I've read from different places that the header phenomenon known as scavenging is caused by the exhaust gas from one primary tube moving by the opening of the other tubes and creating a vacuum on those other tubes. (The whole moving air is less pressurized theory) However, this just doesn't seem right to me. I would figure that the pressurized exhaust gas would partially enter the other primary tubes and actually interfere with exhaust flow? I have an engineering degree so feel free to tell me how this works (or that it doesn't work) in the most scientific terms you need to use.

 

i cant explain how it works but all the drag racers use it to clear the crank pressures out,youll see the hoses running from the headers to the valve cover.i guess with the high rate of flow the exhaust has escpecially at wide open it will create a vacuum in the hose and suck air into it,thus releiving crank pressure

 

Think of it this way: It works in the same way a sandblaster works. As fast moving air passes over (or by)an opening it creates a low pressure area. That inturn acually makes vacuum . which pulls on the other header tube makeing it more efficient. I'm not to good at explaing things so I hope this helps, and doesn't make it worce.
Darin

 

2 reasons

1st being, the exhaust gases have momentum, they're flying past the opening of another cylinder, they dont feel like stopping to say hello
and second reason was already stated, bernoulli's principle
as a fluid flows it creates a low pressure area around it

 

bernoulis??????

 

In simpleton terms, scavenging means sucking the exhaust pulses out, like a siphon.

 

http://www.ssheaders.com/header.htm
This all you need. Just read and learn.

 

I believe it is the momentum of the gas that I was not taking into account. I'm sure some of the gas will still try to enter the other primary tubes which are lower pressure, but by the time it stops and tries to turn around the other cylinder fires. Thanks for the clarification.

 

did i spell it wrong, i dunno

bernoulli's principle
http://home.earthlink.net/~mmc1919/venturi.html

 

That web site was very informative. It also does a good job of explaining why having a smaller header primary tube diameter to increase air velocity is a good idea.