tooter's manifold adventure...

[CrankyOldMan]

Originally Posted by tooter

I've been reading about another manifold principle in play called resonance. Sound waves created by the valves slamming closed travel back up the runner and are reflected back to the valves by the plenum. This tuning can pressurize the incoming flow as it enters the valves providing a supercharging effect. These sound waves travel back and forth many times even within one valve cycle. So the plenum is a kind of echo chamber, and the size determines the frequencies of rpms that it favors. This principle is somewhat like the principle of exhaust scavenging which uses resonant pulses of adjoining runners to draw the exhaust down the header.

Aah yes, good old Helmholtz Resonators. My first understanding of the Helmholtz resonance was that it applied to the runners and plenum, but that proved to be incorrect since all of the calculators that I found used the volume of the cylinder at bottom stroke. That's where the confusion over the volume of the plenum came in--none of the calculators seemed to care about it--and some calculators even seemed to be based on the runners alone. There was anecdotal evidence that a larger plenum can flatten the torque curve, and that it should be "about 1.5x the displacement of the engine" but no empirical studies or documentation. Perhaps Mr. Holdener had some more definitive results from his 50+ prototypes?

I tried modeling the intake as a series of cylinders in Finite Element Analysis (FEA) software, but wasn't able to get any sensible results. I haven't taken a class in how to do FEA for fluids, a.k.a. Computational Fluid Dynamics (CFD), so it was a rather fruitless effort.