doug

ok, I got bored so I ran some numbers, hoping to explain why a 33% increase in pipe size is so effective at reducing EGTs and why it sounds lower.<br><br>A 6 cylinder 4 stroke engine completes 3 exhaust strokes per revolution. This means the CTD expells 180 cubic inches (1/2 of its displacement) every time the crank goes around. 2000 rpm is equivalent to 33.3 revs per second, so that means the CTD spits out about 6000 cubic inches, or about 3 1/2 cubic feet of air per second at 2000 rpm. <br><br>In order for a 3&quot; diameter pipe to carry away that much volume of air per second, the air has to travel about 48 mph through the pipe. However, in a 4&quot; diameter pipe, the air has to travel only 27 mph. thats a 78% decrease in air velocity. However, the real story with regards to EGTs is that exhaust back pressure is reduced by the same percentage -- you get a 78% decrease in pressure as seen by the exhaust manifold. That much reduction in air pressure means that evacuating the cyliders is more efficient.<br><br>So why does the 4&quot; pipe sound lower? At 800 rpm, the engine produces a fundamental frequency of exhaust pulses of somewhere around 40 Hz. Thats a lot of energy at a very low frequency -- lower than any human male can speak, and closer to the frequencies produced by an orchestral bass drum. There is tons of energy above 40 Hz as well, of course, but my point is that even very small changes to the ability of the pipe to carry or enhance low frequency energy will result in hearing a &quot;lower&quot; sound. for example:<br><br>1. anything that makes the pipe appear longer to a sound wave will tend to support lower frequency acoustics. For example, a larger diameter pipe of the same physical length will appear to be very slightly longer, acoustically. its also possible that the new 4&quot; pipe might be longer physically, depending on the install; if there is a new tail section, etc. When the pipe supports the lower frequency components of the exhaust sound (which itself is very complex) you notice a more throaty sound.<br><br>2. the speed of the air exiting the exhaust pipe will tend to compress the sound waves (air pulses) coming from the engine, so that you hear a higher note than the engine actually produces. Since the air moves in a 4&quot; pipe slower than a 3&quot; pipe, the larger pipe will sound lower by a small amount (its about a half step, musically speaking, which is very obvious). <br><br>3. Other things being equal, the walls of a 3&quot; pipe will tend to be stiffer and will support higher pitched mechanical vibrations of the pipe itself. what we hear outside the truck is a combination of pure acoustic energy from the engine's exhaust pulses, and the vibration of othere things including the pipe itself. A larger pipe will contribute less high frequency energy in that regard, and cause the 4&quot; pipe to sound lower.<br><br>4. The larger pipe diameter means that the pipe now holds 78 percent more air. The increased mass of this air also has some acoustic properties relating to resonant ports, and will result in supporting lower acoustic frequencies already present in the engine's output.<br><br>happy exhausting -- Doug

doug

cool. This presents an interesting rule of thumb: it suggests that the back pressure presented by a 4&quot; pipe at 29lbs boost is about the same as that of a 3&quot; pipe at 14 lbs boost. bombs away!<br><br>Now if only I knew how to quantify the flow characteristics of the stock airbox...<br><br>doug

phox_mulder

[quote author=Doug link=board=7;threadid=14778;start=0#138549 date=1053023645]<br>Now if only I knew how to quantify the flow characteristics of the stock airbox...<br><br>doug<br>[/quote]<br><br>I think CRAP or any other censorable derivative sums it up nicely. ;D <br><br>phox

w4xtc

Doug, please tell me you are an engineer or physicist. I have a college education and had trouble reading your post. That has to be the most expansive exhaust dialogue I have seen to date. I loved it, keep up the great work. BTW, can you help me with this question? If a plane is traveling 3000 mph with no tail wind and looses 33% of its lift how fast will it fall?? Just kidding....

GrandpasRam

Nice calculations...however, 48 mph - 27 mph is a reduction of 21 mph. Divide the 21 (the amount reduced) by the original amount (48), to get the decimal ratio (.4375), then multiply by 100 to get percent - that is a reduction of 43.75%, not 78%. Still... a considerable reduction in back pressure and EGT's, all other variables being equal.

doug

yea, I said that wrong. I should have said, &quot;the 3 inch pipe (48mph) presents 78% more pressure than the 4&quot; pipe (27 mph). <br><br>see how percentages can be expressed differently, depending on your objective?<br><br><br>

doug

[quote author=w4xtc link=board=7;threadid=14778;start=0#138600 date=1053031962]<br>Doug, please tell me you are an engineer or physicist. I have a college education and had trouble reading your post. That has to be the most expansive exhaust dialogue I have seen to date. I loved it, keep up the great work. BTW, can you help me with this question? If a plane is traveling 3000 mph with no tail wind and looses 33% of its lift how fast will it fall?? Just kidding....<br>[/quote]<br><br>fast enough to hurt when it hits the ground . yea, Its just the engineer in me talking. when there are lots of numbers and complex relationships, its does get hard to follow. kind of like reading someone else's programming code -- its a lot easier to write a program than it is to read and understand someone elses ;D

BigBlue

You must have been very bored one day.

2manydogs

Boy, that was good reading!! What if you were able to mount a small pressure gauge right in the turbine side before the outlet. I have no idea why I want to know this, I'm WAY over my head. I was thinking (oh-oh) that the real back pressure creator is the actual outlet size of the turbo, before it enters the down pipe, as long as there is no muffler involved. Is the turbine also being used as a pump, so to speak, to pump the exhaust through the pipe? If so, lets run a 5'' tube behind the right front tire to minimize the length!! I'm just trying to better understand all this big pipe stuff! Jim

Bart Timothy

There is no measurable back pressure in the stock exhaust sytem until about the 350 hp level, and then it's not much above that.

AlpineRAM

Well a fact that seems interesting to me is that the most guys seem to do a cat back exhaust system on the 12V, or a resonator back system on the 24V. I do not think that the overall back pressure of the system is the main culprit of high EGT or slow spool until some very high hp numbers. Personally I blelieve that a lot of these problems do come from the resonance properties, resulting in pressure spikes from the exhaust valve meetine resonance exhaust spikes from the exhaust pipe directly at the turbine. Since the turbine gets its energy out of the differences in pressure and temperature of the exhaust gas a little less restriction post turbo will mean a faster spoolup. <br>Has anybody ever tried to wrap the stock exhaust post turbo with some kind of insulation to keep the temps up and to make the exhaust gas flow easyer?<br><br>Pondering abouut the next steps <br><br>AlpineRAM

doug

There's a company who's name escapes me at the moment that sells sound reducing wraps for the 2nd gen CTD, including valve cover, oil pan, hood liner, and a turbo wrap. They claim the turbo wrap lowers EGTs. The only explanation I see for that is to raise the temp differential across the turbocharger, which might create more boost with less fuel = lower overall EGT. seems like a stretch to me, but maybe there's something to that. <br><br>so yea, a reduction in post turbo restriction should help spool up. when you increase the temp differential you in effect &quot;lower&quot; the restriction post turbo and increase airflow across the turbo. that can only be good. But part of the restriction &quot;post&quot; turbo is the restriction on other side of the engine -- what's attached to the the exhaust manifold. <br><br>You raise an interesting possibility relating to standing waves though, which I think could occur in the system between the manifold and the resonator/cat. sound wave reflections, and the potential for a standing wave, occur at ubrupt physical changes to the pipe -- a sudden diameter change or &quot;discontinuity&quot; as we say (thats what resonators, mufflers and cats do). I think you're saying that changing the back 3/4 of the system might change the resonance properties of the front 1/4. I don't yet see compelling data to support that, but you might be thinking of something I've missed. Based on the acoustic properties of pipes and standing wave theory, I'm not (yet) convinced that a substantial standing wave could exist within the entire length of the system (tip--&gt;resonator--&gt;manifold). The resonator/cat/muffler is supposed to break those things up, so I would tend to think that resonances from tip to cat would be distinct from resonances that exist from cat to turbo. In that regard, a cat back system would not (materially) affect the resonance properties upstream.<br><br>also, if a standing wave were to occur, it would by definition be a high pressure point at the &quot;endpoint&quot; in its wavelength-- the &quot;discontinuity&quot; or boundary such as the resonator and turbo itself. so a high pressure point would actually be an advantage as regards turbo spoolup because it's not physically possible for a boundary (like the turbo blades) to be a low pressure point in a standing wave.<br><br>Back to resonances in/near the downpipe between cat and turbo. Consider that straight pipe conversion is a different story -- but note that when guys change out their exhaust systems from stock to straight pipes, they are increasing the chances for tip-to-turbo resonances. Now there is no resonator or cat to break up standing waves, and note that these guys get good EGT reductions as well. that is, the cat-back guys don't have an EGT advantage (that I know of) over the full straight pipe guys. If anything, its the other way around, which I think is due to bulk air flow and the physics of moving lots and lots of air around.<br><br>The physics that describe EGT reduction has to do with moving lots of air molecules. They (the molecules, not the physicists) don't just sit around and cool off extemporaneously; they have to be expelled from the cylinders and pushed out. In other words, a hot air molecule must leave the cylinder and be relaced by a cooler one. Such air movment is best explained by bulk properties of air flow, and in the quntities we're talking about (14 cubic feet of air per second for modest boost levels) the only way to evacuate the molecules is to remove restrictions. I'm not saying resonances are not present -- they certainly are, but the small vibrational movements and instantaneous pressure differences (due to standing waves) upstream of the cat/resonator I don't believe will significantly affect the ability to move that much air out the tailpipe or across the turbo to be cooled by the intercooler. The speed of sound is many times faster than the average speed of the air molecules in the exhaust system, so indeed a standing wave can certainly exist during high volume air exchanges without affecting the aggregate flow itself or the cooling properties of that flow.<br><br>I can, however, see a potential system overkill or even an &quot;imbalance&quot;, if you will. would it not be true that changing only the exaust pipe (say, straight 4&quot; pipe from tip to turbo) would make the stock turbo and stock airbox less efficient? I mean -- from the standpoint of temperature differential across the turbo, You may be right here -- you don't want to cool off the exhaust manifold so much that you loose the optimium balance between air flow and pre-turbo EGTs. Suppose for example I take my stock 02 ETH + EZ box and I hang a straight pipe 4&quot; system tip to turbo. Well, I may get a 100 degree EGT reduction but is that the best measure of success?

BigBlue

Yall really need to get a life. This is very interesting reading though. Brings up a lot of questions.

TXRAM2

OMG ???