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Help calculating plenum volume

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dawesy:
I'm currently making an inlet manifold for my MR2.
So far I've cut the old plenum off and milled the surface flat

Welded a plate onto it and shaped the tops of the port runners

I'm now sorting the plenum area.
The manifold is going to be side fed from the right hand side and I've been told that it needs a taper from the throttle body end to the other and this taper needs to go down in size so each port has a 1/4 less volume than the one before it. This is where I'm stuck.
This is what we have at the mo


Has any one got a formula/ way of working this out. Ive divided the height by 4 and Then deducted this measurement from each port which gives me a taper but I'm not sure that it is a quarter less volume above each port.
Any help appreciated.

Lew_Merrick_PE:
Lee,

The plenum is the inlet area feeding the (4 port) manifold, correct?  The inlet at each manifold is "blah" inches long.  Your plenum is a piece of tubing cut by a chord to fit your manifold plate, correct?

If so, then you have a problem.  Your "tube" is a cylinder.  It needs to be some other shape.  There are many it could be.  The "issue" is the Ideal Gas Law (PV = nRT where "P" is (absolute) pressure, "V" is volume, "n" is the number of molecules, "R" is the "gas constant," and "T" is the (absolute) temperature).  At "steady state" the "nRT" side of the equation is a constant.  Thus, what you are trying to do is to maintain a constant "PV" across each of the manifold inlets.  Correct?

So, the "PV1" at the first inlet needs to be 4* the "PV4" at the fourth inlet.  [You also need to maintain a "smooth transition" in between each inlet to avoid "flow losses."  Otherwise you lose the "assumption" that "P" is constant along the length.]  Etc.

The "issue" is going to be figuring out the shape that fits your application.  I normally do this type of design for hydraulic circuits that are quite a bit simpler than pneumatic circuits.

Pete.:
I don't think it's as critical as all that. Make a taper by cutting the top at a slope and weld a flat plate on but don't go to any great lengths to 'do calculations' because there are so many variables that have nothing to do with just volume you could spend your life doing sums and be no better off than if you had just winged it.

Jonny:
Think what they meant by 1/4 less volume is by having four separate contained units fed by one source. Last one in line will always have a pressure drop eaten up by the first three unless centre fed easier to sort where opposite ends 1 and 4 will then have a drop.
What your after is to speed the flow at furthest away, easiest taper the tube with no back pressure at feed in which I think you will have being end fed. Need to smooth out the flow will get better quicker throttle response and do the same with throttle body.

Over whelmed by Lew you can help me with pneumatics and flow rate, haha.

dawesy:
This is a forced induction setup so not looking for tuned lengths as such. The stock manifold is centre fed and the centre two cylinders are biased and as such run leaner than the outer two.
Some of the manifolds I see for this engine have no taper at all.
Thanks for the advice guys, wasn't sure if there was a formula for this kind of thing

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