The science of exhaust size diameter .......
Discussion
Nothing on tv tonight so having a surf on you tube and came across this interesting video and back to back Dyno testing on open vs 2.5 inch vs 3 inch exhaust systems
If you can't be arsed to watch......open exhaust wins followed by 3 followed by 2.5 lol
https://www.youtube.com/watch?v=_PVXvHkr-Vs&eb...
Makes me sort of glad I've got a 3" system with a 4" exit lol.....god it's going to be loud
If you can't be arsed to watch......open exhaust wins followed by 3 followed by 2.5 lol
https://www.youtube.com/watch?v=_PVXvHkr-Vs&eb...
Makes me sort of glad I've got a 3" system with a 4" exit lol.....god it's going to be loud
It's almost as complicated as cylinder head design !!
Basically flow is a function of cross section (=area of circle) which is pi*radius*radius, but then every bend has an effect, and the biggest restriction is the muffler(s), because they HAVE TO restrict flow to some degree to actually work, even the straight through ones.
2.5 inch = 4.9 square inches
3.0 inch = 7.07 square inches
3.5 inch = 9.62 square inches
4.0 inch = 12.56 square inches
So there's a much bigger difference than you might think, even 2.5 to 3.0 inch is almost 50% larger ....
Basically flow is a function of cross section (=area of circle) which is pi*radius*radius, but then every bend has an effect, and the biggest restriction is the muffler(s), because they HAVE TO restrict flow to some degree to actually work, even the straight through ones.
2.5 inch = 4.9 square inches
3.0 inch = 7.07 square inches
3.5 inch = 9.62 square inches
4.0 inch = 12.56 square inches
So there's a much bigger difference than you might think, even 2.5 to 3.0 inch is almost 50% larger ....
Edited by RCK974X on Tuesday 17th May 22:43
I hope this doesn't turn into a conversation about "Back pressure"..
...A good way to think about it is by representation of the human digestive system...I said..."Human" not "Mcvities"..Right you need a great big No.2...
"YOU DO NOT NEED BACK PRESSURE"...
you need to let it all out man....However saying this there will always be a restriction in an exhaust system ...
The shape of a mid or rear section exhaust box can have the desired effect ...But it can also ruin a note...Ill keep my ears open for a B52 SEAC arriving at the fest....
...A good way to think about it is by representation of the human digestive system...I said..."Human" not "Mcvities"..Right you need a great big No.2... "YOU DO NOT NEED BACK PRESSURE"... you need to let it all out man....However saying this there will always be a restriction in an exhaust system ...The shape of a mid or rear section exhaust box can have the desired effect ...But it can also ruin a note...Ill keep my ears open for a B52 SEAC arriving at the fest....
Zig, I'll go further than that ...
For best efficiency you don't WANT ANY BACK PRESSURE AT ALL !
But it's impossible to achieve in real life.
Even those straight pipes pointing upwards from a drag racer have back pressure. Not a lot, I agree, but it's there, honest.
When you add the passage in the cyl head, a few bends, even a welding ridge, etc....
Mufflers HAVE TO stop the 'pulse' nature of the exhaust, which is the major noise, and they do this by capturing those peaks, and that by it's very nature, causes some back pressure.
It's PHYSICS dude.......
For best efficiency you don't WANT ANY BACK PRESSURE AT ALL !
But it's impossible to achieve in real life.
Even those straight pipes pointing upwards from a drag racer have back pressure. Not a lot, I agree, but it's there, honest.
When you add the passage in the cyl head, a few bends, even a welding ridge, etc....
Mufflers HAVE TO stop the 'pulse' nature of the exhaust, which is the major noise, and they do this by capturing those peaks, and that by it's very nature, causes some back pressure.
It's PHYSICS dude.......
Diameter is not the whole story, big is not necessarily good. There is a significant pressure change when the velocity of the air changes suddenly - this will occur when the cross sectional area of the pipe changes; the effect is not so different to a bend in the pipe. Good exhaust design will maintain the same velocity throughout the system to avoid this. It does get difficult when you're merging pipes from 4-2-1, the changes have to be gradual, empirical data says a taper of 7:1 is the most severe you should use when changing cross sectional area.
It's a fascinating subject, Physics indeed! Most designs are still conceived using good old fashioned empirical (experimental) data; and then finessed using CFD (computers...). However even the computers struggle, as the boundary conditions are so difficult to define, and the air dynamics within a running engine system are extremely difficult to model. A Cray supercomputer is used to do this and it's left to run overnight.
It's a fascinating subject, Physics indeed! Most designs are still conceived using good old fashioned empirical (experimental) data; and then finessed using CFD (computers...). However even the computers struggle, as the boundary conditions are so difficult to define, and the air dynamics within a running engine system are extremely difficult to model. A Cray supercomputer is used to do this and it's left to run overnight.
I think that going from a smaller diameter into a larger one, even suddenly, is OK.
Going from larger to smaller is a big no-no.
After all that's what happens at the end of the tailpipe, you do from 4 inches to over a metre (if you think of the road as one side of a "pipe"), if that didn't work we'd all have to have infinite length exhaust pipes.
Going from larger to smaller is a big no-no.
After all that's what happens at the end of the tailpipe, you do from 4 inches to over a metre (if you think of the road as one side of a "pipe"), if that didn't work we'd all have to have infinite length exhaust pipes.
adam quantrill said:
I think that going from a smaller diameter into a larger one, even suddenly, is OK.
I'm not completely sure even that is always OK - you can get some really weird stuff going on with pressure waves and resonance where you get a kind of 'pressure block' thing. It is VERY complicated.As Hatter said, there's all sorts of odd effects caused by the fact that you don't get a smooth flow in the system, but a series of big pulses running down the pipes.
The whole subject of gas flow is complex - there's even an air flow meter based on turbulence (Karmann vortex)
Hi Adam,
Suddenly going from a small pipe to a large one is good for sound reduction (ie a silencer) but bad for pressure drop due to the reduction in air flow velocity; think of it as a line of people walking quickly through a door into a large room full of people that are moving slowly, the queue will back up into the doorway. You need to go with a 7:1 taper, Anything sharper than 7:1 allows the flow to break away from the side walls and provokes turbulence which reduces the effective diameter of the pipe.
You're right that going from large to small is a no-no; except again for some silencers, where that's what you want to do. You can minimise the pressure loss using a bell mouth entry to the smaller pipe but that will reduce the acoustic effect - it's all a compromise.
Suddenly going from a small pipe to a large one is good for sound reduction (ie a silencer) but bad for pressure drop due to the reduction in air flow velocity; think of it as a line of people walking quickly through a door into a large room full of people that are moving slowly, the queue will back up into the doorway. You need to go with a 7:1 taper, Anything sharper than 7:1 allows the flow to break away from the side walls and provokes turbulence which reduces the effective diameter of the pipe.
You're right that going from large to small is a no-no; except again for some silencers, where that's what you want to do. You can minimise the pressure loss using a bell mouth entry to the smaller pipe but that will reduce the acoustic effect - it's all a compromise.
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