Originally Posted by motodave
Same as std James aluminum inlets, dia is 4.8"
Got it. I do hope you'll measure upper and lower pressures after you get flying. I'll be happy to help you with a standardized method which allows comparison with similar applications.
I asked about the inlet ring size because of the shape of the plenum inlet necks. Cowl exit area sets mass flow, so given two cowls with the same exit, inlet size sets inlet velocity.
Our goal is to convert dynamic pressure to increased static pressure by smoothly slowing the flow. It can be done externally, out in front of a large area, low velocity inlet, or internally, with a smooth expansion of duct area. Given significant velocity, sharply diverging expansions have the potential for flow separation, resulting in less static pressure (sketch below).
So returning to inlet size, a 4.8" inlet is 18 sq inches. A 6" inlet (for example) has roughly 50% more area, while a 4" inlet is roughly 50% smaller. Given the same cowl exit, the three inlet velocities are very different, smaller being much higher.
In practical application (i.e. cowls for airplanes like ours), inlets in the 6" range are so insensitive to separation that they are often seen with no duct at all, just a nicely shaped lip around a hole dumping into the upper cowl volume. At the other end of the spectrum, applications with small inlets require carefully shaped ductwork.
Take a look at one of the plenum lids Bill Lane has done, like Rockwood's in post#15. I think they use the same 4.8 ring, but expand more gradually. I'm not saying yours is not going to work. It will, the question being how well, i.e. coefficient of pressure per CR3405. I'd love to see pressure data for the two otherwise similar installations.