Right, let’s pick this up — Page 143, Lesson 176

Right, let’s pick this up. We’re looking at how the pilot knows exactly how much pressure is being pushed into the cylinders when the throttle is opened — especially when the engine is being overboosted. The pilot needs an indication of the amount of pressure he or she is allowing into the cylinder with the throttle. That pressure, measured between the throttle valve and the inlet valve, is called manifold pressure. And it’s shown to the pilot on one of two types of gauge. The first is the boost pressure gauge. Boost pressure is defined as the pressure in the induction system relative to sea level standard pressure. The gauge is calibrated in pounds per square inch — psi — above or below standard sea level atmospheric pressure, and that standard sea level pressure is marked as zero on the gauge. So the zero point isn’t no pressure at all; it’s the reference point of standard sea level pressure, which is 14.7 psi. Let me make that concrete with the numbers from the text. If the boost gauge reads -3 lb of boost, that means the absolute pressure in the induction system is 14.7 lb minus 3 lb, which equals 11.7 lb absolute. If instead the gauge reads +4 lb of boost, the absolute pressure is 14.7 plus 4, which equals 18.7 lb. So a negative reading means you’re below sea level standard pressure, a positive reading means you’re above it. Now the second gauge, which is the American practice, uses the term Manifold Absolute Pressure, abbreviated MAP. Here the manifold gauge indicates the absolute pressure, and it’s measured in inches of mercury, written as inches Hg. The key relationship is this: when atmospheric pressure is 14.7 lb, it will support a column of mercury 29.92 inches high. So a boost pressure of 0 lb is the equivalent of a manifold pressure of 29.92 inches Hg. That’s the same reference point, just expressed in a different unit. And to make a quick comparison between the two systems, you can assume that two inches of Hg is approximately equal to one pound of boost. So if you see a MAP reading and you want to think in boost terms, roughly every two inches of mercury corresponds to one pound of boost. So the core idea here is that both gauges are telling you the same thing — the pressure in the induction system — but one is referenced to sea level standard pressure and reads in psi above or below zero, while the other reads absolute pressure in inches of mercury. The zero on the boost gauge is 29.92 inches Hg on the MAP gauge, and the two-inch-per-pound rule lets you convert between them.