high pressure point -- IF it came reflected back from the resonator. If it came from the end of the pipe (a low pressure point by definition) then it can't end up at the turbo as a high pressure point.<br><br>I thinking at this point that something other than sound waves may be at work here. Air is pretty springy and maybe a flow restriction could &quot;reflect&quot; (but not in the audio sound wave sense) back into the system in the form of pressure variations that are not governed by the speed of sound. then we have a fluid dynamics issue and not a sound wave issue. That is, maybe the air can bunch up on itself without being a wave. inquiring minds want to know! I'd be interested in your thoughts on that.

 

I'm glad I spend most of my time chasing light beams. lambda/20 @833nm wavelength I understand.(41 millionths of a millimeter, better known as an LRCH).<br><br>chasingtinkerbellagaintodayShortroun d out

 

[quote author=KATOOM link=board=7;threadid=14778;start=15#139134 date=1053141300]<br>1. hot air moves slower which doesnt flow as freely as cool air. <br>[/quote] just explain what you mean here in the context of exaust. the laws of physics say that warm air will move faster, but it is more chaotic. perhaps what you may mean here is that heat transfer out of the engine is faster when the exhaust air is cooler. thats just good thermodynamic theory. so we can say that hot air move &quot;the heat&quot; slower, but we can't say that hot air itself moves slower than cold air. <br><br>well, it moves faster not because it is cool, but because of the pressure drop due to the cavity size increase. It does cool, certainly, but that itself doesn't make it move faster. Again, cool air is more dense (number of molecules per cubic volume) and less chaotic (molecule movement is slower) so it is more efficient at transfering heat out of the engine. But the cool air doesn't &quot;move&quot; faster. If anything, it moves slower because of the larger pipe size.<br><br>the serial (consecutive) nature of cylinders firing into the exhaust system has nothing to do with the volume of air that needs to be evacuated from the system to perform cooling. Its all about moving large amounts of air over a long period of time (compared to a single exhaust stroke one at a time in order). <br><br><br>I think in order to be meaningful we would have to compare cubic displacement. remember that the effective displacement of the 5.9L CTD grows exponentially with boost and becomes huge at moderate levels. For example, at 28 psi of boost the engine displaces about the same as a 23L (normally asperated) engine and a 4&quot; system is plenty large enough.

 

I belive that the exhaust would have to exit the turbo at &quot;sonic levels&quot; to start interfering with the sound waves, around 1088 FEET PER SECOND!!! Then you would be dealing with &quot;sonic barrier&quot;, in which the exhaust would be trying to force its way past the sound waves and if it accomplished this then you would get a &quot;sonic boom&quot;!!! <br>Come on guys...Where talking about exhaust out of a small diesel engine. Its fun to be all technical BUT this is scrapping the bottom of the barrel. I would think your exhaust is exiting around 80 mph close to the turbo. Hmmm...thats about 3,494,400 FPS to slow to cause a problem. Isnt there an aeronautical forum we could be chating in?

 

[quote author=KATOOM link=board=7;threadid=14778;start=30#139881 date=1053362730]<br>I belive that the exhaust would have to exit the turbo at &quot;sonic levels&quot; to start interfering with the sound waves,around 1088 FEET PER SECOND!!! <br>[/quote]<br>the question we're dealing with is not &quot;does the exhaust interfere with the sound&quot;. We all know that sound waves can and do exist in the exhaust pipe, precisely for the reason you mention -- sound waves travel a lot faster than the air in the pipe and are largely unaffected by the speed of the air in the pipe. the question we're dealing with is &quot;do those sound waves (or anything else) interfear with the exhaust and its ability to cool EGTs&quot;. <br><br>BTW, you're right of course about the approximate speed of sound. FYI, at std temp/pressure (sea level) sound travels 343 meters per second which is about 1114 ft./sec. <br> <br>the dynamics of exhaust, pressure variations, standing waves, etc., are all meaningful at current exhaust gas speeds which are on the order of 40-80 mph.

 

<br>What??? The fact that every piston not firing at the same time has a lot to do with the size of exhaust pipe needed! Yea, boost does increase the displacment of the cylinder but that doesnt change the fact that you are exiting the exhaust in a sequential order. Why do you think they stagger the tube lengths of a header? So that the exhaust pulses dont collide with each other and exit in a flowing maner which means the exhaust pipe doesnt flow a big chunk of air at once! Larger amounts of hot air take longer to cool than smaller amount thus affecting the total volume of air needed to be moved. And when you figure displacment increase with boost dont forget we are living in 14.2 lbs pressure. So 28psi would get you around 18L.<br><br> Exhaust cools at it moves through the pipe which in turn becomes more dense and SMALLER. Smaller air moves through an area &quot;easier&quot; with less pressure. As the air is moved down the pipe it cools and becomes smaller by volume. Which is why heat is bad in exhaust systems! <br>As for sound waves again. Try this. Light a fire cracker 720 ft away. When it goes off will you feel a 720 mph gust of wind? If someone claps their hands in you face is that going to cause a sound wave pressure spike on your face and impede your ability to breathe?<br> <br>Enough of this nonsence!!!

 

[quote author=Diesel Freak link=board=7;threadid=14778;start=15#139267 date=1053192824]<br>The 3&quot; to 4&quot; diverget section of a 4&quot; downpipe does not lower the pressure at the turbine outlet. <br>[/quote]<br>thats right. the sudden increase in diameter itself might have some sonic effects, but would not affect the aggregate air flow through the system. If, however, the rest of the pipe were also 4&quot;, then this would certainly represent a decrease in the pressure as seen by the turbo outlet, since the aggregate restriction in several feet of 3&quot; pipe is higher than that of the same length of 4&quot; pipe.

 

<br>Thank you for pointing this out...The sound waves are traveling at the speed of sound. No kiding. BUT you have no proof that the sound waves and exhaust air are intereacting in any varying degree!!! Take in consideration the Flowmaster design. Creating barriers in the muffler so that sound waves bounce into each other canceling out them selves, and the air just flows right by. Now if there was a intereaction then this system would just cause a big air/sound wave traffic jam!

 

[quote author=KATOOM link=board=7;threadid=14778;start=30#139916 date=1053367859]<br>The fact that every piston not firing at the same time has a lot to do with the size of exhaust pipe needed! <br>[/quote]<br>for the purpose of understanding the practical control of EGTs with 4&quot; versus 3&quot; pipe, in real, bombed CTD trucks no -- this (sequential firing thing) is a red hearing point that is not germane to the subject. <br><br>yes, I understand all that. it just doesn't help us understand the 4&quot; versus 3&quot; issue for engines that are already sequentially fired. <br><br><br>so far, we're saying the same thing. you say &quot;smaller&quot; and I say &quot;more dense&quot;. I'm not sure what your point is.<br><br>As you read through my posts, you will find that that I do not believe that sound waves themselves materially affect aggregate air flow. thats just because sound waves travel much faster than the air in the pipe does. so I'm not sure what you are trying to say to me.

 

Well, OK now my &quot;shockwaves&quot; seem to give away that my mother tongue is not english. ;D<br>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. We had construed this device for testing muffling efficiency against flow for racecars. (Getting thrown out of the ranking and being banned from the next 3 races can ruin your season). The main differences where observed when we used exhausts with catalytic converters. In some cases it seemed that there were some areas in resonators where we had very local high temperature in the gas, which lead to less flow like a restriction. <br>Due to the bankrupcy of my employer at this time I couldn't get on with this research. <br>In this case when there was a heat buildup of an area of gas we assumed that this was due to some kind of compression due to soundwaves coming from the exhaust pulses. ( Which are nowhere near sine waves to make your hair turn grey), the resonator became a high pressure point in the system and could reflect sound. What we did find out too was that this had some very odd effects on the exhaust turbine. We assumed that this had some connection to a disruption of the flow, maybe a kind of stall condition on the turbine blades due to turbulences. As I said we were interrupted in our experiments.... :'( <br><br>I am sorry that I really can't explain the results and the theories developed then in english. I think that I may sound a little to you. <br><br>AlpineRAM

 

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)

Here's my nickel:

It's important to remember that the sound aspect and FLOW aspect are very different animals. The confusion arises because they share a common conduit. There is a certain infinitesimal amount of interaction, but it is really of little consequence when designing the exhaust system (conduit).

It's useful to think of exhaust like radio. There is a carrier frequency, and there is a modulated signal. The carrier frequency is only affected by the extent and type of modulation. In exhaust, the airflow column is the carrier frequency, and the exhaust pulses (pressure waves) are the &quot;modulation&quot;.

To correct misinformation: the ideal header is EQUAL length. That's why they go through the trouble of bending the pipe all up in weird shapes. Unequal length header tubes are easy to design, yet they do it the &quot;hard way&quot;. How come? It's because equal lengths ensure that the exhaust pulses are evenly spaced, which is important on a hi-po gasser that relies so much on scavenging and overlap to breathe. This is especially true of the v-8 configuration, which almost always has two neighboring cylinders firing in succession.

But we have diesels. And they are inline-6s. TOTALLY DIFFERENT ANIMALS ALTOGETHER.

Our engines have one cylinder firing every 120° of crank rotation. There's plenty of time for exhaust to evacuate. Exhaust pulses CANNOT collide with one another-- they are all traveling at the speed of sound. The only way we could have &quot;colliding&quot; exhaust is if we had two cylinders fire at the same time.

So, let's ignore the sound aspect, since it has little to do with flow. It used to be that people that glasspacks were powerful because they were loud. We now know better.

Since EGT is the end game here, let's start with that as a reference point. To cool EGTs, we have to increase the mass flow of air through the whole engine (that's why boost cools egt). Assuming that the exhaust is the biggest restriction to flow in the system (*which is not true on our trucks*), we can then determine how to ease &quot;backpressure&quot;. First consider that the single largest restriction to flow is the turbine side of the turbo. It takes WORK to get that turbine spinning. So now we add another assumption: that the turbine is spinning fast enough to not be the largest exhaust restriction.

We are quickly arriving at the point where we can see that 4&quot; exhaust may not be the best way to lower egt. For one thing, the intake is proprtionately MUCH weaker (lower flow potential) than the exhaust.

Morever, installing a free-flowing intake can reduce backpressure. How, you say? Because the turbo compressor has to fight intake restriction to spool up, and what's connected to that? Right, the turbine. The turbine is designed so that the most efficient way for exhaust to move in that part is THROUGH it, not AROUND it. Thus, less intake restriction means less exhaust restriction.

Back to exhaust design. Let's also establish something else. Hotter air is less dense, and therefore flows more easily. It's simply because it's less viscous. Just someone already said. HOWEVER, hot air will NOT transmit wave energy as efficiently. Partly because of its unique properties of being elastic and compressible. Mostly, though, due to the molecules being farther apart. This is why the speed of sound is faster in water than in air-- water is more dense, and transmits wave energy more efficiently. But we have to remember, we don't care about wave energy, since the low density of the medium (exhaust) and its subsonic speed mean that it can only insignificantly affect airflow.

Why do jet aircraft fly so high? Because the lower density at high altitude greatly lowers the drag on the plane, and saves them money on FUEL. The air &quot;flows&quot; more easily over the plane. True, they also get less lift for the wings, but the ratio favor fuel savings right up until near that A/C service ceiling. Cruising altitude is designed to optimize the ratio of decreased drag to decreased lift (which is complex, since lift is a cause of drag).

The way to ensure optimum airflow is to FIRST ensure that the flow is laminar for every flow stream within the pipe. Thus, one constant-diameter straightpipe would be almost ideal for flow (and horrible for sound level). In a street system, there's not much you can do. Make sure the transitions are smooth from one component to another, for example.

Quality and Quantity of flow are the two components we have to deal with. Quality is what we are going for with mandrel-bends and smooth transitions. Quantity is pipe size.

Expansion: having a transition to a larger area will slow the flow down, cool it down, and increase density. Unfortunately, as velocity goes down, pressure goes UP!! That means large resonator or some other expansion area HURTS FLOW. As the exhaust is humming along it hits this wall of higher pressure, and that hurts efficiency. Like when you come up fast on slower traffic, you have to hit the brakes and waste energy.

I personally think that exhaust size should be somewhat relative to the peak boost you see. If a 3in stock system (7 in^2) is adequate for 34PSIA, then 4&quot; (based on that reasoning) should feed 60PSIA. This is 46PSI gauge. This will keep backpressure at stock spec (ratio of boost to exhaust volume).

So, a 4&quot; system will feed 46psi of boost with no more pressure than the stock system with stock fuel. Unfortunately, the fueling means that we will have a LOT more mass to flow (proportionately) than the stock system.

I would say that unless you have enough fuel to hit over 40PSI of boost (non-wastegated), then your exhaust is not the biggest restriction.

I would look to the intake to improve things. It's harder to suck than the push. That's why lift pumps die. That's why intake valves are bigger. That'w why you should look there FIRST, then look to POSSIBLE exhaust improvements to reduce EGT.

Sorry so long.

HOHN

 

Doug, I was speaking general to the post, not specificaly to you.<br>Yes it does. Yes it does. Your exhaust sees 60ci at a time instead of 360ci like I think some guys think.<br> This applies to any multiple cylinder engine that exits with the same pipe. Many V-8 motors cylinders dont see the pulses from the other bank since true dual exhaust.<br>

 

Well I have been consulting my dictionary a bit and made some sketches etc. <br>I think that some of the phenomena observed then where caused when a low frequency soundwave with a big amplitude had to pass a &quot;bunch of small pipes&quot;. In this situation the pressure gradient of the sound wave meant a change in the velocity of the gas in the pipe. This change of velocity meant a different behaviour, like the flow being laminar or turbulent. And since very small influences in a turbulent media will have great effects on the position of the turbulence I assume that sound waves even of higher frequencies can influence the position of turbulent swirls within the system. <br>Maybe I did not express myself correctly when I said sound wave. I meant a pressure curve. ( Like you can observe when watching the exhaust stroke you will see a rise of the pressure in the exhaust (due to the resistance) until the end of the cycle and then a return to outside pressure. ( for a single cylinder)<br><br>Well hoping to have cleared up some of my former postings... <br><br>AlpineRAM