Exhausts-back pressure
Discussion
BTW - no high flow cats are no a gimmick. But you need to know if they'll actually help your car or not.
Back pressure is what it says. If you had a drinking straw and blew down it. Now mostly cover the end and try again. There'll be much greater resistance and it'll be harder to blow. You've just created back pressure.
On an engine you want as minimal back pressure as possible while not sacrificing gas flow velocity. However reducing back pressure will alter the cars air/fuel ratio. So you need to 'tune' the car to run correctly and make use of the reduced back pressure.
However these are only 2 parts of exhaust tuning. The exhaust gas pressure wave and on n.a. cars particulary exhaust scavenging is a highly potent part of exhaust tuning and power gains.
Google 'David Vizard'. He's written some first rate articles on this subject (among many others).
Back pressure is what it says. If you had a drinking straw and blew down it. Now mostly cover the end and try again. There'll be much greater resistance and it'll be harder to blow. You've just created back pressure.
On an engine you want as minimal back pressure as possible while not sacrificing gas flow velocity. However reducing back pressure will alter the cars air/fuel ratio. So you need to 'tune' the car to run correctly and make use of the reduced back pressure.
However these are only 2 parts of exhaust tuning. The exhaust gas pressure wave and on n.a. cars particulary exhaust scavenging is a highly potent part of exhaust tuning and power gains.
Google 'David Vizard'. He's written some first rate articles on this subject (among many others).
It just refers to how much the system restricts the flow of exhaust gasses.
Obviously the more restricted it is, the more exhaust gasses remain in the cylinder after each stroke, the less air enters the cylinder on the intake stroke, the less fuel can be burnt; which reduces power. I assume modern cars will automatically adjust the fueling based on the data it's seeing from the sensors in the exhaust will tell it how complete combustion has been, in which case anything that reduces back pressure will increase power to some extent.
Modern exhausts tend to be pretty good, though.
Obviously the more restricted it is, the more exhaust gasses remain in the cylinder after each stroke, the less air enters the cylinder on the intake stroke, the less fuel can be burnt; which reduces power. I assume modern cars will automatically adjust the fueling based on the data it's seeing from the sensors in the exhaust will tell it how complete combustion has been, in which case anything that reduces back pressure will increase power to some extent.
Modern exhausts tend to be pretty good, though.
Edited by kambites on Wednesday 20th June 09:28
300bhp/ton said:
However reducing back pressure will alter the cars air/fuel ratio.
Care to expand on that one old chap? Air/fuel ratio is controlled by the carb or fuel injection system - changing the exhaust won't change that (unless I'm missing something obvious?)It's good to note that on a turbocharged engine you want as little backpressure as possible but on a NASP you do need some as it helps scavenge the exahust gases out of the combustion chamber.
Assuming the ECU can't adapt, reducing back pressure will allow more air into the cylinders for a given throttle opening and engine speed, but the fuel injection system wont know so it will still be fueling for the amount of air it expects. You just end up running lean.
Pretty sure that the entire point of lambda sensors is that the ECU can adapt, though.
Pretty sure that the entire point of lambda sensors is that the ECU can adapt, though.
LeoZwalf said:
300bhp/ton said:
However reducing back pressure will alter the cars air/fuel ratio.
Care to expand on that one old chap? Air/fuel ratio is controlled by the carb or fuel injection system - changing the exhaust won't change that (unless I'm missing something obvious?)It's good to note that on a turbocharged engine you want as little backpressure as possible but on a NASP you do need some as it helps scavenge the exahust gases out of the combustion chamber.
I often read about how altering the back pressure (usually positively - i.e. reducing it) can have an adverse affect on a car when the opposite is true.
Air fuel ratio of an injected car is controlled by the ECU. If your scavenging is good, you will actually suck air out of the cylinder. On a natasp car, you will have an element of valve overlap which means that during the last few degrees of rotation while the exhaust valve is open, the inlet will start to open so you can actually start to increase the velocity of the incoming air flow even before the piston starts to go down. This aids with cylinder filling. If you tune for it, you can get massive gains.
kambites said:
Assuming the ECU can't adapt, reducing back pressure will allow more air into the cylinders for a given throttle opening and engine speed, but the fuel injection system wont know so it will still be fueling for the amount of air it expects. You just end up running lean.
Pretty sure that the entire point of lambda sensors is that the ECU can adapt, though.
Most ECUs cant adapt to a point. Most lambda sensors in cars are pretty basic narrowband sensors and are only used when you are cruising on a steady throttle, in "closed loop" mode, allowing the ECU to take a voltage from the lambda sensor to see how much it can lean the mixture. Under harder acceleration the car will run open loop and refer to the fuel map.Pretty sure that the entire point of lambda sensors is that the ECU can adapt, though.
Most engines have an Air flow meter (or MAF) which will tell the ECU about the rate of air flow, however this combined with the fixed fuel map does mean that there is limited adaptation (for many ECUs, some are more advanced than others). With this in mind, its worth remapping an engine if any significant modifications are made the to the engines intake and exhaust, or at least check the air/fuel ration with a wideband lambda.
RobCrezz said:
Most ECUs cant adapt to a point. Most lambda sensors in cars are pretty basic narrowband sensors and are only used when you are cruising on a steady throttle, in "closed loop" mode, allowing the ECU to take a voltage from the lambda sensor to see how much it can lean the mixture. Under harder acceleration the car will run open loop and refer to the fuel map.
Most engines have an Air flow meter (or MAF) which will tell the ECU about the rate of air flow, however this combined with the fixed fuel map does mean that there is limited adaptation (for many ECUs, some are more advanced than others). With this in mind, its worth remapping an engine if any significant modifications are made the to the engines intake and exhaust, or at least check the air/fuel ration with a wideband lambda.
Fair enough. Most engines have an Air flow meter (or MAF) which will tell the ECU about the rate of air flow, however this combined with the fixed fuel map does mean that there is limited adaptation (for many ECUs, some are more advanced than others). With this in mind, its worth remapping an engine if any significant modifications are made the to the engines intake and exhaust, or at least check the air/fuel ration with a wideband lambda.
Watchman said:
LeoZwalf said:
300bhp/ton said:
However reducing back pressure will alter the cars air/fuel ratio.
Care to expand on that one old chap? Air/fuel ratio is controlled by the carb or fuel injection system - changing the exhaust won't change that (unless I'm missing something obvious?)It's good to note that on a turbocharged engine you want as little backpressure as possible but on a NASP you do need some as it helps scavenge the exahust gases out of the combustion chamber.
I often read about how altering the back pressure (usually positively - i.e. reducing it) can have an adverse affect on a car when the opposite is true.
Air fuel ratio of an injected car is controlled by the ECU. If your scavenging is good, you will actually suck air out of the cylinder. On a natasp car, you will have an element of valve overlap which means that during the last few degrees of rotation while the exhaust valve is open, the inlet will start to open so you can actually start to increase the velocity of the incoming air flow even before the piston starts to go down. This aids with cylinder filling. If you tune for it, you can get massive gains.
300bhp/ton said:
benny 61 said:
Hi can anyone explain what is meant by back pressure on exhaust's?
And are high flow cats a gimmick, or are they worth the expense?
Benny
You really couldn't be bother to try Google first, just encase someone might have asked something similar before? And are high flow cats a gimmick, or are they worth the expense?
Benny
benny 61 said:
No it never crossed my mind, I do hope to be perfect like you one day.
Funny, that despite my little (truthful) quip. You decide only to respond to that and totally ignore my 2nd reply which would seem to answer a lot of your questions as well as some pointers for further reading. 
But you're very welcome all the same.LeoZwalf said:
300bhp/ton said:
However reducing back pressure will alter the cars air/fuel ratio.
Care to expand on that one old chap? Air/fuel ratio is controlled by the carb or fuel injection system - changing the exhaust won't change that (unless I'm missing something obvious?)It's good to note that on a turbocharged engine you want as little backpressure as possible but on a NASP you do need some as it helps scavenge the exahust gases out of the combustion chamber.
Watchman said:
Scavenging is not a function of back-pressure. It's to do with the harmonics created. Also where branches of the manifold meet, you're aiming for the harmonics at those intersections to assist with exhaust velocity, so that the pulse of the exhaust exiting through one branch creates a negative pressure directly in front of the pulse of exhaust coming down another.
I often read about how altering the back pressure (usually positively - i.e. reducing it) can have an adverse affect on a car when the opposite is true.
Air fuel ratio of an injected car is controlled by the ECU. If your scavenging is good, you will actually suck air out of the cylinder. On a natasp car, you will have an element of valve overlap which means that during the last few degrees of rotation while the exhaust valve is open, the inlet will start to open so you can actually start to increase the velocity of the incoming air flow even before the piston starts to go down. This aids with cylinder filling. If you tune for it, you can get massive gains.
Thanks Watchman - every day is a school day I often read about how altering the back pressure (usually positively - i.e. reducing it) can have an adverse affect on a car when the opposite is true.
Air fuel ratio of an injected car is controlled by the ECU. If your scavenging is good, you will actually suck air out of the cylinder. On a natasp car, you will have an element of valve overlap which means that during the last few degrees of rotation while the exhaust valve is open, the inlet will start to open so you can actually start to increase the velocity of the incoming air flow even before the piston starts to go down. This aids with cylinder filling. If you tune for it, you can get massive gains.
I though for optimal power, individual tuned straight runners from each exhaust port give the best results, similar to tuned ITBs on the intake side?
Obviously this is not suitable for a road car, but nor is it a system that creates back pressure, making me skeptical about anyone claiming you need some back-pressure for performance.
Obviously this is not suitable for a road car, but nor is it a system that creates back pressure, making me skeptical about anyone claiming you need some back-pressure for performance.
Watchman said:
Scavenging is not a function of back-pressure. It's to do with the harmonics created. Also where branches of the manifold meet, you're aiming for the harmonics at those intersections to assist with exhaust velocity, so that the pulse of the exhaust exiting through one branch creates a negative pressure directly in front of the pulse of exhaust coming down another.
I often read about how altering the back pressure (usually positively - i.e. reducing it) can have an adverse affect on a car when the opposite is true.
Air fuel ratio of an injected car is controlled by the ECU. If your scavenging is good, you will actually suck air out of the cylinder. On a natasp car, you will have an element of valve overlap which means that during the last few degrees of rotation while the exhaust valve is open, the inlet will start to open so you can actually start to increase the velocity of the incoming air flow even before the piston starts to go down. This aids with cylinder filling. If you tune for it, you can get massive gains.
So in other words, witchcraft I often read about how altering the back pressure (usually positively - i.e. reducing it) can have an adverse affect on a car when the opposite is true.
Air fuel ratio of an injected car is controlled by the ECU. If your scavenging is good, you will actually suck air out of the cylinder. On a natasp car, you will have an element of valve overlap which means that during the last few degrees of rotation while the exhaust valve is open, the inlet will start to open so you can actually start to increase the velocity of the incoming air flow even before the piston starts to go down. This aids with cylinder filling. If you tune for it, you can get massive gains.
Like 2-stroke exhaust design.
busta said:
I though for optimal power, individual tuned straight runners from each exhaust port give the best results, similar to tuned ITBs on the intake side?
I don't think that's necessarily true because with a manifold you can use the velocity of the exhaust gas from the previous cylinder's combustion to generate negative back-pressure (no idea if that's the right term) to help suck exhaust gasses out of the cylinder, and indeed help suck air in if you have overlapped valves. I don't know whether that works in practice, though?
On a turbo car however, you don't need back pressure at all, the bigger the pipe the better, serious gains to be made by removing flow restrictions from behind the turbine.
The bonus is that a turbo acts as a natural silencer ( to a point!) so off boost, there is no huge increase in noise.
The bonus is that a turbo acts as a natural silencer ( to a point!) so off boost, there is no huge increase in noise.
Modern exhausts are carefully developed by the car manufacturer to work efficiently. Most supposed add-on "sports" exhausts are not as efficient as OE. In other words, you sound louder but go slower or suffer increased fuel consumption. Sticking a bigger pipe on your car guarantees nothing.
Ozzie Osmond said:
Modern exhausts are carefully developed by the car manufacturer to work efficiently. Most supposed add-on "sports" exhausts are not as efficient as OE. In other words, you sound louder but go slower or suffer increased fuel consumption. Sticking a bigger pipe on your car guarantees nothing.
Not true IMO. One of the primary drivers now is emissions control - hence multi-cats, etc.- so modern exhausts are far from being optimised.Gassing Station | General Gassing | Top of Page | What's New | My Stuff