Originally Posted by radomirthegreat
Is this practically how exhaust gases should flow as well?
I like describing the process as water in a pipe, but the highway analogy works pretty well too.
One trick to getting a good basic exhaust system is to make an effort to have all the header tubes the same length between the flange and the collector. The rationale being that the velocity of the gasses comming out of each cylinder port are about the same, and since no two cylinders fire at the same time, each pulse of moving/expanding gasses hit the collector alone and have full use of the entire diameter of the pipe to expand in. That'* just a starting point though. The worst exhaust is the log styles with unequal lengths...they usually only work right in a narrow rpm range before the pulses start hitting each other.
Scavaging the exhaust gases out of the cylinders is good, but it is more a function of the camshaft and exhaust runners within the head rather than the header tubes. The best way I can describe this is a siphon hose... the expanding exhaust gasses behaves like a fluid..get it moving in the right direction and it'* own mass will keep it moving. This happens within the heads. It also needs an open intake valve to let air into the cylinder at the same time the exhaust gasses are trying to suck it out. (Overlap of the intake & exhaust cam lobes allows this to happen.) A good header system can promote it or kill it, but the action of scavaging the cylinders takes place at the exhaust and intake valves.
As a demonstration of exhaust scavaging, listen to the next rice rocket that passes you and downshifts into a corner...lots of time you can hear a popping noise in the exhaust note.. that'* the sound of scavaging the cylinders...not only is hot exhaust gasses going out the heads, but a little of the fresh mixture is getting sucked across the head and out the exhaust port too, lighting off inside the hot headers and making the POP! noise.