Can anyone explain how scavenging works on a full exhaust system?

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pingu
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Can anyone explain how scavenging works on a full exhaust system?

Post by pingu » Mon 12 Feb, 2018 02:16

The video shows the dilemma. I can see how it works when the system has very little back pressure, but how does it work on a full system?

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siwilson
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Re: Can anyone explain how scavenging works on a full exhaust system?

Post by siwilson » Tue 27 Feb, 2018 08:41

A full exhaust with cats and silencers will create some back pressure. However, in the test he only applies pressure to one pipe so there is nothing in any of the other inputs to fit against the back pressure and help force the flow down the full exhaust. The even with a small amount of back pressure the easiest route must still be out of an unrestricted opening. I suspect if all cylinders were pumping then the combined flow would start to create a draw through the pipe, which would then start to help with the scavenging.

A bit like when you removed the baffle from your moped and actually went slower.
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pingu
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Re: Can anyone explain how scavenging works on a full exhaust system?

Post by pingu » Tue 27 Feb, 2018 14:28

I've solved the problem with more learning.

What you can see on the video is inertia scavenging. Inertia scavenging only works on systems with little or no back pressure (as you said).

Wave scavenging is the one to go for. The wave can't be seen, but is generated when the exhaust valve opens. The positive pressure wave is partially reflected (and inverted into a negative pressure wave) when it exits the header into the collector. The non-reflected part of the wave continues to the exit of the tail-pipe. The remainder of the wave is partially reflected (and inverted into a negative pressure wave) when it exits the tail-pipe.

The trick is to tune the length of the header (and to a much lesser extent) the full exhaust to make the reflected negative pressure wave(s) hit the exhaust valve when it about 1.25mm from closing. The negative pressure in the reflected wave reduces the pressure that the exhaust in the cylinder is pushing against. This allows more gas to leave the cylinder.

An additional benefit is that the inlet and exhaust valves are in overlap when this happens, so inlet gas is pulled into the cylinder, even although the piston is rising.

Tuning the inlet tract to the same RPM as the header is tuned will further enhance this effect, as a positive pressure wave from the inlet tract and a negative pressure wave from the exhaust is the best of both worlds :D .


The positive pressure wave in the inlet is caused by the inlet valve closing. When the valve is open, the gas is moving into the cylinder. When the valve closes, the gas still has inertia and wants to move toward the valve, but can't. This creates a positive pressure wave that is reflected back towards the plenum. When the wave reaches the plenum it is inverted into a negative pressure wave that is reflected back towards the valve. When the wave hits the valve it is reflected (but not inverted) back towards the plenum. When the wave hits the plenum, it is reflected (and inverted into a positive pressure wave) and returns towards the valve.

If this can be timed to hit the back of the inlet valve when it is open, but less than 1.25mm open, the pressure wave will help to push air into the cylinder that wouldn't flow at ambient pressure.
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Gazza
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Re: Can anyone explain how scavenging works on a full exhaust system?

Post by Gazza » Tue 27 Feb, 2018 16:31

Would your initial observation have something to do with the amount of air being forced in ?

I just watched a similar demonstration on Wheeler Dealers (s14 ep8) where Ant showed the flow on a 3 into 1 collector, he placed tissue over two inlets, he used compressed air at a low flow in one inlet and the the other two inlets actually sucked on the tissue due to scavenging.
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pingu
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Re: Can anyone explain how scavenging works on a full exhaust system?

Post by pingu » Tue 27 Feb, 2018 18:21

Gazza wrote:
Tue 27 Feb, 2018 16:31
Would your initial observation have something to do with the amount of air being forced in ?

I just watched a similar demonstration on Wheeler Dealers (s14 ep8) where Ant showed the flow on a 3 into 1 collector, he placed tissue over two inlets, he used compressed air at a low flow in one inlet and the the other two inlets actually sucked on the tissue due to scavenging.
An engine (especially at the headers) is not constant flow. It is a series of pulses. It almost becomes constant flow further down the system. His explanation was how I understood scavenging until it was explained to me by someone who knew how it really worked.

Inertia scavenging is only relevant when open pipes are used, such as racing. Stick a cat in the system, or a muffler, and it becomes irrelevant.
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