Another good question to add to this is
How much HP is it going to take to turn a bigger blower vs a smaller blower to produce the same boost.

In reading Dustin's posts it seems that more HP will be build with a bigger blower vs a smaller one.

Another question, what factors determine when an engine starts making boost? My old Top Gun (468's/671's) started making boost right around 4k. The new boat, modified 575SCI's (flat tops, 525 roller cams) starts making boost at 4800, rev limiter is at 5600.

I like the fact that these new motors don't make any boost until 4800, it's pretty efficient on fuel at that and below.

Without having graphs and flow numbers for a particular engine, I think the answer will go something like this:

When the engine is turing at low RPM, you have to move X amount of air per revolution through the intake, manfold and into the cylinders. At low RPM's the air velocity is slow, and the pressure drop is minimal, and the blower doesn't do much to compress the air.

When the engine starts going faster, you still need X amount of air into the cylinder (assume for a minute you are running normally aspirated). That volume of air now has less time to get to the cylinder, causing air velocities in the intake to go up. Flow resistance will increase with the square of the velocity, so, as mentioned, pressure drop at low rpms is minimal. At some point, the velocity increases to the point where a significant pressure drop occurs. The blower, (assume its a roots constant volume), now starts to pack air because it is trying to overcome the increased restriction. That's when the boost starts. As the motor turns faster and faster, this effect increases, building more and more boost. At some point, the blower, whose power consumption is rising cubically with speed, starts to suck more and more power from the driveline. At some point, the power increase derived from the extra boost pressure minus the power required to spin the blower peaks out. Beyond that, the blower power requirements rise rapidly until the point where it is consuming ALL the extra power it is helping to make, which is the CROSSOVER point for the system.

Small blowers have a lower crossover point than a big one. Usually, due to the lower gearing, the bigger blower won't start making any observable boost until a higher RPM is reached.

Keep in mind that if you are running a supercharger, and reading "0" boost, it may be already doing quite a bit , since otherwise, you'd be running at a significant manifold vacuum.