I'm sure these are the very kinds of discussions that go on with the engineers who design these wonderful trucks. Physics is a good thing to understand; the better you understand it, the less you spend needlessly on experimentation. Every time you mod something in the power train, you are changing the operating specs of the vehicle. Since this costs money, it would be good to consider whether you know what the results will be before you waste that money. It's all economics, man...

 

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>Your argument about the mass of air molecules moving is valid in my opinion. I have experienced buildups of shockwaves across mufflers that caused restrictions to flow that were much higher than for a flow of the same mass, pressure and temperature without the vibrations. <br><br>I did not want to say that changing the rear 1/4 of the system could change resonance properties of the 1st quarter- but if we are allowed to see the turbo as a discontinuity, and then the resonator as the next then it would be valid (IMHO) to assume that a change in the properties of the rear 3/4 can change the delta p in the turbo. I thought that if a pressure spike coming back from the open end ( or the resonator) hits on a spike from the exhaust manyfold you have less delta p to drive the turbine than when the pressure spike from the exhaust manyfold does not hit on a pressure spike. <br>In your last paragraph you state something about an imbalance- I do not think that this will occur. Basically I believe that if we were to reduce EGT by getting so much more air (for a given load )that the turbo would spin down, we would get rid of this problem because there is less air with the turbo going slower. <br><br>Hmm , pondering and thinking...<br><br>AlpineRAM

 

Geez, I think my parents were right. I SHOULD have gone to College. I am now convinced I do not need to know any more about the size of my exhaust pipe. It's only 3'' but it works well and makes me happy. Jim

 

Two things I have to add are (and I will make it short ) 1. Most reason stainless is not used for high performance exhaust systems is because stainless dissipates heat at a much slower rate than regular steel and hot air moves slower which doesnt flow as freely as cool air. The only air that needs to stay warm is close to the exhaust valve so the valves dont cool rapidly and warp. 2. Turbo motors dont have the exhaust pulses that naturally asperated motors have. The turbo breaks them up. One reason I think 4&quot; systems allow cooler EGT's is because the fact that the turbo exit size is 3&quot; and going to 4&quot; makes for an expansion area. When air expands it cools which inturn moves faster. I know this is kind out there but look at how a two stroke exhaust is built. The massive expansion chamber works as a vacuum to pull the exhaust out of the cylinder. I belive the reason that adding 4&quot; to only the back half of the exhaust doesnt give the lower EGT's guys are after is because the heat the resonator or muffler holds. Guys mentioned a ways back the volume of air the 3&quot; system is capable of flowing in comparision to the ci. size of the motor.<br><br>Cant find the first post but this will do.<br><br>This is a way to figure it but you must realize that all 6 pistons are not firing at the same time. They go in a balanced order. Which means that, the exhaust only has to flow the air produced by cylinders in the firing order. Semi's have HUGE displacements and they use 5&quot; exhaust. If this theory was true then they should be using 8-10&quot; systems! ???<br><br>OK, so I cant be short!!! :-X

 

;D Just got the 2k ctd back from the muff shop with the 4 inch system. to much noise 2K RPMS INSIDE THE CAB.need to quiet it down. pulled the 30ft rv home at the same time can go up a long bridge in od, 3% grade that i had to do in third b4.thats a good improvment.seems like the egt's are lower to.i like the sound but want to have it quieter in the cab any idea's besides gettin the sound deadener for under the carpet?

 

[quote author=jigdog link=board=7;threadid=14778;start=15#139159 date=1053147238]<br> ;D Just got the 2k ctd back from the muff shop with the 4 inch system. to much noise 2K RPMS INSIDE THE CAB.need to quiet it down. pulled the 30ft rv home at the same time can go up a long bridge in od, 3% grade that i had to do in third b4.thats a good improvment.seems like the egt's are lower to.i like the sound but want to have it quieter in the cab any idea's besides gettin the sound deadener for under the carpet?<br>[/quote]<br><br>Yeah, a muffler.<br><br>Get something like the Walker DynoMax round welded race muffler (straight through). Or be different, like me , and get a FlowMaster.<br><br>I think the pipe acoustics can be explained fairly simply- pipe organ. Big diameter pipe = low note.<br><br>What's that old formula? When you double the diameter, you increase the cross sectional area by a factor of four.<br><br>BTW, who says cold exhaust gasses are faster? I've always thought the idea was to keep the heat in to maintain velocity. Ala header wrap, HPC coating, etc...<br><br>Lastly, you can't tell me there aren't any appreciable pulses to the diesel exhaust. If that were the case, there would just be a whooshing sound of air coming out the pipe.

 

An interesting thing to do would be to test some different systems on one truck. Starting off with a stock system, going to a straight through muffler on stock diameter etc. Recording all the parameters for each combo. <br>This would be a nice study to do for somebody who has the time and cash to spend <br>One of the main advantages that I see in stainless 4&quot; or even 5&quot; exhausts is that they look cool ;D <br>I already mused about the construction of an expansion chamber right behind the turbo. Some problems like available space, heat to the PCM and air intake kept me from doing it. <br>Another interesting test would be to move the resonator from its stock position somewhere closer to or further from the turbo and watch what happens. <br><br>Regarding the dreaded drone we could start a poll about it: <br>Which engine, which cab-bed combo, which mods. I think the drone may have something to do with the wheelbase....<br><br>AlpineRAM

 

There is absolutely no reason to go with a 4&quot; exhaust (except for the cool sound) on a CTD when under 350 HP.<br><br>3&quot; pipe has a cross sectional area of 7.065&quot;<br>4&quot; pipe has a cross sectional area of 12.56&quot;<br><br>this is a difference of 77%<br><br>Katoom, I have to disagree with your previous statement about hot vs. cold gas flow. <br><br>a hot gas flows easier than a cool gas. The reason is this.<br><br>Hot gas molecuels are moving and spinning very rapidly. Their molecular colisions are elastic...meaning that they recoil with almost the same energy. As a gas cools these colisions become more inelastic, and because of this gas flow becomes more viscous. More energy most be expended to move a cool gas than a hot gas. <br><br>Wrapping the downpipe to keep the gases from cooling off is a great idea! I have the first four feet of mine wrapped.<br><br>I understand how a Two Cycle expansion chamber works with respect to scavenging, and pressure waves but this annalogy can not be used with a turbocharger.<br><br>The change in diameter from 3&quot; to 4&quot; aft of the turbo elbow does not create a venturi. A venturi has a reduction in diameter first..this reduction raises gas velocity, and according to Bernoulli &quot;as the velocity of a fluid increases, its pressure decreases&quot; the faster moving gas is at a reduced pressure....but only at the smallest diameter of the venturi....NOT the turbine outlet like some exhaust vendors would like us to believe. &gt;<br><br>The 3&quot; to 4&quot; diverget section of a 4&quot; downpipe does not lower the pressure at the turbine outlet. The only way to create a low pressure area at the turbine outlet is to<br><br>1. put a vacuum pump on the tail pipe<br>2. have the exhaust exit the truck where a low pressure area exists while it is moving at a high tate of speed.

 

Maybe I should have stayed awake during physics. I don't care what yall say about 3&quot; and 4&quot; exhaust. Where I live, BIGGER is always BETTER. ;D ;D

 

my new 4 inch and magna flow sounds great and it has boosted the power most notedly at the lower end must be the quicker turbo spin-up

 

<br><br>Who said anything about a venturi??? Thats going in the other direction of what were looking for! Venturies deal with velocity and turbo charged or blown engines dont need that. Thats why we dont need to mess with the cam or head porting unless you are going out of the flow ranges of the motor (BIG power).<br><br>And yes the turbo does eliminate the pulses of the engine. <br> Remove the turbo and you will understand the difference. You &quot;are&quot; hearing a whooshing sound, if you will. Just cause you hear sound waves doesnt mean you feel the pulses of air exiting the pipe. <br><br> <br>Yes, its true about cool air flowing faster than hot air. They used to wrap headers because of the burn factor when working on the motor, coat them for looks and corrosion, not for performance reasons! And they dont wrap anymore since the intense heat was detroying the pipes!!! Do you notice any gains with your wrapped down pipe? If this was the case then wrap the whole system. <br>And yes expanding air does cool itself. Look at our cylinders for example. They dont need spark plugs, right? WHY...because the air being compressed by the piston gets really really HOT!!! Look at the turbo...really HOT air comes out of the compressed turbine, thus the need for a cooler. Air expanding allows the molecules to slow and inturn be cooler. Same for a supercharger for those of you saying the heat comes from the exhaust. <br><br>I realize this and I wasnt trying to say we could do this on the Cummins, I was just showing how expanding air pulls the air behind it along.<br><br>Dont get me wrong here. I agree big exhaust is a good thing. But I dont think anything more than straight 3&quot; for anyone under 400hp is not necessary unless you like the sound.<br>Now I forgot the reason of this post.

 

[move]YOU GUYS ARE A TRIP!!!!![/move]

 

So far I am following most of what has been discussed here. Highly technical and I love it! It is for reasons like this thread that I enjoy this forum. Keep it up. Not necessary for me to add my .02 technically as you guys are covering most (all?) of the bases.

 

I had the wife out in the ctd today and she didn't think the new exhaust was any louder than the stock so she's happy.the more i drive it the more i like it sound.I notice that the egts go down faster when it is in park now waiting to shut it off.