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One of the things I start thinking abut in the middle of the night.
OK We do extensive head work to improve intake air flow, right? On the exhaust side a little port and polish , match a couple ports , add some headers and we're good to go.
How much HP is lost to restricted flow around the valve? I know that the gas is forced out by the rising piston but the exhaust side is handling 14 times the volume(if memory serves me) of the intake side yet the exhaust valve is usually smaller. Wonder if there may be some way of measuring cylinder pressure on the fourth stroke. Just seems that there ought to be some good HP gains to be made there. Any thoughts? :)
 

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You probably want to make sure you get a decent amount of velocity traveling out the exhaust valve. The less resistance, the less velocity, but more total flow. I'm sure you could design an exhaust to evacuate immediately, but most larger cams count on the exhaust flow to create a low pressure area behind the exhaust valve to suck in the intake air. I'm sure this could be affected by porting-

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The flow ratings are rated at the same depression, usually 28" H2O, but in a running engine the flow through the intake is generated in a much less efficient manner than the flow through the exhaust.
It takes energy to develop the velocity in the exhaust port that creates the scavenging. At the point you are saving pumping energy with a large exhaust system and but losing more energy due to decreased efficiency from reduced scavenging, your power goes down. If you have no scavenging, about 10% of your spent exhaust gases remain in the cylinder, reducing the available space for new air/fuel mixture.
 

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In a Popular Hotrodding magazine there was a great article on Head porting, very very thorough. The October 06 Volume 46 NO10. you should check it out. The throat diameter as measured just below the valve opening should be right around 85 percent of the valve. You also want a nice long slow turning bottom turn. Air has density and mass, just like a car it is easier to go around a long straighter turn, and also easier if slowed down first. The ideal exhaust port would be a straight venturi nozzle but we have to have curves in our ports. Also a mirror finish on the exhaust makes no more performance difference than a slight polishing (although it still may help in carbon buildup) With later model heads like the Vortecs there is little flash material left in the ports and they have great shape. The biggest work then is done to the valve guide boss. Rounding all sharp edges and making it as small of a "bump in the road" as possible. With older heads you have allot more work to be done. I have a set of Vortec cylinder heads I just put on my 350. The only thing we did to the intake side was remove a line of flash that circled around the middle of the port. The biggest amount of work was done to the exhaust ports, we had larger valves cut for it, and worked on the valve guide boss. These heads flow excellent on intake but worse than a set of 882 heads on the exhaust side. So the exhaust intake efficiency stock is only like around 60 percent. Good heads will be around 75% There is still so much more to it. I strongly encourage everyone to read the Pop hot rod article. You would be amazed at how many common things done to heads actually hurt flow.
 
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