I admit to not knowing ANYTHING about the powerplate specifically (I have a hotair, so it doesn't apply to me). But I DO know a LOT about fluid mechanics. I have a Masters of Science in Mechanical Engineering (not bragging, just stating) and am working towards a doctorate. Besides the several semesters of coursework in fluid mechanics, I have done a fair bit of research in the area, and messed around with CFD (computational flow dynamics) a little bit.
As far as Ross' specific question, "why does the plate matter, isn't the entire plenum at the same pressure?" the answer is yes and no. There are two components of pressure you must consider: static pressure and dynamic pressure. You can think of this as stationary pressure and moving pressure. If your plenum had no flow through it, if it were essentially a tank filled with still air pressurized to 24 psig, and you opened the intake valves, then you'd be correct,they would all flow the same volume of air (well, not exactly, there would be slight differences even then due to differing geometries in the intake, etc). The problem with this simplification is it does not account for the 'flow' (movement) of air through the intake. This is where dynamic pressure comes into play. Dynamic pressure is the result of moving fluid. Think of it like this: If you're driving down the road on a 'standard day' at sea level, the air pressure around you is about 14.7 psia. Now if you stick your hand out the window into the airstream, you will feel a large amount of pressure on your hand. The air inside and outside your car are at the same static pressure (14.7 psia) but the motion of the air relative to your hand creates a dynamic pressure which is added to the static pressure to find the total pressure. A simple device called a pitot tube uses these principles to determine flow characteristics, for example flow velocity. It consists of two small tubes with their openings 90* to each other. One tube is oriented so its opening faces into the flow, and the other is therefore perpendicular to flow (and therefore sees only static pressure). The difference in pressure seen by the two tubes can be used to determine flow velocity (many aircraft use these to determine airspeed). The greater the velocity of a fluid, the greater the dynamic pressure (specific gravity of the fluid also obviously plays a role, but since we're not comparing different fluids here, thats kind of a moot point).
How this comes into play in your intake is this: for reasons that would be difficult to explain easily and concisely (hell, its tricky for me to understand this stuff and I do it for a living) the air moving along the back wall of the intake moves at a higher velocity. This increased velocity means the dynamic pressure (in the direction of flow) is also increased. This means the total pressure in that area will be higher. Since fluid flow is driven by pressure differential, intake runners/valves in that area will flow more air when opened. As an illustration, consider this example: (disclaimer, I'm picking these numbers right outta my a$$, I can guarantee you they are waaaaay off, i'm just using them to illustrate a point) Lets say the static pressure is at the 24 psig you mentioned. Lets further say that the air at the back of the intake is moving so fast that it has a dynamic pressure of 10 psig, while the air at the front of the intake was so slow it only had 2 psig dynamic pressure. The net effect would be that while the whole plenum was pressurized to "24 psig" as indicated on your boost gauge (which measures static pressure), when the intake valves opened onto cylinders (that are at 0 psig), the pressure differential at the rear would be 34 psig, while only 26 at the front. This difference in the pressure differential across the intake valves will cause the the flow difference (more air and fuel to some cylinders than others).
As to whether or not the powerplate will "restrict flow" or not, its not a simple answer in any way. As stated previously, if you have 2 'restrictions' in a system, one with a cross sectional area of 10 sq. in. and one of 20, changing the second one to 15 sq. in. will still likely hinder flow, even though its still larger than the first restriction. In a system as complex as our intake, it would be virtually impossible to arrive at an analytical solution. You could model the system and make some approximations, or use a finite elements/CFD approach, and that might shed some light on it. Or the simplest way would be to test an intake setup on a flow bench with and without the plate and get some empirical data. The fact is though that as I understand it, it's a moot point whether or not it reduces flow or not. I don't think any of us compare whose intakes flow more CFM at a certain pressure differential, do we? We generally compare power and torque numbers, or 1/4 mile trap speeds and ET's. If the plate allows for more boost and more power on a given setup (which I think the consensus and the limited testing i've come across says it does) then does it matter if in the process of doing that it reduces flow through the intake?
Think of it this way, we all know that better exhaust flow means more power in an internal combustion engine (thats why headers and 3" exhaust systems are made). By bolting a turbo into that exhaust stream, you are adding one hell of a restriction to exhaust flow. Do any of us care? No, because the net result is more power and more speed. The powerplate seems to make people's cars run faster. If in the process it reduces intake volumetric flow slightly (which it very well could, and certainly seems like it would) does anyone care?
If I sounded like I was talking down to anyone on technical issues by over-simplifying, my apologies, it certainly wasn't my intent. If I sounded like I was talking over anyones heads, same thing goes. It's hard for me to put this stuff into words when I barely understand it myself, much less make sure other people can make sense of those words
Fluid mechanics is tricky stuff, very little of it is intuitive. I was just trying to shed some light on a subject that for once I actually know something about
