Science behind Exhaust ideal exhaust size

Have been thinking about this for a few nights lying in bed, can't sleep. :(

Basically in Fluid Mechanics, there is a law stating that what goes into a closed system (pipe) must come out, if the flow is incompressible! I am sure Everyone knows that one.
However the rate of flow i.e M (Mass flow rate)in = M (mass flow rate)out - (M = p (Density) X v (fluid velocity) X A (cross sectional area)), now since the flow is incompressible we can cross out p (density) on both sides of the Mass flow rate formula ^, as density remains constant.
When you cross out Density you are left with, v (fluid Velocity) X A (cross sectional area) = V which is volumetric flow rate.
If you think about it Vol rate in must = Vol rate out same as M(in) = M(out), so to keep the Vol rate constant, you must either increase the cross sectional area which will slow down the flow or do the opposite which will increase the velocity of the flow. Same reason why people tapper the openings of their Throttle bodies!

Also when you increase velocity at that point the pressure is lower than any other point in the system, causing the gasses or fluid behind to get sucked towards the velocity direction creating a chain reaction, that is why 4-1 headers work well at high RPM compared to 4-2-1.

Knowing that you can experiment with different diameter exhaust pipping to reduce Head losses, which is caused by pressure loss due to loss in velocity, and increase efficiency = more power!

Gonna try this with my Build after my uni exams.
Any thoughts?

have you done any reading? There are millions of explanation sof exhaust diameter sizing all over the place... but you're on the right track

After you decide where you want your power gain, you can decide what size you want

Companys like Spoon have already done all this research

Always good to use some initiative though and have a good ponder in bed or shower over these sciences

Mugen has also.

If you buy a spoon or mugen exhaust you can't go wrong.

Hope u kunts are trolling.

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Originally Posted by curtis265

Mugen has also.

If you buy a spoon or mugen exhaust you can't go wrong.

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Spoon > fork ?

Obviously a second year engineering student. Wait another 2 years until you learn a little more about heat transfer and gas dynamics to really answer your questions.

Also, I don't always believe that spoon, mugen, etc do a great job, it's a very sensitive part of the engine and is dependent on nearly everything in and around the engine. You can't make a one size fits all exhaust.

Keep up the research mate, however it's probably easy to technical for the average member on here...

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Originally Posted by Chr1s

Obviously a second year engineering student. Wait another 2 years until you learn a little more about heat transfer and gas dynamics to really answer your questions.

Also, I don't always believe that spoon, mugen, etc do a great job, it's a very sensitive part of the engine and is dependent on nearly everything in and around the engine. You can't make a one size fits all exhaust.

Keep up the research mate, however it's probably easy to technical for the average member on here...

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Yep, second year Mechanical engineering student, unfortunately I am going to transfer to civil after this year.

But I recon there might be some more power to be freed with this variable diameter exhaust system.

Yeah, but try to engineer one that that works properly, and isnt to expensive, while having soemthing that responds to revs.

Itd be impossible.

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Originally Posted by curtis265

have you done any reading? There are millions of explanation sof exhaust diameter sizing all over the place... but you're on the right track

After you decide where you want your power gain, you can decide what size you want

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I am not sure you understood what I was explaining above
I want two or more different diameters, to over come the head loss caused by the change in pressure = (F X L X p X v^2)/2D, ( f is calculated by working out Reynold's number, to find out whether the flow is turbulent or laminar) which is caused by the velocity of the exhaust gasses slowing down as it travels further away from the exhaust ports which is caused by the relative roughness of the pipe. To overcome this inefficiency, you must lower the pressure again by increasing velocity which is achieved by decreasing the pipe's diameter, VOl rate = v X a. You can actually calculate the extra work the motor has to do to over come the change in pressure with, W(engine) = v X (change in)Pressure, this work in (Kw) is the power loss of the engine loses because of the exhaust design.

Also to correct you earlier, which I see you have done by editing your post, you cannot use Bernoulli's equation as the flow is viscous, the head loss in this situation is caused by the relative Roughness of the pipe, which causes a (Change in) Pressure etc etc.

To answer your question I have done my reading, but not on Forums. :)

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Originally Posted by trism

Yeah, but try to engineer one that that works properly, and isnt to expensive, while having soemthing that responds to revs.

Itd be impossible.

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I'll give it a try, I am always up for a challenge and got some piping lying around.

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Originally Posted by curtis265

Mugen has also.

If you buy a spoon or mugen exhaust you can't go wrong.

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Sure, if you are a millionaire!

Rightio, I had a feeling you were an engineer, your writing is unclear.

I have faith in your technical understanding but from your original post i had no idea what your intention was.



You've also missed that the exhaust flow is probably not necessarily going to be viscous - most probably turbulent at high RPM and viscous at idle.

and it'll be pulsing too, not constant

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Originally Posted by MassiEk4

Sure, if you are a millionaire!

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lol i'm just mocking fanboys, don't worry.

There are no formulas to predict the performance or optimise the performance of the design of a header. This is because the exhaust gases are moving at, or have portions moving at transonic speeds. At these speeds there’s no way of knowing exactly how the exhaust gases are going to behave. The pressures can increase and decrease at the same time inside a pulse moving through a high velocity point. There are just too much dynamic pressures and density events happening inside a proper exhaust system to even try and apply maths formulas to. You can try… but it isn't going to help your header design. They call it a dark art for a reason…

My advice to you or anyone building/designing an exhaust header would be to spend most your time concentrating on the collector. Remember choke is GOOD! Choke = power!

IF you get the collector right, which is very difficult to do even with alot of experience, you can use long primaries to your advantage. Don’t worry about all that equal length primaries nonsense.

By the way, do you know how to weld or fabricate? It would make building the header a lot easier.

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Originally Posted by mrwho

There are no formulas to predict the performance or optimise the performance of the design of a header. This is because the exhaust gases are moving at, or have portions moving at transonic speeds. At these speeds there’s no way of knowing exactly how the exhaust gases are going to behave. The pressures can increase and decrease at the same time inside a pulse moving through a high velocity point. There are just too much dynamic pressures and density events happening inside a proper exhaust system to even try and apply maths formulas to. You can try… but it isn't going to help your header design. They call it a dark art for a reason…

My advice to you or anyone building/designing an exhaust header would be to spend most your time concentrating on the collector. Remember choke is GOOD! Choke = power!

IF you get the collector right, which is very difficult to do even with alot of experience, you can use long primaries to your advantage. Don’t worry about all that equal length primaries nonsense.

By the way, do you know how to weld or fabricate? It would make building the header a lot easier.

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Those are very good points on header design, but my main focus is on improving efficiency after the headers.

I agree that it is hard to predict the behavior of exhaust gases leaving the ports with a high Mach Number, but the formulas and theories can still be applied to help understand and improve designs, to make them flow more efficiently.

You say, Choke = More power, don't you think that is linked to Vol rate = v x A(cross sectional area) ?

Well I have been wielding and fabricating since I was 12 years old, but I am not at all experienced in to make my own headers (I could design one using calculations, provided with the correct data);).

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Originally Posted by curtis265

Rightio, I had a feeling you were an engineer, your writing is unclear.

I have faith in your technical understanding but from your original post i had no idea what your intention was.



You've also missed that the exhaust flow is probably not necessarily going to be viscous - most probably turbulent at high RPM and viscous at idle.

and it'll be pulsing too, not constant




lol i'm just mocking fanboys, don't worry.

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I wouldn't call myself an engineer, I still got a long way to go.
But from my understanding the exhaust gasses will most defiantly be viscous, and turbulent at low RPM and laminar at higher RPM.

i thought laminar meant that viscous forces > momentum forces resulting in a smooth flow, and that turbulent flow is was where the momentum dominated the viscous forces

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Originally Posted by curtis265

i thought laminar meant that viscous forces > momentum forces resulting in a smooth flow, and that turbulent flow is was where the momentum dominated the viscous forces

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Damn, what am I saying? "and turbulent at low RPM and laminar at higher RPM"
Sorry it is the other way around, Turbulent at higher RPM! Re = (p x v x L)/dynamic viscosity, if Re > 4000, flow is turbulent, if Re < 2300 flow is laminar.

I'll post some info soon, abit of false info going on..

Half of you are talking about static and half are talking about transient conditions, and no, it's not a black art either. That's what idiots say about camshaft design too, pick up a book and read it.

I also don't understand how engineers speak/write in a manner unclear?!

I dont understand how someone thats in the 2nd year of an engineering degree can come in and think they can rewrite the books on exhausts.

I see it constantly, on all sorts of car forums.

Guys, the research and development on this sorta stuff has been going on for over half a century, dont come in and try and think you know more than people who spend millions designing this stuff.

Just read up information thats already out there.

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Originally Posted by trism

I dont understand how someone thats in the 2nd year of an engineering degree can come in and think they can rewrite the books on exhausts.

I see it constantly, on all sorts of car forums.

Guys, the research and development on this sorta stuff has been going on for over half a century, dont come in and try and think you know more than people who spend millions designing this stuff.

Just read up information thats already out there.

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Mate, no one is trying to write a book here, the book has already been written I am trying to use the info to my advantage.
If I knew everything I wouldn't come on a forum and share this stuff I would publish a Thesis.

I am sure a company who has spent millions of dollars on research would not share their findings with the public.
Don't confuse me with other people on other forums or be offended by my Idea. Just wanted to know what other people thought on this or could correct me on, to help me with this idea.

It just seemed like you were trying to reinvent the wheel

Do you know much about late model ferrari exhaust systems?

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Originally Posted by Chr1s

I'll post some info soon, abit of false info going on..

Half of you are talking about static and half are talking about transient conditions, and no, it's not a black art either. That's what idiots say about camshaft design too, pick up a book and read it.

I also don't understand how engineers speak/write in a manner unclear?!

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Good, please do. It seems like a good ides, but I don't know enough to 100% back it up.

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Originally Posted by curtis265

It just seemed like you were trying to reinvent the wheel

Do you know much about late model ferrari exhaust systems?

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Yes I guess so, but wheels were made of wood and stone in the early days, and now days it is far more refined and efficient with light weight alloys.
Just trying to refine the exhaust to be more efficient.

Late model Ferrari exhaust system? Never was a fan of Ferrari, but Please let me know more about that, does it use the same idea?

Bit of a strange defense mechanism going on lol..no offense but the reaction you are looking for is not to be found on this forum, you better off bringing this up on physicsforums or engtips.

Also, an undergraduate Thesis is far from an "expert" publication

Out of curiosity, what uni do you go to?

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Originally Posted by Chr1s

Bit of a strange defense mechanism going on lol..no offense but the reaction you are looking for is not to be found on this forum, you better off bringing this up on physicsforums or engtips.

Also, an undergraduate Thesis is far from an "expert" publication

Out of curiosity, what uni do you go to?

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"an undergraduate Thesis is far from an expert publication" That is exactly why I am on here. LOL
I am going to RMIT.

I am not sure about physics forums, I recon there would be a problem with people on that forum not seeing how the idea would apply to a car.
I thought that since there are people who are experienced with cars they could also have a better understanding than me regarding this stuff; so I can learn more off them or improve/correct this I idea I have.

Instead of going so technical bro and coming on all big shot [Just saying what most others would feel when they read your thread man, no hate] check out what others have said and the discussions that ensued.

I'm no engineering student [Switched from Civil to Finance after 2yrs], don't need to go screw ya head over it man...

Science behind Exhaust design? Flow v Backpressure for the absolute basics... For example [Disclaimer: Not 100% so yeah...]

For Small NA: For most application, you want to retain a certain amount of backpressure to ensure that you have low-mid range torque otherwise you will lose whatever torque the engine can muster and if it really is too big ie too free flowing, then you'll lost power too!

eg - Majority of the hypo JDM 1.6L - 2.0L NAs run 60.5mm centre pipe while the ends are smaller than 100mm.

Too big? Saw the result in practice and on the dyno... Kid had a nice custom system, 60.5mm with a 90mm tip, produced some ~120PS @ the wheels like the rest of us, switched to a HKS Muffler made for a Supra, definitely too big and the result? Loss of 15PS+, loss 20Nm+ and the curve had much less area than the rest of us. That's how sensitive small NA engines are...

Turbo: Bigger usually is better [within reason obviously], much less emphasis on retention of backpressure [unlike NA, force induction has mechanical aid so lack of torque low down not much of an issue]

As others have talked about, pulse, scavenger effect, etc is more to do with the path and shape etc...

Anyway, no need to go all physics 101 man =)

Well, I don't want to burst your bubble, alot of the guys on here are very educated but not exactly educated on literature or on an academic basis such as ourselves, this is not to say that we know more than them and they know nothing. At times it's quite the contrary... at your level of studies I wouldn't come on here and fight with other guys, you will find yourself digging a hole because they don't like someone coming here and derivating the maths behind their engines, it's a shame but it's a fact of life (for forums lol), don't know why.

I have some real advice for you, stop wasting your time on here (not that it's a bad place, but it's bad for what you are trying to do), go down to the workshop at uni, introduce yourself to the FSAE guys there and get on the powertrain team. You will find yourself in a puddle of mud quick smart and you can have a ball designing the exhaust for their IC car and not use any of your money!!

PS - don't switch to civil, stick with mechanical.

Edit: http://www.fsae.rmit.edu.au/ - see you guys in Melbourne in 30 days!

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Originally Posted by Chr1s

Well, I don't want to burst your bubble, alot of the guys on here are very educated but not exactly educated on literature or on an academic basis such as ourselves, this is not to say that we know more than them and they know nothing. At times it's quite the contrary... at your level of studies I wouldn't come on here and fight with other guys, you will find yourself digging a hole because they don't like someone coming here and derivating the maths behind their engines, it's a shame but it's a fact of life (for forums lol), don't know why.

I have some real advice for you, stop wasting your time on here (not that it's a bad place, but it's bad for what you are trying to do), go down to the workshop at uni, introduce yourself to the FSAE guys there and get on the powertrain team. You will find yourself in a puddle of mud quick smart and you can have a ball designing the exhaust for their IC car and not use any of your money!!

PS - don't switch to civil, stick with mechanical.

Edit: http://www.fsae.rmit.edu.au/ - see you guys in Melbourne in 30 days!

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Thanks man, it is really frustrating when you try to explain something or your ideas and people taking offense and calling you a douche for expressing you ideas. :(

In no way am I trying to show that I know so much, in fact I know nothing and thought I could learn more off someone on here.

I had been wanting to do mechanical engineering ever since I knew it existed, but looking at the news and hearing from family friends about the lack of Jobs for mech engineers and even chemical engineers, I am a little worried, so I would rather do Civil and have a solid Job that can financially support my passion for cars and such. Who knows I might go back and pick up mech engineering once I work for a few years.

No way! Don't listen to people who "can't find jobs" - finding work is entirely up to yourself and you can't base your direction/career because someone else can't get a job. I have friends who struggle getting work and some have 3 or 4 offers coming at them.

Not to bash the FSAE drum again, but do FSAE and your chances of finding work will increase dramatically, go ask the guys who have done FSAE before. Our team leader last year was offered every single job he had an interview for - hard to find work apparently?

Every career right now is doing it hard.. stick to what you love mate.

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Originally Posted by Chr1s

No way! Don't listen to people who "can't find jobs" - finding work is entirely up to yourself and you can't base your direction/career because someone else can't get a job. I have friends who struggle getting work and some have 3 or 4 offers coming at them.

Not to bash the FSAE drum again, but do FSAE and your chances of finding work will increase dramatically, go ask the guys who have done FSAE before. Our team leader last year was offered every single job he had an interview for - hard to find work apparently?

Every career right now is doing it hard.. stick to what you love mate.

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I pm'd you more about this.

id love for your text books to find there way to my house lol
your always going to get the mechanic v engineer animosity so take it with a grain of salt, your heads in the right place but theres alot of by products created from different varients in exhaust design, if you a really looking to get into the exhaust design start from the newest design of car exhaust and start reverse engineering and reading up on how different problems are resolved, someone mentioned transonic speeds as an example, now there are car campanies spending millions of dollars and using teams of engineers looking into things like that and really without the proper tooling or equipment at your desposal its going to be hard.
theres always going to be new ideas getting thrown into the market and it will never be a 100% efficient exhaust throughout the exhaust but if you pick a point at which you want to start ie high rpm, torque etc you wil find how differet setups work and what there draw backs and keep trying to push the envalope untill your resonably happy.
so keep studying and keep playing around, i quote my old electronics teachers " you know the formulas, now prove it to yourself "