Exhaust Question. Genius Required!!!
Discussion
Hi all. There is a question that has been nagging me for a while as nobody has been able to give me a straight answer; so felt it was time to tap the knowledge of all you good PHeads…
How does having a blow in your exhaust affect the performance?
We all know, or are told at least that a hole in your exhaust is a bad thing; it reduces fuel economy, performance and increases CO2 emissions (in turn, again we are told, contributing to the death of baby polar bears and the fact Pakistan is 8 foot under water now)… But why is this the case? Why does having a hole in your exhaust effect performance etc etc? I’m met with answers like “because there’s no backpressure,”; OK, but why does no backpressure do that?
I can understand how having an exhaust with a “good” flow will allow for exhausts to travel through it easier, meaning less energy is required to “pump” the air meaning more can be used to move the car… I’m rambling now…
Thanks
How does having a blow in your exhaust affect the performance?
We all know, or are told at least that a hole in your exhaust is a bad thing; it reduces fuel economy, performance and increases CO2 emissions (in turn, again we are told, contributing to the death of baby polar bears and the fact Pakistan is 8 foot under water now)… But why is this the case? Why does having a hole in your exhaust effect performance etc etc? I’m met with answers like “because there’s no backpressure,”; OK, but why does no backpressure do that?
I can understand how having an exhaust with a “good” flow will allow for exhausts to travel through it easier, meaning less energy is required to “pump” the air meaning more can be used to move the car… I’m rambling now…
Thanks
Depends on the design of the exhaust and where the hole is.
If (big if) your car has an exhaust designed to optimise the power/torque at particular revs, it will resonate and form pulses which will help extract the gas at those revs. Putting a hole in this will spoil the effect. Think of a tin whistle, it makes different notes according to how the holes are covered, the notes are formed by the resonance of the pipe length. So having a hole in your exhaust will move the resonance.
For most cars, the effect will be small on power, the nearer the back, the less the difference.
If (big if) your car has an exhaust designed to optimise the power/torque at particular revs, it will resonate and form pulses which will help extract the gas at those revs. Putting a hole in this will spoil the effect. Think of a tin whistle, it makes different notes according to how the holes are covered, the notes are formed by the resonance of the pipe length. So having a hole in your exhaust will move the resonance.
For most cars, the effect will be small on power, the nearer the back, the less the difference.
jimbobsimmonds said:
Hi all. There is a question that has been nagging me for a while as nobody has been able to give me a straight answer; so felt it was time to tap the knowledge of all you good PHeads…
How does having a blow in your exhaust affect the performance?
We all know, or are told at least that a hole in your exhaust is a bad thing; it reduces fuel economy, performance and increases CO2 emissions (in turn, again we are told, contributing to the death of baby polar bears and the fact Pakistan is 8 foot under water now)… But why is this the case? Why does having a hole in your exhaust effect performance etc etc? I’m met with answers like “because there’s no backpressure,”; OK, but why does no backpressure do that?
I can understand how having an exhaust with a “good” flow will allow for exhausts to travel through it easier, meaning less energy is required to “pump” the air meaning more can be used to move the car… I’m rambling now…
Thanks
IT is likely to have little affect in reality. Flow is a good thing, reducing back pressure is a good thing. The only time it causes issues is if you affect the backpressure significantly that it alters the a/f ratio which would mean a re-tune to take benefit or the reduced back pressure.How does having a blow in your exhaust affect the performance?
We all know, or are told at least that a hole in your exhaust is a bad thing; it reduces fuel economy, performance and increases CO2 emissions (in turn, again we are told, contributing to the death of baby polar bears and the fact Pakistan is 8 foot under water now)… But why is this the case? Why does having a hole in your exhaust effect performance etc etc? I’m met with answers like “because there’s no backpressure,”; OK, but why does no backpressure do that?
I can understand how having an exhaust with a “good” flow will allow for exhausts to travel through it easier, meaning less energy is required to “pump” the air meaning more can be used to move the car… I’m rambling now…
Thanks
The only thing a hole might do is affect a thing called the exhaust pressure wave, which might affect performance, but it's unlikely that it'd really be noticeable IMO, unless again you'd need a re-tune to take advantage/compensate.
what actually happens in an exhaust ? Why do cars have them ?
The answer is that an exhaust acts as a kind of flywheel for the gas flow in a car. It allows it to have momentum, and uses the already expelled gas to suck air into the manifold, during the moment the inlet and outlet ports are open at the same time.
The "pulse" of air expelled travels down the exhaust pipe, creating a area of low pressure behind it as it travels.
This low pressure area is what sucks additional inlet charge into the engine, and this is the effect known as scavenging.
If you collapse this low pressure, by having a hole in the exhaust, then you reduce the scavenging and reduce the engine power.
It is very dependant on the engine and how well it is designed. I had an engine with a beatiful 4-2-1 manifold and header, and no cat. On this car, any hole was immediately apparent as the engine ran like a dog. On other cars even quite large holes were not that obvious.
The answer is that an exhaust acts as a kind of flywheel for the gas flow in a car. It allows it to have momentum, and uses the already expelled gas to suck air into the manifold, during the moment the inlet and outlet ports are open at the same time.
The "pulse" of air expelled travels down the exhaust pipe, creating a area of low pressure behind it as it travels.
This low pressure area is what sucks additional inlet charge into the engine, and this is the effect known as scavenging.
If you collapse this low pressure, by having a hole in the exhaust, then you reduce the scavenging and reduce the engine power.
It is very dependant on the engine and how well it is designed. I had an engine with a beatiful 4-2-1 manifold and header, and no cat. On this car, any hole was immediately apparent as the engine ran like a dog. On other cars even quite large holes were not that obvious.
ExPat2B said:
what actually happens in an exhaust ? Why do cars have them ?
The answer is that an exhaust acts as a kind of flywheel for the gas flow in a car. It allows it to have momentum, and uses the already expelled gas to suck air into the manifold, during the moment the inlet and outlet ports are open at the same time.
The "pulse" of air expelled travels down the exhaust pipe, creating a area of low pressure behind it as it travels.
This low pressure area is what sucks additional inlet charge into the engine, and this is the effect known as scavenging.
If you collapse this low pressure, by having a hole in the exhaust, then you reduce the scavenging and reduce the engine power.
It is very dependant on the engine and how well it is designed. I had an engine with a beatiful 4-2-1 manifold and header, and no cat. On this car, any hole was immediately apparent as the engine ran like a dog. On other cars even quite large holes were not that obvious.
thanks... any more details would be appreciated...The answer is that an exhaust acts as a kind of flywheel for the gas flow in a car. It allows it to have momentum, and uses the already expelled gas to suck air into the manifold, during the moment the inlet and outlet ports are open at the same time.
The "pulse" of air expelled travels down the exhaust pipe, creating a area of low pressure behind it as it travels.
This low pressure area is what sucks additional inlet charge into the engine, and this is the effect known as scavenging.
If you collapse this low pressure, by having a hole in the exhaust, then you reduce the scavenging and reduce the engine power.
It is very dependant on the engine and how well it is designed. I had an engine with a beatiful 4-2-1 manifold and header, and no cat. On this car, any hole was immediately apparent as the engine ran like a dog. On other cars even quite large holes were not that obvious.
i love learning this sort of stuff...
ExPat2B said:
what actually happens in an exhaust ? Why do cars have them ?
The answer is that an exhaust acts as a kind of flywheel for the gas flow in a car. It allows it to have momentum, and uses the already expelled gas to suck air into the manifold, during the moment the inlet and outlet ports are open at the same time.
The "pulse" of air expelled travels down the exhaust pipe, creating a area of low pressure behind it as it travels.
This low pressure area is what sucks additional inlet charge into the engine, and this is the effect known as scavenging.
If you collapse this low pressure, by having a hole in the exhaust, then you reduce the scavenging and reduce the engine power.
It is very dependant on the engine and how well it is designed. I had an engine with a beatiful 4-2-1 manifold and header, and no cat. On this car, any hole was immediately apparent as the engine ran like a dog. On other cars even quite large holes were not that obvious.
The only trouble is, scavenging is promoted at the collector of the header, not at the rear box.The answer is that an exhaust acts as a kind of flywheel for the gas flow in a car. It allows it to have momentum, and uses the already expelled gas to suck air into the manifold, during the moment the inlet and outlet ports are open at the same time.
The "pulse" of air expelled travels down the exhaust pipe, creating a area of low pressure behind it as it travels.
This low pressure area is what sucks additional inlet charge into the engine, and this is the effect known as scavenging.
If you collapse this low pressure, by having a hole in the exhaust, then you reduce the scavenging and reduce the engine power.
It is very dependant on the engine and how well it is designed. I had an engine with a beatiful 4-2-1 manifold and header, and no cat. On this car, any hole was immediately apparent as the engine ran like a dog. On other cars even quite large holes were not that obvious.
Also most standard exhaust systems simply don't scavenge, anything with a "log" style exhaust manifold and single downpipe, which covers the vast majority of standard road cars.
Also scavenging is non existent on forced induction engines.
Depends on the engine.
A blow might increase backpressure slightly due to causing turbulance in the exhaust.
But with turboed cars, no exhaust is the best exhaust for performance.
Also if the blow is before the CATs then you will actually be giving out less CO2 emissions (CATs convert nastier gasses into CO2), and if its after, there will be pretty much no difference.
A blow might increase backpressure slightly due to causing turbulance in the exhaust.
But with turboed cars, no exhaust is the best exhaust for performance.
Also if the blow is before the CATs then you will actually be giving out less CO2 emissions (CATs convert nastier gasses into CO2), and if its after, there will be pretty much no difference.
PaulG40 said:
The boy racer exhaust 'popping' always grates on me! Chavs in there 1.4s love it, even though as ive always understood it, it loses them power putting a free flowing exhaust on, decreasing back pressure too much.
Popping is fine, my 5.7 V8 does it. And you only lose power if you late the a/f ratio too much and don't/can't tune for it. In short you really can't have to little back pressure on na engine. PaulG40 said:
The boy racer exhaust 'popping' always grates on me! Chavs in there 1.4s love it, even though as ive always understood it, it loses them power putting a free flowing exhaust on, decreasing back pressure too much.
As said you can't decrease back pressure too much. It's a myth possibly perpetuated by people putting on badly designed or poor quality aftermarket exhaust systems on who find their car produces less power.Also unless I'm mistaken the AFR problem only affects certain types of fuel injection
The Wookie said:
PaulG40 said:
The boy racer exhaust 'popping' always grates on me! Chavs in there 1.4s love it, even though as ive always understood it, it loses them power putting a free flowing exhaust on, decreasing back pressure too much.
As said you can't decrease back pressure too much. It's a myth possibly perpetuated by people putting on badly designed or poor quality aftermarket exhaust systems on who find their car produces less power.Also unless I'm mistaken the AFR problem only affects certain types of fuel injection
PaulG40 said:
The boy racer exhaust 'popping' always grates on me! Chavs in there 1.4s love it, even though as ive always understood it, it loses them power putting a free flowing exhaust on, decreasing back pressure too much.
My exhaust pops on over-run and I love it! I don't think the exhaust loses me power (as Supersprint ones are all custome designed for each car), but I certainly didn't notice a power increase when I put it on.I didn't mean popping is a bad thing on say a performance car, I was getting at, in a sort of round about way, you need back pressure but not too much or too little of it, a fine balance of it if that makes sense tuned to your engine for ideal performance characteristics therefore increasing power/torque through the rev range or indeed moving the power/torque range about. With a 2 1/2" exhaust straight through, 4-2-1 manifold on my Polo G40 popped alot with throttle lift off, moving to a 2 1/4" system stopped the popping and increased torque response. Top end power only lowered by 2-3bhp, but had a better power delivery curve throughout the rev range.
What grates on me is the 1.0l Saxos or corsas drivers (sporting the usual baseball cap or beanie hats) that rag it around town with big bore exhausts on, revving up, then let it 'pop'. Just winds me up... get a proper car. LOL. Even more annoying is the chavs up the road from me that think working/diagnosising a fault on a car meaning revving the tits off it constantly... winds the engineer inside me right up, but i'll leave that rant for another day!
What grates on me is the 1.0l Saxos or corsas drivers (sporting the usual baseball cap or beanie hats) that rag it around town with big bore exhausts on, revving up, then let it 'pop'. Just winds me up... get a proper car. LOL. Even more annoying is the chavs up the road from me that think working/diagnosising a fault on a car meaning revving the tits off it constantly... winds the engineer inside me right up, but i'll leave that rant for another day!
PaulG40 said:
I didn't mean popping is a bad thing on say a performance car, I was getting at, in a sort of round about way, you need back pressure but not too much or too little of it, a fine balance of it if that makes sense tuned to your engine for ideal performance characteristics therefore increasing power/torque through the rev range or indeed moving the power/torque range about. With a 2 1/2" exhaust straight through, 4-2-1 manifold on my Polo G40 popped alot with throttle lift off, moving to a 2 1/4" system stopped the popping and increased torque response. Top end power only lowered by 2-3bhp, but had a better power delivery curve throughout the rev range.
What grates on me is the 1.0l Saxos or corsas drivers (sporting the usual baseball cap or beanie hats) that rag it around town with big bore exhausts on, revving up, then let it 'pop'. Just winds me up... get a proper car. LOL. Even more annoying is the chavs up the road from me that think working/diagnosising a fault on a car meaning revving the tits off it constantly... winds the engineer inside me right up, but i'll leave that rant for another day!
I'm sure your popping problem could have been cured in another way, as several others have said above engines don't *need* backpressure. OTOH, given that the proper fix would probably involve having new cams developed to properly match the rest of the system, just reducing the exhaust flow may well have been the most cost effective.What grates on me is the 1.0l Saxos or corsas drivers (sporting the usual baseball cap or beanie hats) that rag it around town with big bore exhausts on, revving up, then let it 'pop'. Just winds me up... get a proper car. LOL. Even more annoying is the chavs up the road from me that think working/diagnosising a fault on a car meaning revving the tits off it constantly... winds the engineer inside me right up, but i'll leave that rant for another day!
As for making your car pop, in my youth I knew someone who used to turn his engine off whilst driving down the high street in his Midget (they were quite cheap back then, yes it was a long time ago). On turning the engine back on again it would backfire, making everyone in the street jump. Needless to say one day it blew the exhaust manifold off. Kids in cars, they never change.
Popping is not necessarily a good thing.
It's usually caused by unburnt fuel in the exhaust system being ignited.
Which means that isn't good for the cat.
On pre cat engines it's absolutely fine however.
Fuel consumption can suffer with a blowing exhaust, because of back pressure loss.
Loss of back pressure invariably leads to loss of torque, forcing the driver to accelerate harder to maintain the same level of progress, and hence using more fuel. This will apply to NA cars more so than turbos.
It's usually caused by unburnt fuel in the exhaust system being ignited.
Which means that isn't good for the cat.
On pre cat engines it's absolutely fine however.
Fuel consumption can suffer with a blowing exhaust, because of back pressure loss.
Loss of back pressure invariably leads to loss of torque, forcing the driver to accelerate harder to maintain the same level of progress, and hence using more fuel. This will apply to NA cars more so than turbos.
Edited by TonyRPH on Sunday 22 August 20:11
funwithrevs said:
As for making your car pop, in my youth I knew someone who used to turn his engine off whilst driving down the high street in his Midget (they were quite cheap back then, yes it was a long time ago). On turning the engine back on again it would backfire, making everyone in the street jump. Needless to say one day it blew the exhaust manifold off. Kids in cars, they never change.
I remember when I was a youngster going to the Ace Cafe for the Max Power night. There were loads of Nova's etc all parked up revving their engines and using the technique you describe to make their exhaust pop. They were doing it over and over again until the exhaust glowed red!#Embarressingly at the time i thought it was quite cool. I'm glad I grew out of being a chav.
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