Backpressure in exhaust (how to add and remove)
Discussion
Ive seen dyno charts testing factory exhaust vs free flowing or chambered aftermarket exhaust. The difference is usually 3000 rpm and down the backpressure adds HP but after 4000 the free flowing just takes off and backpressure stays steady in gaining HP.
has anyone ever made an aftermarket muffler that changes flow given a certain rpm or vacuum ? quiet when you just drive but free flowing when you are wot or above a certain load
Yea I've seen some car manufacturers have something like that and I've had Motorcycles with the flap in the exhaust that opens at like 6-8k and adds a lot to the bottom end. I took it off when I did a muffler on my 600 and could tell a difference
But was thinking more like a flowmaster
But was thinking more like a flowmaster
I've never understood the claim that somehow, "back-pressure increases power at low rpm". Maybe someone could explain the mechanism by which this could ever be true?
It simply doesn't make any sense: an exhaust system that was already restrictive at low rpm, when there's (relatively-speaking) very little exhaust gas volume to flow per unit time, would completely strangle the engine as rpm increased. Any exhaust system that works reasonably well at high rpm (i.e. is capable of flowing the required volume of gas per unit time without too much restriction) just won't generate any back pressure at lower rpm. This must include the majority of ex-factory exhausts, as most engines these days offer good specific power outputs that a really inefficient exhaust wouldn't allow.
I'm sure the differences in how various aftermarket systems perform at low/high rpm is far more to do with pipe lengths and diameters than back-pressure. I'd guess optimising scavenging for high rpm may reduce its efficiency at low rpm and that's the effect being seen.
It simply doesn't make any sense: an exhaust system that was already restrictive at low rpm, when there's (relatively-speaking) very little exhaust gas volume to flow per unit time, would completely strangle the engine as rpm increased. Any exhaust system that works reasonably well at high rpm (i.e. is capable of flowing the required volume of gas per unit time without too much restriction) just won't generate any back pressure at lower rpm. This must include the majority of ex-factory exhausts, as most engines these days offer good specific power outputs that a really inefficient exhaust wouldn't allow.
I'm sure the differences in how various aftermarket systems perform at low/high rpm is far more to do with pipe lengths and diameters than back-pressure. I'd guess optimising scavenging for high rpm may reduce its efficiency at low rpm and that's the effect being seen.
Edited by Ian_UK1 on Monday 30th January 17:22
You want the pressure just the other side of the exhaust valve to be as low as possible.
At high revs, the flow rate is pretty much constant, but at lower revs, the flow is pulsed as the time difference between each cylinder firing is longer.
These pulses can be 'tuned' with the use of back pressure to maximise the low-pressure by bouncing the pulses back along the pipe. It has the most effect in 2 strokes, but NA 4 strokes can also benefit. Turbo charged cars gain nothing.
The pulse has a leading edge of high pressure, and a trailing edge of low pressure. It's timing the pulse's low pressure tail to pass the exhaust valve at the right moment that aids in the evacuation of the exhaust gas from the cylinder
This might be an oversimplified description, but it's how I've always understood it
At high revs, the flow rate is pretty much constant, but at lower revs, the flow is pulsed as the time difference between each cylinder firing is longer.
These pulses can be 'tuned' with the use of back pressure to maximise the low-pressure by bouncing the pulses back along the pipe. It has the most effect in 2 strokes, but NA 4 strokes can also benefit. Turbo charged cars gain nothing.
The pulse has a leading edge of high pressure, and a trailing edge of low pressure. It's timing the pulse's low pressure tail to pass the exhaust valve at the right moment that aids in the evacuation of the exhaust gas from the cylinder
This might be an oversimplified description, but it's how I've always understood it
Ian_UK1 said:
I've never understood the claim that somehow, "back-pressure increases power at low rpm". Maybe someone could explain the mechanism by which this could ever be true?
It simply doesn't make any sense as an exhaust system that was already restrictive at low rpm, when there's (relatively-speaking) very little exhaust gas volume to flow per unit time, would completely strangle the engine as rpm increased. Any exhaust system that works reasonably well at high rpm (i.e. is capable of flowing the required volume of gas per unit time without too much restriction) just won't generate any back pressure at lower rpm. This must include the majority of ex-factory exhausts, as most engines these days offer good specific power outputs that a really inefficient exhaust wouldn't allow.
I'm sure the differences in how various aftermarket systems perform at low/high rpm is far more to do with pipe lengths and diameters than back-pressure. I'd guess optimising scavenging for high rpm may reduce its efficiency at low rpm and that's the effect being seen.
Two strokes live and die by the back-pressure waves generated by their expansion chambers but that's bugger all to do with the discussion at hand It simply doesn't make any sense as an exhaust system that was already restrictive at low rpm, when there's (relatively-speaking) very little exhaust gas volume to flow per unit time, would completely strangle the engine as rpm increased. Any exhaust system that works reasonably well at high rpm (i.e. is capable of flowing the required volume of gas per unit time without too much restriction) just won't generate any back pressure at lower rpm. This must include the majority of ex-factory exhausts, as most engines these days offer good specific power outputs that a really inefficient exhaust wouldn't allow.
I'm sure the differences in how various aftermarket systems perform at low/high rpm is far more to do with pipe lengths and diameters than back-pressure. I'd guess optimising scavenging for high rpm may reduce its efficiency at low rpm and that's the effect being seen.
Ian_UK1 said:
I've never understood the claim that somehow, "back-pressure increases power at low rpm". Maybe someone could explain the mechanism by which this could ever be true?
I've driven cars where peak torque at low rpm would be with the throttle partially closed and opening the throttle past that point would reduce power. I suppose it must be something to do with resonance or flow reversal or something. It's unintuitive but definitely happens. I suppose exhaust backpressure could have a similar unintuitive effect under some conditions.on the dyno ive put restrictions in the tailpipe outlet to add back pressure and guess what it loses power at every single rpm so dont trust most of the internet is nonsense alot of the time. back pressure doesnt give lowend torque its bad physics to say so as you actually dont want back pressure. the best way to get lowend torque is to ensure there is a scavenging wave during overlap and this is a low pressure region not high pressure
if they got more lownend torque then the the way the pressure waves in the system behaved was changed in a manner other than just adding restriction, so the equivalent length changed or the change in ratio of cross sections was different etc etc such that one configuration reduced pumping losses and/or scavenged better by virtue of a geometric difference between the systems not actual back pressure.
if they got more lownend torque then the the way the pressure waves in the system behaved was changed in a manner other than just adding restriction, so the equivalent length changed or the change in ratio of cross sections was different etc etc such that one configuration reduced pumping losses and/or scavenged better by virtue of a geometric difference between the systems not actual back pressure.
Edited by Inline__engine on Monday 30th January 20:25
GreenV8S said:
I've driven cars where peak torque at low rpm would be with the throttle partially closed and opening the throttle past that point would reduce power. I suppose it must be something to do with resonance or flow reversal or something. It's unintuitive but definitely happens. I suppose exhaust backpressure could have a similar unintuitive effect under some conditions.
A bimodal intake is great for helping low end torque as the air flow speed is increased and helps cylinder filling. On the exhaust side you would need variable length headers not restrictions to help extraction aiui.^^ yes with intakes the same theory applys.
That the free flowing intake doesn't have the velocity down low but up top it can move much more air. Factory intake versus a FAST 102, HP down low will drop but up top will gain an unreal amount.
So how can you get the best of both worlds? Besides getting an intake not to big or not to small... There's still meat on the bones up top and down low that way
That the free flowing intake doesn't have the velocity down low but up top it can move much more air. Factory intake versus a FAST 102, HP down low will drop but up top will gain an unreal amount.
So how can you get the best of both worlds? Besides getting an intake not to big or not to small... There's still meat on the bones up top and down low that way
An exhaust system with pipe diameters sized for a low volume of flow will increase gas velocity and aid cylinder scavenging. But, the smaller diameter will become restrictive (increase in backpressure) at higher rpm. Likewise, if the exhaust were sized for high flow volumes the lower speed scavenging effect will be reduced.
In these cases the backpressure is a function of the diameter more than the muffler design. If you designed an exhaust with diameter appropriate to high speed high gas flow, and then added a restrictive muffler, it will perform poorly at all engine speeds.
In these cases the backpressure is a function of the diameter more than the muffler design. If you designed an exhaust with diameter appropriate to high speed high gas flow, and then added a restrictive muffler, it will perform poorly at all engine speeds.
I was always led to believe that the engine speed for max torque could be altered by changing the distance to the 'nodes' of the exhaust system. (The points where the diameter of the pipe changes such as when the pipes in the manifolds join, then the points where these pipes are combined and when they encounter the first silencer.) These nodes form tuned sections which will produce back pressure when the distance is at the wavelength of the exhaust pulses (or multiples thereof).
Anywhere near the truth?
Anywhere near the truth?
Dave Walker at Emerald has done quite a bit of work on variable length intake trumpets
http://www.emeraldm3d.com/articles/emr-adj-length-...
http://www.emeraldm3d.com/articles/emr-adj-length-...
i wouldnt waste my time doing that unless you just want it to be quieter. thats how those dynamax valves work in their mufflers to keep it quiet
restricting the flow doesnt give you torque, you need to tune the waves to provide suction at overlap which is a length thing, the longer the effective length the lower the rpm it will be intune at
restricting the flow doesnt give you torque, you need to tune the waves to provide suction at overlap which is a length thing, the longer the effective length the lower the rpm it will be intune at
stevesingo said:
An exhaust system with pipe diameters sized for a low volume of flow will increase gas velocity and aid cylinder scavenging. But, the smaller diameter will become restrictive (increase in backpressure) at higher rpm. Likewise, if the exhaust were sized for high flow volumes the lower speed scavenging effect will be reduced.
In these cases the backpressure is a function of the diameter more than the muffler design. If you designed an exhaust with diameter appropriate to high speed high gas flow, and then added a restrictive muffler, it will perform poorly at all engine speeds.
That is how I see it.In these cases the backpressure is a function of the diameter more than the muffler design. If you designed an exhaust with diameter appropriate to high speed high gas flow, and then added a restrictive muffler, it will perform poorly at all engine speeds.
There appears to be a best gas speed for exhaust scavenging (determined by experience / experiments). As the volume of exhaust gas goes up at higher rpm, you need a larger diameter pipe to get the same gas speed.
Tuned wave scavenging has a different issue with changing rpm : the reflected waves arrive back at the exhaust valve a (roughly) fixed time after the valve opens/closes, so one exhaust can't be optimal for all rpm.
Manufacturers have had variations on variable length / diameter inlets for decades, but I can't recall an "active" exhaust of that type.
The closest I can think of is Honda - on the last of their 2-stroke GP bikes they were injecting water into the exhaust, which changed the temperature and density of the gas. This allowed them to broaden the power band by effectively altering the tuned length of the pipe.
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