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A while back I read an article in a performance mag (HCI) about do's and don'ts of head porting. The principle being less is more. In other words, the idea of increasing air flow is not to completely open up the head intake chambers and valve bowls because beyond a certain point you're causing a disruption to the "jet" effect of air flowing into the combustion chamber. According to the article, the ideal air flow passage as far as engine performance is concerned includes a restriction just inches from the combustion chamber. In head design, that restriction is at the point just before it enters the valve bowl. The idea being that once it passes the restriction point it expands and propels itself into the combustion chamber. Too large a valve bowl area after the restriction would cause a slight lag in the air expanding into the combustion chamber. That makes sense.
What I don't understand is the exhaust side of the equation. Other sources I've read refer to back pressure just pass the exhaust side of the head and into the manifold. As far as porting goes on that side, it is not recommended to eliminate the "step" of the head mating surface with respect to the manifold interior. In other words to not open up the exhaust ports to match the larger size of the exhaust manifold interior. Can someone explain how that step and any back pressure is important for optimum turbo performance? Mind you, I'm refering to the stock manifold design. Which leads to another question - with a performance exhaust header, is that step also part of the design? TIA.
posted by 205.188.116...
, Wed, 4 Aug 2004 17:49:23, Thu, 5 Aug 2004 08:26:18, Thu, 5 Aug 2004 07:28:37
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