ive heard this term come up alot on other fourms when discussing manifold design for turbos. I have "ideas" as to what it is, but i really dont know.

Anyone care to enlighten me?

 

Exhaust scavenging is being able to use the exiting exhaust charge to either suck the exhaust out of the opposite rotor, or suck the intake charge into the engine via port overlap. Or both.

The key is to tune header diameter and length to time the pulses so that the the negative pressure created by one pulse will pull the exhaust discharge from the other rotor.

In the same sense, this is why high overlap ports (bridge, peripheral) work so well at high RPM. The exiting exhaust charge pulls in more of the intake charge. However this causes exhaust dilution at lower RPMs which can cause issues as well (low torque, poor idle, bucking, etc.).

 

Scavenging is MUCH more important in an N/A than a turbo exhaust. With a turbo the exhaust is under significant pressure and the pressure pulses get screwed up because of the turbo and everything converging at the turbo. Its more important to have everything come together as calmly as possible at the turbo than it is to worry baout scavenging.

In an NA scavenging can help to evacuate the exhaust from the engine, pull in the intake charge (increasing intake velocity), and reduce pumping loses.

BC

 

I would say it is more of a delta P thing. Exhaust scavenging occurs only when intake pressure is high the exhaust pressure at the rotor/housing interface. The residual exhaust gas is displaced through the exhaust port by a higher pressure intake charge durring overlap. The effectivness of this scavenging directly effects the VE.

It is possible to have a turbocharger setup with more exhaust pressure than intake. No scavenging happening there.

A supercharger would be the best for exhaust scavenging. I want a supercharged PP.

 

Exhaust scavenging doesnt occur because the intake is at a higher pressure, but the intake is at a higher pressure because of scavenging.

In a properly built header, at a given RPM, the exhaust pulses will reach the collector at the the correct time to cause the other primary to drop in pressure. This is what causes the intake to be at a 'relatively' higher pressure than the exhaust.

With a supercharger this effect is multiplied by the intake being under artifically higher pressure than the exhaust, which, with a tuned header, will also be under a relatively low pressure.

BC