Originally Posted by Strongjeff
well its getting late so ill finish up this little "class" tomarow with headers and exhaust scavenging.
Well her it is. Part 2. Headers and scavenging:
Headers or extractors as they can be referred as are essentially a manifold designed for performance. they promote the scavenging effect.
"What is scavenging
Put simply, scavenging is the vacuum effect created when the exhaust pulse from one cylinder helps "pull" exhaust gasses out from another cylinder during valve overlap. (That brief amount of time in which both the exhaust and intake valves are open. when the exhaust valve is closing the intake is opening
) this also applies for a cross pipe like an x pipe or an h pipe. The effect is minimal in a y pipe setup. the reason behind that is because you are merging the 2 exhaust streams (V type motors or H type) causing the pulses to slow down.
What makes headers special over a stock "Log" style manifold is that instead of dumping the exhaust into one common tube, each cylinder has its own tube. inside a stock manifold there is a lot of sharp 90* turns the pulses have to make. this slows down the pulse and created turbulence in the exhaust stream. there is also the possibility for the exhaust pulses to travel backwards.
aftermarket headers come in 3 flavors. Long tubes, which produce the best overall gains and the most torque. Mid-length, which is a compromise between a long tube and short tube, it shares many advantages of both and some disadvantages as well. and lastly short tubes. short tubes don't make the power that a long tube does but still inherently increases it from improving the scavenging effect. short tubes are also the most easily installed as they are a direct fit to stock mid pipes. shorties also tend to make there power higher in the rpms. Mid-length headers have a broader power band then short tubes but may require new mid pipes and possibly a tune. long tubes will require tuning and new mids but make the best and widest power gains.
having longer tubes allows more time for the gasses to fully escape the chambers and increases the speed of the pulse. long tubes have a greater scavenging effect. however, you can buy headers with tubes too big and not make the power that ones with a slightly smaller tube could make. so to make the best power at the rpms you want, you need a properly sized header. this goes back to Size vs Velocity
again. another advantage to headers is that they prevent revision, the effect of the exhaust pulse traveling backwards.
But as a recap as to why velocity is so important, tho I really haven't touched on it is this. A fast moving pulse creates a low pressure area behind it. this vacuum effect (scavenging) will speed up the pulses that are behind it. think of how nascar does drafting. the car behind is faster then the lead car, but a train of cars is faster the one car on its own.