Science behind Exhaust ideal exhaust size

[MassiEk4]

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?

[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

[EKVTIR-T]

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

[curtis265]

Mugen has also.

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

[u mad?]

Hope u kunts are trolling.

[ceekay101]

Mugen has also.

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

Spoon > fork ?

[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...

[MassiEk4]

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...

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.

[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.

[MassiEk4]

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

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.

[MassiEk4]

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

Itd be impossible.

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

[MassiEk4]

Mugen has also.

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

Sure, if you are a millionaire!