Wanted to discuss Exhaust Thermodynamics (wisemen welcomed)....

[Boosted98gsx]

Alright, here's the deal. I've been running a 3" pressbent catback for over 2 years now. i'm running 12.4's with it as such, and haven't had any problems with it. The exhaust BLOWS out of my tailpipe (has a high velocity) but is cool by the time is does so.

I've taken my college level chemistry courses and i know the basics of thermodynamics. I know as air cools it becomes more dense and so takes up less room and at higher temps the molecules are further apart and are much more excited from the kinetic energy (thus why they move faster at higher temps).

Now, that out of the way, here's my question. Me and a friend were talking exhaust thermo today over the phone and he's convinced (for some reason) that a true, full 3" mandrel bent exhaust all the way back would be better for the car's performance. I dont agree. I think that having a true 3" catback would actually hinder performance because of the decreased exhaust velocity from the colder/denser exhaust after it (wouldn't it have less momentum?).

I think that the way i have it would actually help the car increase performance and the exhaust velocity exiting the car as the tighter piping would try to contain and keep the molecules excited and moving.

For arguements sake, theoretically, exhaust temps and kinetics would depreciate linearly, correct? So to maintain PERFECT velocity out of an exhaust system (if it was straight) you would need a cone that tapers down. So, my theory w/ the more "restrictive" catback makes sense.

Please, i didn't mean this to be a rhetorical statement. I would like some ppl who know about thermodynamics to come in here and correct me where (if) i'm wrong.

 

[Rob10_99]

ever see one of those funny car drag racers? they run(or at least when I saw a couple years ago) a header 4-1 collector, then it tapers down from 6" to 4" and back out. Im not sure if thats a pressure wave thing or something to do with the atoms like you are talking about.

good idea but I think the open exhaust is sticking around

 

[Boosted98gsx]

ever see one of those funny car drag racers? they run(or at least when I saw a couple years ago) a header 4-1 collector, then it tapers down from 6" to 4" and back out. Im not sure if thats a pressure wave thing or something to do with the atoms like you are talking about.

good idea but I think the open exhaust is sticking around

Well yes, granted no exhaust is better than any exhaust on a turbo'd car. But for those ppl that would rather run a full exhaust w/ less noise than an open d-pipe, that is what this post is for.

 

[Thomas91169]

rule of thumb, turbo'd cars like the least amount of backpressure possible. helps in spool time and top end flow. backpressure only helps create low end power. if we wanted low end power, wed all be driving supercharged mustangs.

while moving to a 3" mandrel bent exhaust system might not equate actual ET gains(though you never know), it would prolly make a dent in the butt dyno.

btw do you run no cat? could be why the escape velocity is so strong. i run a cat and i can stick my hand over the whole exhaust tip with little more than a slight put put sound from it trying to escape through my fingers, but my buddy who runs a cat delete i can barely keep my hand 2 inches away from the pressure.

 

[Defiant]

http://www.dsmtuners.com/forums/showthread.php?t=192811&highlight=wiseman

 

[Turbocharged]

Exhaust velocity is not the important thing here. The important thing is mass flow rate. You can easily change the velocity of your exhaust gas by changing the diameter of pipe of exhuast pipe (Bernoulli's equation), but your mass flow rate will remain constant throughout the exhaust. The idea is to find an exhaust setup that maximizes mass flow rate.

If you are looking to increase exhaust velocity, you need the smallest possible exhaust that will fit on your car. This is more of a fluid dynamics problem than it is a thermodynamics problem.

Larger exhaust = slower, less pressure
Smaller exhaust = faster, more pressure

 

[LessIsMore]

The goal when designing an exhaust system is to optimize turbine power output. Turbine power output is a function of two variables, mass flow rate and change in internal energy of the exhaust gases through the turbine.

Turbine Power=(Energy Change)(MassFlowRate)

Analyzing an exhaust system as a control volume, we can assume the mass of exhaust entering and leaving the system will remain constant per unit time. Therefore mass flow rate at the entrance of the exhaust must equal the mass flow rate at the exit.

Mass flow rate is porportional to density, velocity, and cross-sectional area:

m=pVA

Increasing the mass flow rate at the end of the exhaust will increase mass flow rate at the begining of the system, in our case through the turbine. An increase in cross-sectional area will result in a decrease in density and velocity according to bernouli's pronciple. Therefore maximizing mass flow rate is a balancing act.

The other factor in generating power through a turbine is energy change. The energy of exhaust gases at the turbine inlet minus the energy at the outlet determines this change in energy. The energy of exhaust gases increase with pressure and temeprature. Pressure differential is where the majority of the energy change occurrs through a turbine.
This is where the statement that "the least backpressure after the turbine will create the most power".

This statement is not completely correct. "The least backpressure after the turbine will create the greatest change in exhaust energy" would be the correct statement. Though too little backpressure from increased pipe diameter may adverslety effect the mass flow rate of exhaust gases.

In conclusion, to acheive maximum power output form a turbine one must optimize mass flow rate and energy change through the turbine. This is most easily accomplished by varying pipe diameter(cross-sectional area).

 

[96blacktalon]

http://www.dsmtuners.com/forums/showthread.php?t=192811&highlight=wiseman

Except he didn't give a nondescript title asking for help. Nor did he ASK for wisemen help. He quite clearly says "(wisemen welcomed)" letting them know that it is a topic they might be interested in. And anybody that actually wants to learn anything isn't going to not open the thread just because they think they're not smart enough b/c they're not wisemen. You learn from the advice of others more advanced on a topic than yourself. Boosted98 knew this and therefor asked EVERYBODY for input. He just also wanted to see if perhaps a "wiseman" would be interested in such a topic.

Sorry for interrupting your topic Boosted98, that just bothered me a bit.

 

[Boosted98gsx]

Sorry for interrupting your topic Boosted98, that just bothered me a bit.

Perfectly alright. Thanks for not making me say it

 

[1stgensleeper]

If the exhaust coming out of the back of your car is cool, than it is a 100% fact that you are not getting the most power possible out of it. If you remember the rules that govern all engine and motors, a simple way to measure efficiency is by measuring the difference in temperature that it creates for example in a steam engine if the steam goes in at extremely high temperatures and comes out at a low temperature it is running extremely efficiently (because a steam engine works by extracting the heat), these rules also apply to our car motors (But our motors make energy through the creation of heat), if your intake is taking in cold air (ambient) and your exhaust is putting out near ambient temp air, your losing efficiency. To get the most power you have to create the largest temp diff between intake and exhaust. All of this stuff can easily be related to when you take an NA car and turbo it, what happens...The EGT's go up thats why the more power you make the better cooling you need, but if modern metals could withstand those temps the engine would make more power the hotter it is ya dig

 

[desolateboosted]

Take piece of tube that narrows down on one end and blow into it. Take another tube that doesn't narrow down and blow into it. I'm pretty sure the tube that narrows down will be more restrictive than the one that is straight. On a turbo car you don't want that backpressure.

 

[SethA]

If the exhaust coming out of the back of your car is cool, than it is a 100% fact that you are not getting the most power possible out of it. If you remember the rules that govern all engine and motors, a simple way to measure efficiency is by measuring the difference in temperature that it creates for example in a steam engine if the steam goes in at extremely high temperatures and comes out at a low temperature it is running extremely efficiently (because a steam engine works by extracting the heat), these rules also apply to our car motors (But our motors make energy through the creation of heat), if your intake is taking in cold air (ambient) and your exhaust is putting out near ambient temp air, your losing efficiency. To get the most power you have to create the largest temp diff between intake and exhaust. All of this stuff can easily be related to when you take an NA car and turbo it, what happens...The EGT's go up thats why the more power you make the better cooling you need, but if modern metals could withstand those temps the engine would make more power the hotter it is ya dig

Some of what you have said is true, but some of this is incorrect. In a perfect world the exhaust would be as cold as possible. Ideally all of the energy produced in the combustion process would be converted to mechanical energy by the engine. The exhaust would be cold because there would be no energy left to heat it up.

From a thermodynamic standpoint engines can make more power at higher operating temperating. Conservation of energy dictates that the energy released from combustion has to go somewhere. A hot engine will absorb less of the energy than a cooler engine. If the combustion chamber was at the same temperature as the gasses it contains, no energy would be transferred into the combusiton chamber and then to the cooling system. Less energy would be lost to the radiator and more will be available to accelerate the car.

This is off the topic slightly. Sorry Boosted98gsx.

As for the tapered exhaust idea, I agree with turbocharged that mass flow rate is the controlling factor. Having the greatest pressure drop across the turbine will result in the highest flow. That is accomplished by having a constant diameter pipe after the turbine.

Seth

 

[1stgensleeper]

Some of what you have said is true, but some of this is incorrect. In a perfect world the exhaust would be as cold as possible. Ideally all of the energy produced in the combustion process would be converted to mechanical energy by the engine. The exhaust would be cold because there would be no energy left to heat it up.

From a thermodynamic standpoint engines can make more power at higher operating temperating. Conservation of energy dictates that the energy released from combustion has to go somewhere. A hot engine will absorb less of the energy than a cooler engine. If the combustion chamber was at the same temperature as the gasses it contains, no energy would be transferred into the combusiton chamber and then to the cooling system. Less energy would be lost to the radiator and more will be available to accelerate the car.

i know and thats why i made the comment about the meterials being able to handle it, in an ideal world the combustion chanber pistons oil and coolent wouldnt warm up and the air would so that everything in the car would be cold but the gasses contained within it would he hot, however that would represent a near 100% efficiency which just plain is not possible, sorry i was unclear i just kind of left it vague because sometimes when you get technical people get confused

 

[SethA]

i know and thats why i made the comment about the meterials being able to handle it, in an ideal world the combustion chanber pistons oil and coolent wouldnt warm up and the air would so that everything in the car would be cold but the gasses contained within it would he hot, however that would represent a near 100% efficiency which just plain is not possible, sorry i was unclear i just kind of left it vague because sometimes when you get technical people get confused

Cool, I think we are on the same page here. Really we would like the gases to be cold too though. In our mystical engine even the energy that was expended to heat up the exhaust gases would somehow get converted to mechanical energy helping us accelerate. The we would have 100% effieciency.

Seth

 

[Defiant]

He quite clearly says "(wisemen welcomed)" letting them know that it is a topic they might be interested in.

Well, so nice that he did. They never look unless invited.
Thank you so much for the scolding, too. Next time, PM it.

 

[Boosted98gsx]

As for the tapered exhaust idea, I agree with turbocharged that mass flow rate is the controlling factor. Having the greatest pressure drop across the turbine will result in the highest flow. That is accomplished by having a constant diameter pipe after the turbine.

Seth

I would think that you would need a pipe that is equally ratio'd in reducing diamter to the increasing density of the exhaust gases. If you have a constant size i would think that the air would stagnate and slow down rather than keep its speed and actually hinder the flow of the exhaust coming down the pipe.

Think of a river (although it has a "constant" temp). If you start opening up the river, to simulate cooling down of gases, the river slows down. Although it has a larger body to fill up, once it is filled up, wouldn't it technically reduce the flow rate and increase the water column upstream?

Please, correct me with examples like this if i'm wrong.

 

[DGajre777]

Well, so nice that he did. They never look unless invited.
Thank you so much for the scolding, too. Next time, PM it.

Do you have anything to add to this thread or are you just going to try to show off your 'moderator' power in here like all the other threads? WTF

And yeah, that link bothered me too If someone else did the same thing, you would close the thread. Too bad there isn't a 'Defiant' block on here.

 

[SethA]

I agree that the river would slow down if the channel got wider. But maybe instead of getting wider, imagine some of the river evaporating or maybe getting diverted into irrigation ditches. There is a smaller volume of water further downstream. The volume of water flowing past would decrease and the river would not backup.

Imagine making the channel getting narrower at the same rate as the river evaporates so that the depth stays constant. Next imagine the channel staying the same width. As the river evaporates the depth decreases. Doesn't it make sense that in this scenario the depth of the water at the head of the river would be lower in the constant width channel than the decreasing width channel?

In this model, the depth of the river is pressure in the exhaust pipe.

Does that sound right?

Seth

 

[suicidal2af]

I would think that you would need a pipe that is equally ratio'd in reducing diamter to the increasing density of the exhaust gases. If you have a constant size i would think that the air would stagnate and slow down rather than keep its speed and actually hinder the flow of the exhaust coming down the pipe.

Think of a river (although it has a "constant" temp). If you start opening up the river, to simulate cooling down of gases, the river slows down. Although it has a larger body to fill up, once it is filled up, wouldn't it technically reduce the flow rate and increase the water column upstream?

Please, correct me with examples like this if i'm wrong.

Well, you're definitely thinking, but your logic is a little off.

A widening river will have less velocity, yes. But overall flow -- gallons of water -- will be greater.

You want velocity/pressure before the turbine. After the turbine, it's going to create backpressure...which will slow the rate that the turbine can spin. Think of it this way: if you have 75psi of pressure pre-turbine, and you bring pressure in the o2 housing to 75psi, is the turbine spinning anymore? Reduce o2 housing pressure to 0psi, and suddenly that pressure in the manifold is going to equalize violently.

Back to the river analogy: Imagine a hydro power plant. It controls the flow of water through the turbines by closing off paths water can flow through -- effectively narrowing the dimension of "pipe" available for flow.

Now if they open up the floodgates, you'll flow a lot more water and make a lot more electricty(read: power).

Make anymore sense?

 

[Defiant]

Do you have anything to add to this thread or are you just going to try to show off your 'moderator' power in here like all the other threads? WTF

Yes, I'm so big and bad. I'd have no life if it wasn't for internet thuggery. If you'd take a look, you'd find I do more helping than sweeping-up around here. In fact, I do more of everything than anyone else here. Put your focus where you choose.

And yeah, that link bothered me too If someone else did the same thing, you would close the thread. Too bad there isn't a 'Defiant' block on here.

There's an "ignore" button on your User Control Panel. Please help yourself to it.

The next off-topic in this thread will lock it.

 

[LessIsMore]

You guys need to stop making comparisons between exhaust flow and water flow. Water is a virtualy incompressible fluid, meaning density does not change with volume. In this case mass flow is only dependant upon two factors, velocity and cross-sectional area where density remains constant.

Gas on the other hand is a compressible substance, meaning density will change with volume. Therefore mass flow is dependant upon density, velocity, and cross-sectional area. Maximizing mass flow in this case involves opitimizing the three different variables. Two large of a cross-sectional area will result in decreased density and velocity and may ultimately decrease mass flow.

 

[pboglio]

I would think that you would need a pipe that is equally ratio'd in reducing diamter to the increasing density of the exhaust gases. If you have a constant size i would think that the air would stagnate and slow down rather than keep its speed and actually hinder the flow of the exhaust coming down the pipe.

Think of a river (although it has a "constant" temp). If you start opening up the river, to simulate cooling down of gases, the river slows down. Although it has a larger body to fill up, once it is filled up, wouldn't it technically reduce the flow rate and increase the water column upstream?

Please, correct me with examples like this if i'm wrong.

I'd recommend you purchase "Scientific Design of Exhaust & Intake Systems" by Philip H. Smith. Maybe a college level Fluid Dynamics textbook too. Making assumptions on "fluid flow mechanics" is bound to set you in the wrong direction. I've not noticed a 3" exhaust making LESS horsepower than a smaller exhaust, quite the opposite.

 

[JimiHO 81]

You want velocity/pressure before the turbine. After the turbine, it's going to create backpressure...which will slow the rate that the turbine can spin. Think of it this way: if you have 75psi of pressure pre-turbine, and you bring pressure in the o2 housing to 75psi, is the turbine spinning anymore? Reduce o2 housing pressure to 0psi, and suddenly that pressure in the manifold is going to equalize violently.

This makes the most sense to me as far as creating power via turbo charger. To get the most power (pressure on the intake side) from the turbo you need it to spin at as high a velocity as possible. Given that we can not change the pressure on the engine side of the turbo (besides adding throttle), the next best way to increase the velocity would be to create a very low/vaccum pressure on the exhaust side of the turbine; so your goal for your exhaust would be to move as much pressure out of/to the back of the exhaust system. However how you would do that is beyond what you learn in New York State HS physics/chemistry so i cant really help ya there. Hope that makes sense to someone

 

[Boosted98gsx]

However how you would do that is beyond what you learn in New York State HS physics/chemistry so i cant really help ya there. Hope that makes sense to someone

There is a company that makes a "jet" exhaust injector that injects Co2 into the exhaust and will SPIN the exhaust to make a more laminar flow (kinda like the "turbonator" does for the intake, but it actually injects air into the exhaust to increase exhaust velocity).

 

[JimiHO 81]

Wow I have never heard of such a thing. It seems like that would create an extreme tempature differnece, to the point where i would worry about having one side of the turbo "cold" and the other at engine temp. This is just my guess cause whenever i used CO2 for my paintball gun the tank would get very cold if several shots were fired off quickly. Not only that, but as you are increasing the velocity of the gasses, the pressure at and behind the point of injection must spike, right? Once again this is jsut what iv come up with using my slight understanding of physics and such

Can you get a link to a web site or something about this CO2 injection, Im curious as to how it works. Thanks!