John Reid wrote:I was considering a 2bular exhaust manifold after seeing the superb quality of the ehaust he made for me. The janspeed maniflold is starting to look a bit grotty so it wont be too long before I find another hole I think.
I would imagine Jim would know how to spec the exhaust to match the engine/cam set up. I was also thinking about ceramic coating for the manifold. I have read somewhere that it increases power as well by changing the cooling down characteristics of the exhaust gasses.. kind of ties in with the above about velocity I think.
One of the guru's manage to explain it to me in laymans terms?
Simple answer: you can get the piper narrow bore flexi manifold for ~380 IIRC - it's good for K-series engines <190BHP.
Long answer:
I'll bet Jim would be able to explain this all better ... my understanding is not even theoretical, let alone practical ...
Things to consider:
(1) Gas will travel down the pipe as you push more in from the engine end - it mostly only moves one way (towards the tail pipes) - cools down and slows as it goes.
(2) Pressure waves (positive and negative) fly around inside the pipe (like the aftermath of chucking a hand full of pebbles into a pool). They can move in either direction and travel faster than the gas.
(3) Negative pressure waves travelling towards the exhaust ports encourage exhaust gas scavenging, and also if you have overlapped cams, encourage cylinder refilling (that's because you end up with a lower pressure at the port than in the cylinder, so gas flows to equalise the pressure).
(4) Positive pressure waves travelling towards the exhaust ports block exhaust flow and discourage filling on overlap.
(5) Pressure waves of either type travel more quickly in hot gas than cold gas.
Now, whenever the exhaust port opens, a positive pressure wave is transmitted into the pipe (along with the gas itself, travelling more slowly).
For as long as the pipe retains a constant diameter and temperature (and include the actual exhaust port in this) the wave will continue.
When the positive wave hits a widening or a drop in temperature, it will reflect a negative wave back up the pipe.
When the positive wave hits a narrowing or a rise in temperature, it will reflect a positive wave back up the pipe.
The opposite is true for negative waves, but we don't need to consider this too much I hope.
The wave front is blurred by bends in the pipe, and some reflections are caused; the sharper the bend, the worse it is.
The ceramic coating is an insulator, so it allows you to keep the gas hotter for longer (and maintain higher wave speeds).
However, while the gas is hot and the wave speeds are high, any reflected negative pressure waves will have little or no effect (they cannot penetrate the advancing high pressure wave).
So, your mission, should you choose to accept it, is to construct a set of pipes that allow the exhaust gas to be expelled with minimum resistance (i.e. that offer least resistance to steady state flow, ignoring waves) and that arrange for a negative pressure wave to arrive at the exhaust port towards the tail end of the exhaust cycle and well after the initial wave has left.
For any given pipe design there will be round trip time (positive wave out to negative wave in); that round trip time will vary with exhaust gas temperature but not with RPM (much) because wave speed is a function of gas temperature not piston speed.
It follows then that there will be a sweet spot in the rev range where the negative wave arrival will coincide with the tail end of the exhaust stroke nicely; below that speed the manifold will work acceptably but there will be limited increase in scavenging because the pressure wave will have dissipated before the piston reaches TDC; it still works at least as well as the standard manifold. Above the sweet spot the valve ends up closing before the peak of the pressure wave can get back and so there is limited gain in efficiency.
If I were doing it and were able to compute all the sums I would aim for a sweet spot of at least 6,000 RPM because that's were the standard manifold becomes the bottle neck when you're running enough valves and cam to maintain air flow into the cylinder at that speed.
Cheers,
Robin
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