Exhaust back pressure??

[ShastaBlasta14]

Ive read some stuff in this forum that vaguely mentions exhaust back pressure, and how it affects non-turbo engines, but Im still a little confused. Does anyone have some good links that can explain this better to me?

 

[The1Bill]

Gah! Backpressure! I hate this word!
What you really want, to produce ideal power, is exhaust airflow velocity. It doesn't matter how much pressure is behind the exhaust valves, what matters is how fast you can get the gasses out of the combustion chamber. A motor is basically an air pump.
Air/fuel mix is sucked in by the piston going down when the intake valves are open. The mix is then compressed when the piston comes back up, ignited near the top of the piston's travel, and makes power by expanding and pushing the piston down (and pushing/turning the crank at the same time). When the piston comes back up, it shoves the air out of the exhaust valves, through the port in the head, through the manifold (or header, or manifold and turbo if you are lucky).
There is more to this, though. The Otto cycle, however, is modified in a production engine to increase volumetric effeciency. This means that the intake valves will be open at the same time as the exhaust valves for a short period. This is called overlap. This makes the pulse of exhaust leaving the head pull in some fresh air/fuel mix with the period of low pressure that follows the exhaust pulse. The engine kinda tosses the exhaust gasses out, and in doing so, reduces the pressure right behind the exhaust, which is used to help tug the fresh air in.
At low RPMs, however, overlap is bad because something called reversion can happen. Reversion occurs when the overlap is too great for the exhaust to stay out as the piston starts it's travel back down. Overlap decreses the velocity of the exhaust gas at low RPM, which is a sacrifice that is made to increase the exhaust gas velocity at high RPM. The only reason why we can run overlap at all and still idle is the scavenging effect of an exhaust manifold or a header. Headers and manifolds are designed with this in mind, and they scatter a pulse (a packet of exhaust gas that was just tossed by the motor) that wants to go back into the cylinder head due to the drop in pressure from the piston going back down. This means that we can run more overlap with less reversion at low RPMS.
This is why when a car gets cams, a couple of other issues can follow. Idle quality will suffer, as the motor will be more prone to reversion because the intake and exhaust cams are open at the same time for longer. In other words, there is more overlap. Also, vacuum pressure can drop, as the motor has the tendency to have a vacuum leak through the exhaust port (because they are open at the same time). This can reak havoc with such things as vacuum powered brake boosters, fuel pressure regulators, and other things that rely on vacuum.
On a turbocharged motor, however, overlap is something to be decreased. The turbocharger is an obstruction in the exhaust stream, which means that the pulse will bounce back towards the head sooner and stronger then it would if it were exhaling through a header. This makes the motor more prone to reversion, which lowers the volumetric effeciency of the motor and makes it unhappy, in general.
Hope I didn't lose anyone in that novel up there ^, but contact me if you would like to learn more!
-=B-=