Sorry for another long explanation 
, but I think most people get restrictive backpressure confused with header tuning and exhaust velocity. I thought I would add my $0.02 again.
Backpressure caused by restriction decrease power because the engine has problem forcing the exhaust gases out of the cylinder. Such a restriction would be catalytic converters. They are a restrcition since instead of an open pipe you are now pushing the air through cat's internal stuff (I have no idea what that materail is). Basically, the same way you intake filter can impede air flow. This causes power loss due to the contamination of the intake air with the exhaust. Not all the exhaust can get out so the cylinder is filled with both cool oxygen rich air from the intake and the left over hot oxygen depleted exhaust. Less air/oxygen means less power. Anything that can help get the exhaust out faster will increase power by allowing more intake air/oxygen in. A well designed exhaust system will help the exhaust exit better, and thus makes more power.
Using too larger of a diameter exhaust can actually lose power. Given the same volume of air the larger the diameter pipe that it has to go through the slower it will move. Basically, you have more cross sectional area with the larger pipe so the air does not have to move as fast to evacuate the same volume of air. What happens is that if you slow the air down too much then you decrease the ability of the exhaust to help suck the air out of the cylinder. The basic setup is that the piston pushes out the exhaust gases on its up stroke. As the piston reaches the top of its stoke it cannot push the air out as fast, and when it actually reaches the top it cannot push the air out at all. However, the exhaust can still be removed. It can be removed by taking advantage of the fact that air has momentum. The more momentum the air has the more exhaust that can be removed from the cylinder even though the piston is no longer pushing it out. Momentum = Mass*Velocity. You can increase the momentum by increasing the velocity. If your exhaust piping is too big then as stated before you will lose exhaust velocity, and thus you have less momentum. Less momentum means more exhaust left in the cylinder. More exhaust in the cylinder means less power
This is basically one form of exhaust scavenging. The right exhaust size is a balancing act between getting enough volume of exhaust out via the pipes diameter while maintaining the highest velocity. Basically, larger diameter pipes are better for higher rpms where there is not enough time to allow the exhaust’s momentum to help scavenge out the cylinder. The air will have more velocity from the higher rpms so a larger diameter can be used to get the air out more efficiently. At lower rpms it is exactly opposite. The air has less overall velocity due to the rpm so keeping a small pipe diameter helps to increase scavenging by keeping up exhaust velocity.
Tuning headers works kind of like choosing the right exhaust diameter. Tuning headers is all about reflected pressure pulses and can be seen as another form scavenging. When the exhaust valve is open the exhaust begins to rush out. Basically the air inside the exhaust pipe has gone from atmospheric pressure right before the valve opened abruptly to the higher pressure of the cylinder when the valve opens. This causes a high pressure wave to flow down the exhaust. You can compare this to the water flowing down a hose. By placing changes in the tubing’s diameter one can cause part of this high pressure wave to reflect (this is what the step in step headers does). A change in air pressure is nothing more than sound. So basically there is a sound wave traveling down the exhaust tubing (hence the sound of your car). When it reaches a change in pipe diameter some of this sound is reflected just like it hit a wall. This reflected sound/pressure wave heads back up the exhaust pipe and enters the cylinder again. If the headers are tuned just right the reflected pressure can increase the removal of exhaust in the cylinder and aide in the pulling in of intake air. The pulse can help increase the pressure in the cylinder to help push more air out. The pulse if it is timed right can actually go through the open intake valve (since intake and exhaust valves have over lapping timing due to the cams) and reflect again of the intake to help bring in a more air. The timing of these pulses is a function of RPM of the engine. The lower the rpm you want the scavenging to take place at requires the further down the header that the “step” is placed. Longtube headers are a great example of this. Longtube headers are great at making better low rpm power due to this tuning/scavenging. The step is usually the collector cone where the pipes come together. Don’t confuse this with people that switch to larger diameter long tube headers and loose low rpm power. That problem goes back to the previous paragraph. Step headers actually have multiple steps in them to reflect the pressure at various rpms. Take a look at a set and you will see several changes in pipe diameter in the primaries.
Hope I got all that right. Again, if you have an correction please feel free to let me know.
Mike
, but I think most people get restrictive backpressure confused with header tuning and exhaust velocity. I thought I would add my $0.02 again.Backpressure caused by restriction decrease power because the engine has problem forcing the exhaust gases out of the cylinder. Such a restriction would be catalytic converters. They are a restrcition since instead of an open pipe you are now pushing the air through cat's internal stuff (I have no idea what that materail is). Basically, the same way you intake filter can impede air flow. This causes power loss due to the contamination of the intake air with the exhaust. Not all the exhaust can get out so the cylinder is filled with both cool oxygen rich air from the intake and the left over hot oxygen depleted exhaust. Less air/oxygen means less power. Anything that can help get the exhaust out faster will increase power by allowing more intake air/oxygen in. A well designed exhaust system will help the exhaust exit better, and thus makes more power.
Using too larger of a diameter exhaust can actually lose power. Given the same volume of air the larger the diameter pipe that it has to go through the slower it will move. Basically, you have more cross sectional area with the larger pipe so the air does not have to move as fast to evacuate the same volume of air. What happens is that if you slow the air down too much then you decrease the ability of the exhaust to help suck the air out of the cylinder. The basic setup is that the piston pushes out the exhaust gases on its up stroke. As the piston reaches the top of its stoke it cannot push the air out as fast, and when it actually reaches the top it cannot push the air out at all. However, the exhaust can still be removed. It can be removed by taking advantage of the fact that air has momentum. The more momentum the air has the more exhaust that can be removed from the cylinder even though the piston is no longer pushing it out. Momentum = Mass*Velocity. You can increase the momentum by increasing the velocity. If your exhaust piping is too big then as stated before you will lose exhaust velocity, and thus you have less momentum. Less momentum means more exhaust left in the cylinder. More exhaust in the cylinder means less power
This is basically one form of exhaust scavenging. The right exhaust size is a balancing act between getting enough volume of exhaust out via the pipes diameter while maintaining the highest velocity. Basically, larger diameter pipes are better for higher rpms where there is not enough time to allow the exhaust’s momentum to help scavenge out the cylinder. The air will have more velocity from the higher rpms so a larger diameter can be used to get the air out more efficiently. At lower rpms it is exactly opposite. The air has less overall velocity due to the rpm so keeping a small pipe diameter helps to increase scavenging by keeping up exhaust velocity.Tuning headers works kind of like choosing the right exhaust diameter. Tuning headers is all about reflected pressure pulses and can be seen as another form scavenging. When the exhaust valve is open the exhaust begins to rush out. Basically the air inside the exhaust pipe has gone from atmospheric pressure right before the valve opened abruptly to the higher pressure of the cylinder when the valve opens. This causes a high pressure wave to flow down the exhaust. You can compare this to the water flowing down a hose. By placing changes in the tubing’s diameter one can cause part of this high pressure wave to reflect (this is what the step in step headers does). A change in air pressure is nothing more than sound. So basically there is a sound wave traveling down the exhaust tubing (hence the sound of your car). When it reaches a change in pipe diameter some of this sound is reflected just like it hit a wall. This reflected sound/pressure wave heads back up the exhaust pipe and enters the cylinder again. If the headers are tuned just right the reflected pressure can increase the removal of exhaust in the cylinder and aide in the pulling in of intake air. The pulse can help increase the pressure in the cylinder to help push more air out. The pulse if it is timed right can actually go through the open intake valve (since intake and exhaust valves have over lapping timing due to the cams) and reflect again of the intake to help bring in a more air. The timing of these pulses is a function of RPM of the engine. The lower the rpm you want the scavenging to take place at requires the further down the header that the “step” is placed. Longtube headers are a great example of this. Longtube headers are great at making better low rpm power due to this tuning/scavenging. The step is usually the collector cone where the pipes come together. Don’t confuse this with people that switch to larger diameter long tube headers and loose low rpm power. That problem goes back to the previous paragraph. Step headers actually have multiple steps in them to reflect the pressure at various rpms. Take a look at a set and you will see several changes in pipe diameter in the primaries.
Hope I got all that right. Again, if you have an correction please feel free to let me know.
Mike
