It hit me one night at the shop, while some power-hungry, new-fangled Subaru was being tuned on the dyno. We've been going about this all wrong. Our entire obsession with boost is a moot exercise. All over the forums, even in our editorials, we're obsessed with measuring or talking in terms of boost. What we really care about is flow, not boost or manifold pressure. They aren't even closely related: flow has to do with time, while pressure is an instantaneous measure.
While it says something when comparing boost levels in the same car with all the same hardware, any time the turbo, intercooler charge pipes-or even temperatures-are changed, boost takes on a whole different meaning. Boost, unfortunately, is all relative.
If you remember from basic chemistry or thermodynamics, air (or any gas) has dynamic properties stipulated by temperature, pressure, volume and mass, as well as the properties of the individual components of the gas. For simplification, let's treat air as a uniform substance, instead of a mess of nitrogen, oxygen, carbon dioxide and whatever else. This allows us to ignore things like the compressibility factor, specific heat values and lots of other stuff I've forgotten. The ideal gas law says that for a given gas, its state can be described by the amount (or mass) in a closed system its volume, pressure and temperature. More confusingly, change any one of these four properties and, in a strange kind of co-dependent relationship, at least one other property (or all three) will change proportionately.
Since co-dependent relationships are pretty confusing, we should hold at least one property constant. Let's say we charged a rigid volume like an air tank with air and sealed it. With two properties held constant, only pressure or temperature can change. Heating the tank, which increases the air temperature inside, sees the pressure rise, since all the heat makes the molecules bouncing off each other bounce even faster, exerting more pressure. Cool it down and the pressure drops.
If we only held volume and regulated the pressure, but allowed air to move in and out of the tank, temperature would have to change. In the real world, we see this a lot in propane tanks, since volume is constant, but as you burn off the gas and try to maintain the same regulated pressure, the bottle frosts over because only temperature is allowed to change. The gas temperature drops to compensate, causing the bottle to cool to the point where moisture in the surrounding air turns to ice and frosts the bottle.
If we took a snapshot of an engine, assuming the valves are closed and the turbocharger is boosting, we would have a scenario similar to the air tank (we'll define our air tank or closed system as everything from after the turbocharger up to the intake valves. This way we can ignore the turbo's pressure dynamics or the valves' flow limitations). With the volume of air inside held constant, only pressure and temperature can change. If we were to change to a larger intercooler, piping, or even a larger manifold, the same amount of air would drop in pressure or temperature, or both, on account of the increase in volume.
However, engines are dynamic things, constantly pumping air through, so the snapshot analogy doesn't work so well. Assuming the same system boundaries as before, let's look at the engine in steady-state operation. Since the wastegate is sometimes referenced off manifold pressure, we'll assume pressure is constant and ignore the pressure drop across piping and the intercooler. With larger piping and cooler charges from an intercooler, volume has increased while temperature has dropped. To balance this out, the amount of air in the system (which we now have to call flow rate, since time is passing) has to increase, which is exactly what we want-more air.