Plenums, volume vs density vs velocity. - Page 9 - PowerStrokeNation : Ford Powerstroke Diesel Forum
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post #81 of 81 (permalink) Old 04-13-2011, 01:04 AM
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Re: Plenums, volume vs density vs velocity.

Originally Posted by Chillin View Post
As the valves open and close the manifold pressure falls and rises. Meaning that at any given moment one of them could open into an environment near average manifold pressure, or.... one of substantially lower pressure, depending largely on firing order. In either case, once the valve is open, the pressure drops immediately, and with limited volume to dampen the drop, it can be a substantial dip in pressure, meaning that cylinder will not fill very well comparatively.

If the plenum volume is greater, and the individual cylinders are well shielded from one another with appropriately long runners, the effects of one intake valve's operation on the others is minimized, and each cylinder gets an equal opportunity to receive quality air, and to have that charge capable of sustaining its pressure from the start to finish of the intake stroke, as opposed to erratic fluctuations preceeding, and dwindling pressure soon after the valve opens each time.

With insufficient plenum volume there is inadequate accumulation of charge, and individual cylinders will suffer reduced VE. Depending on firing order and manifold shape and design, each cylinder's efficiency will not degrade evenly either.
Yeah I guess it goes both ways. Just depends on firing order and runner length. The same principal is what tuned length headers are built on. I guess if you don't want to do the math to try to figure out what valves are opening at what time at a given rpm and what the shock wave would be doing at a given RPM to figure out either a way to do a tuned length intake then just making the runners long enough to isolate themselves from each other would be the easier way to go. And would work better overall for any RPM rather than trying to tune them for a specific rpm.

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