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Discussion Starter #1
So how do they increase power output for low rpm, is it :

1) The decreased exhaust port area optimizes flow for the tuned speed to prevent over scavenging
2) The decreased port height gives the engine a longer effective power stroke

With this I was wondering, if its the case that it prevents over scavenging, would it be possible to instead of situating the power valve above the exhaust port to change the port height, that the valve would be situated to the sides of the port to change the port width. Another idea I'd like people's opinion is if a valve was placed after the exhaust port to restrict exhaust flow, would it be as effective as a power valve?

I'd like to thank everyone in advance for their posts
 

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Hello Mansfield; Welcome to E2S. #2, is correct. I don't understand your term over-scavenging? When changing the port area you'd need a porting program which does some calculations for you. Port timing & power valves are are similar but different. You may get some different ideas from someone else.
Tinkering with moving the valve down the pipe would change the pulse timing in the pipe. You're getting too complicated for me.
 

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There have been different forms of Motorsports to use a form of exhaust control after the exhaust. Most of then eccentric albeit kinda cool. However I've never seen it as successful as a power valve. Old small race boats used to do weird stuff with baffles on non PV motors, but now the most common is pipes where the expansion chamber and initial head pipe slide on cables to make them shorter or longer.
 

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Discussion Starter #5
What I mean by over scavenging is that after pushing the spent gasses out of the cylinder, the new charge gets trapped in the exhaust system. On the other hand, under scavenging is when for one reason or another, the new charge is unable to push a significant amount of the spent gasses out, so it becomes trapped inside the cylinder.
 
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