Quote:
Originally Posted by Madact
I was under the impression that a step down in diameter would reflect part of the initial positive pressure pulse back into the cylinder, as well as restrict flow more than a step in the other direction?
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I asked because I didn't know. I suspect a pipe a little smaller than the port would be better for best economy, I would position it off centre so that the main flow didn't hit the step though, I haven't looked at your head but the main flow out of the port is unlikely to be down the centre of the port.
A step up in diameter creates a reverse wave which encourages flow which is why some manifolds have extra steps but the normal explanation for the header being wider than the port is to prevent exhaust going back into the cylinder during overlap on long duration cams which I don't understand but I guess has been found true by experiment. If you don't have long duration cams then there may be no issue.
Quote:
Originally Posted by Madact
Do you know whether the 15" thing is about pressure pulses or flow? I haven't been able to find any explanation of this figure, apart from a couple of old quantitative parametric studies done ages ago on 351ci V8's, and those didn't attempt to answer the 'why' of it  . |
Both are important but I believe flow is by far the most important. You need to work out how long the initial pulse of exhaust gas takes to travel down to the end of the primary at the exhaust flow speed, as it passes the other primary in its pair it will create a strong vacuum in that pipe via the venturi effect (same way airbrushes suck up the paint), the vacuum then takes time, dependent on the speed of sound, to act on the exhaust valve for that pipe and start to suck the exhaust gasses out of that cylinder, it continues sucking until the exhaust flow from the first cylinder slows, thus the time period of the exhaust flow needs to be taken into account to avoid wasting suction but sucking at the time it will have most effect. The same happens as the exhaust reaches the end of the secondary but there it sucks on both primaries of the other pair. The timings all get very complicated, then you have to add in the wave effects!
Most explanations are way off and conflict with each other, then I am sure some experts give the wrong explanation intentionally so that nobody else can work it all out!