Can someone explain exhaust back pressure to me?
Discussion
For a turbo, you want none; that's why turbo cars have such huge exhausts. Back pressure will annoy the turbine which is undesirable.
For N/A and supercharged cars the question is a bit more nuanced. Low back pressure is generally good, but a bit of back pressure allows the scavenging effect to work, which can be worth a few ponies if it's managed properly.
For N/A and supercharged cars the question is a bit more nuanced. Low back pressure is generally good, but a bit of back pressure allows the scavenging effect to work, which can be worth a few ponies if it's managed properly.
frosted said:
If you don't mind, I have a question. Which bit of the exhaust system creates back pressure and what does a baffle do?
Any part of the exhaust system that restricts flow causes a pressure rise. Baffles disrupt the flow of the exhaust gas, causing it to be turbulent, which reduces resonance.You'd be best off posting this in Engines and Drivetrain, OP.
Basically (and probably wrong in some detail), back pressure is a bit of a misleading term when it comes to NA exhaust tuning. Rather, you set up standing sound waves in the pipe, which when done correctly, give a low pressure area just outside the exhaust valve, which helps scavenge the cylinder. The length and size of the pipe affects the rpm at which this occurs.
Basically (and probably wrong in some detail), back pressure is a bit of a misleading term when it comes to NA exhaust tuning. Rather, you set up standing sound waves in the pipe, which when done correctly, give a low pressure area just outside the exhaust valve, which helps scavenge the cylinder. The length and size of the pipe affects the rpm at which this occurs.
You always want the lowest pressure possible in the relevant branch of the manifold as the exhaust valve opens as this will allow as much of the contents of the cylinder to be ejected as possible before the valve closes, thus allowing more oxygen to be drawn into the cylinder on the next intake stroke.
As I understand it, well tuned NA or SC engine exhausts use the kinetic energy of the exhaust gas from the previous cylinder to create a partial vacuum in the manifold at exactly the right moment (this is called exhaust gas scavenging); this isn't really possible on a turbocharged engine because the turbo restricts the exhaust gas flow too much. Thus in a turbocharged car, you want the air to simply flow as freely as possible to the exhaust outlet; however, for other engines back-pressure is used as part of the tuning technique to align the pulses correctly, so fitting a freer flowing exhaust to a N/A car can actually reduce power.
ETA: As a side note, I think this is part of the reason that Imprezas sound as they do - if they were NA, they would need the manifold headers to be of equal length in order for scavenging to work consistently. As they are turbocharged, Subaru couldn't use scavenging anyway, so they could make the headers unequal in length, meaning that gas from different cylinders has to travel a different distance to the exhaust tips, creating that stereotypical off-beat thrum. Theoretically, I see no reason that you couldn't get a turbocharged inline-4 to sound like a Subaru by artificially fiddling with the manifold branch length.
As I understand it, well tuned NA or SC engine exhausts use the kinetic energy of the exhaust gas from the previous cylinder to create a partial vacuum in the manifold at exactly the right moment (this is called exhaust gas scavenging); this isn't really possible on a turbocharged engine because the turbo restricts the exhaust gas flow too much. Thus in a turbocharged car, you want the air to simply flow as freely as possible to the exhaust outlet; however, for other engines back-pressure is used as part of the tuning technique to align the pulses correctly, so fitting a freer flowing exhaust to a N/A car can actually reduce power.
ETA: As a side note, I think this is part of the reason that Imprezas sound as they do - if they were NA, they would need the manifold headers to be of equal length in order for scavenging to work consistently. As they are turbocharged, Subaru couldn't use scavenging anyway, so they could make the headers unequal in length, meaning that gas from different cylinders has to travel a different distance to the exhaust tips, creating that stereotypical off-beat thrum. Theoretically, I see no reason that you couldn't get a turbocharged inline-4 to sound like a Subaru by artificially fiddling with the manifold branch length.
Edited by kambites on Thursday 4th October 11:26
http://www.autozine.org/technical_school/engine/In...
Edited by robmlufc on Thursday 4th October 11:31
What kambites said (which seems to often be the case when it comes to powertrains!). It's a fairly common conception that back pressure is bad, with the idea being that it stops exhaust flowing effectively out of the cylinder and robs you of cylinder space for the fresh charge. However, it's not as simple as that, and while having no back pressure is better than having loads right by your cylinders' exhaust valves, what you actually want to do is shape the manifold (and engine timing) to cause some pressure elsewhere, and a vacuum near the valves. As there's only going to be a finite amount of gas in the exhaust, deliberately raising the pressure in one place reduces it in others, which is beneficial when done correctly.
If you make the exhaust flow more freely, yes you get less obstruction to the exhaust gas escaping, but you also lose the lower-pressure vacuum near the cylinders, and don't scavenge all the exhaust gas out as effectively. Resonance waves exist in the manifold (this applies to the intake one too), and are very finely tuned so that each cylinder's exhaust gas assists the next one in scavenging more thoroughly. Buggering around with it makes you unlikely to achieve the same benefit.
On a turbocharged car, as has been said, you cannot scavenge effectively anyway so needn't worry. But for naturally aspirated engines, while "too much" or "incorrectly placed" back pressure is bad, do not think removing it completely is the best way forward!
If you make the exhaust flow more freely, yes you get less obstruction to the exhaust gas escaping, but you also lose the lower-pressure vacuum near the cylinders, and don't scavenge all the exhaust gas out as effectively. Resonance waves exist in the manifold (this applies to the intake one too), and are very finely tuned so that each cylinder's exhaust gas assists the next one in scavenging more thoroughly. Buggering around with it makes you unlikely to achieve the same benefit.
On a turbocharged car, as has been said, you cannot scavenge effectively anyway so needn't worry. But for naturally aspirated engines, while "too much" or "incorrectly placed" back pressure is bad, do not think removing it completely is the best way forward!
kambites said:
You always want the lowest pressure possible in the relevant branch of the manifold as the exhaust valve opens as this will allow as much of the contents of the cylinder to be ejected as possible before the valve closes, thus allowing more oxygen to be drawn into the cylinder on the next intake stroke.
As I understand it, well tuned NA or SC engine exhausts use the kinetic energy of the exhaust gas from the previous cylinder to create a partial vacuum in the manifold at exactly the right moment (this is called exhaust gas scavenging); this isn't really possible on a turbocharged engine because the turbo restricts the exhaust gas flow too much. Thus in a turbocharged car, you want the air to simply flow as freely as possible to the exhaust outlet; however, for other engines back-pressure is used as part of the tuning technique to align the pulses correctly, so fitting a freer flowing exhaust to a N/A car can actually reduce power.
ETA: As a side note, I think this is part of the reason that Imprezas sound as they do - if they were NA, they would need the manifold headers to be of equal length in order for scavenging to work consistently. As they are turbocharged, Subaru couldn't use scavenging anyway, so they could make the headers unequal in length, meaning that gas from different cylinders has to travel a different distance to the exhaust tips, creating that stereotypical off-beat thrum. Theoretically, I see no reason that you couldn't get a turbocharged inline-4 to sound like a Subaru by artificially fiddling with the manifold branch length.
And there was me thinking it was because they're a flat four, like the Beetle of old. Which sounds similar. As does an Alfasud for that matter. Gosh, every day's a school day.As I understand it, well tuned NA or SC engine exhausts use the kinetic energy of the exhaust gas from the previous cylinder to create a partial vacuum in the manifold at exactly the right moment (this is called exhaust gas scavenging); this isn't really possible on a turbocharged engine because the turbo restricts the exhaust gas flow too much. Thus in a turbocharged car, you want the air to simply flow as freely as possible to the exhaust outlet; however, for other engines back-pressure is used as part of the tuning technique to align the pulses correctly, so fitting a freer flowing exhaust to a N/A car can actually reduce power.
ETA: As a side note, I think this is part of the reason that Imprezas sound as they do - if they were NA, they would need the manifold headers to be of equal length in order for scavenging to work consistently. As they are turbocharged, Subaru couldn't use scavenging anyway, so they could make the headers unequal in length, meaning that gas from different cylinders has to travel a different distance to the exhaust tips, creating that stereotypical off-beat thrum. Theoretically, I see no reason that you couldn't get a turbocharged inline-4 to sound like a Subaru by artificially fiddling with the manifold branch length.
Edited by kambites on Thursday 4th October 11:26
V8mate said:
There's a reasonable body of opinion/evidence which shows that an E36 loses bhp when de-catted. This is surely simply a removal of a 'blockage'.
The outcome has always 'baffled' me though, so I'd be keen to understand what's going on in them there pipes.
I'd guess that the removal of the restriction increases the exhaust gas flow rate marginally, which puts the scavenging pulses out of sync with the exhaust valves. The outcome has always 'baffled' me though, so I'd be keen to understand what's going on in them there pipes.
V8mate said:
There's a reasonable body of opinion/evidence which shows that an E36 loses bhp when de-catted. This is surely simply a removal of a 'blockage'.
The outcome has always 'baffled' me though, so I'd be keen to understand what's going on in them there pipes.
Someone designed them properly..The outcome has always 'baffled' me though, so I'd be keen to understand what's going on in them there pipes.
Which is very irritating, actually
Tyre Smoke said:
And there was me thinking it was because they're a flat four, like the Beetle of old. Which sounds similar. As does an Alfasud for that matter. Gosh, every day's a school day.
I'm not certain, but I think they all run unequal length headers (I'm guessing VW didn't understand scavenging in the 40s, no idea about the Alfa). I can see no reason that the layout of the engine would have any effect on the noise in this case - it still makes two equally spaced bangs per revolution of the engine.Go and listen to a GT86; it sounds much the same as a normal I4 because it has equal length headers.
Edited by kambites on Thursday 4th October 11:47
kambites said:
You always want the lowest pressure possible in the relevant branch of the manifold as the exhaust valve opens as this will allow as much of the contents of the cylinder to be ejected as possible before the valve closes, thus allowing more oxygen to be drawn into the cylinder on the next intake stroke.
As I understand it, well tuned NA or SC engine exhausts use the kinetic energy of the exhaust gas from the previous cylinder to create a partial vacuum in the manifold at exactly the right moment (this is called exhaust gas scavenging); this isn't really possible on a turbocharged engine because the turbo restricts the exhaust gas flow too much. Thus in a turbocharged car, you want the air to simply flow as freely as possible to the exhaust outlet; however, for other engines back-pressure is used as part of the tuning technique to align the pulses correctly, so fitting a freer flowing exhaust to a N/A car can actually reduce power.
ETA: As a side note, I think this is part of the reason that Imprezas sound as they do - if they were NA, they would need the manifold headers to be of equal length in order for scavenging to work consistently. As they are turbocharged, Subaru couldn't use scavenging anyway, so they could make the headers unequal in length, meaning that gas from different cylinders has to travel a different distance to the exhaust tips, creating that stereotypical off-beat thrum. Theoretically, I see no reason that you couldn't get a turbocharged inline-4 to sound like a Subaru by artificially fiddling with the manifold branch length.
All true I think As I understand it, well tuned NA or SC engine exhausts use the kinetic energy of the exhaust gas from the previous cylinder to create a partial vacuum in the manifold at exactly the right moment (this is called exhaust gas scavenging); this isn't really possible on a turbocharged engine because the turbo restricts the exhaust gas flow too much. Thus in a turbocharged car, you want the air to simply flow as freely as possible to the exhaust outlet; however, for other engines back-pressure is used as part of the tuning technique to align the pulses correctly, so fitting a freer flowing exhaust to a N/A car can actually reduce power.
ETA: As a side note, I think this is part of the reason that Imprezas sound as they do - if they were NA, they would need the manifold headers to be of equal length in order for scavenging to work consistently. As they are turbocharged, Subaru couldn't use scavenging anyway, so they could make the headers unequal in length, meaning that gas from different cylinders has to travel a different distance to the exhaust tips, creating that stereotypical off-beat thrum. Theoretically, I see no reason that you couldn't get a turbocharged inline-4 to sound like a Subaru by artificially fiddling with the manifold branch length.
Edited by kambites on Thursday 4th October 11:26
Most production manifolds will be unequal length (because it's cheaper/easier), though perhaps not as extremely as the subaru. Which means, I think, that each cylinder will have a different "sweet spot" in the rev range for best cylinder filling.
McSam said:
V8mate said:
There's a reasonable body of opinion/evidence which shows that an E36 loses bhp when de-catted. This is surely simply a removal of a 'blockage'.
The outcome has always 'baffled' me though, so I'd be keen to understand what's going on in them there pipes.
Someone designed them properly..The outcome has always 'baffled' me though, so I'd be keen to understand what's going on in them there pipes.
Which is very irritating, actually
h to fit though, and I just don't know whether it would be a complete waste of time doing so.Gassing Station | General Gassing | Top of Page | What's New | My Stuff