Bdawg and Babi: The engine gauges on all aircraft provide a measure of the same thing, they just use different units. What they measure is the air pressure in the engine's intake manifold. If i'm not mistaken, this is where the fuel metering valve mixes the fuel with the air according to your chosen mixture settings, if there's a lot of air you get more fuel, the engine "sucks" more air to burn because the throttle valve is more open and you get more power. This is probably just before the fuel-air mix enters the cylinders to burn.
How it's measured in engines that use direct fuel injection inside the cylinders i don't know for sure. However, for most intents and purposes manifold pressure shows you how restricted, unrestricted or even force-fed (in the case of supercharging and/or turbocharging) the air supply is into the engine. This is a measure of power, because the engine is essentially a vacuum pump: without the throttle valve all engines would run on full throttle all the time, but that valve can restrict the amount of air that goes into the engine and thus, it starves it of the needed air to burn fuel and decelerates the rate at which the burn cycle occurs, enabling us to reduce power.
The Luftwaffe ones are metric so they use Ata, which is the atmosphere unit (aka bar). Modern general aviation aircraft and American WWII aircraft use inches of mercury (Hg), while Italian ones use the metric equivalent which is millimeters of mercury.
This unit of measure dates back to Torricelli's experiments. Imagine a vertical lab tube filled with mercury with a repository "dish" below it that holds more mercury...if the atmospheric pressure is high, the air pushes down on the surface of the liquid mercury in the dish and forces it to "climb" inside the tube.
1 Ata is the standard air pressure on sea level for a standard weather day (which is defined by a set of further data like temperature and so on, let's not get even more complicated if we don't have to
).
The rough conversion between units is that 1 Ata (or 1000 millibars) = 760mm Hg = about 30 inches Hg (i think it's actually 29.92 inches, on modern dual-unit altimeters the calibration window shows 29.92 inches when you set it to 1000mBar).
You probably realized i left the RAF dials last. This is because
a) The RAF planes automatically adjust the throttles to maintain your chosen manifold pressure. In a USAF or Luftwaffe aircraft, as you climb higher into thinner air you will need to gradually advance the throttle if you want to maintain the same amount of manifold pressure, if you dive to lower altitude into thicker air you will also need to reduce throttle to prevent the pressure from going too high.
In an RAF aircraft you set it once and it will do its best to maintain it regardless of altitude changes. In that sense the power lever in a Hurricane or Spitfire is not controlling the throttle directly, it's controlling the target value of a system that varies the throttle to maintain the selected manifold pressure.
b) The units are relative and not absolute.
This means that while the boost gauge measures manifold pressure in psi it is not the actual pressure in the manifold. What it measures is the pressure in psi
relative to the standard pressure on sea level ie, it shows you how far above or below you are from standard sea level pressure.
So, 1 Ata = 760mm Hg = 29.92 inches Hg = 0 psi boost
I think that 1 psi = roughly 2 inches Hg (or is it the other way around? i'm a bit fuzzy at the moment).
In any case, it should be enough to show you that zero boost is by no means low power. It's actually full power on sea level for non-supercharged engines and probably why almost no current day general aviation aircraft use this measuring convention: it's neither intuitive nor satisfying for a general aviation pilot to have a big fat zero indicating full power, since most of these aircraft are normally aspirated (that is, they lack superchargers/turbochargers)
