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Chapter 3 - Engines: Knowledge of Engines Is Power

3-29
weighs enough to push a column of
mercury 30 inches into a glass tube
containing a vacuum (see Chapter 5
for more details on barometric pres-
sure). As a measurement of the
atmosphere’s weight, we say that the
outside air pressure is 30 inches of
mercury. Therefore, the engine’s
manifold pressure at full throttle is a
little less than 30 inches (it’s a little
less because of air friction and intake
restrictions within the induction sys-
tem). Clearly, then, manifold pres-
sures near 30 inches of mercury sig-
nifies more power is being developed
by the engine. On the other hand,
low manifold pressures (say 15 inch-
es or so) indicate less fuel and air is
entering the cylinders and less power
is being produced.
As the airplane climbs, you’ll
notice the manifold pressure decreas-
es even though the throttle is fully
opened. Why? Atmospheric pressure
decreases as you ascend. It decreases
approximately one inch of mercury
for every thousand feet of altitude
gain (and increases approximately
one inch of mercury for every thou-
Fig. 49 sand feet of altitude loss). At sea level
you can develop approximately 30
inches of manifold pressure with full
Under normal conditions, the The atmosphere can only push an throttle. At 5,000 MSL, however,
engine’s manifold pressure can’t rise amount equal to how much it weighs. your manifold pressure will be
above atmospheric pressure. Why? At sea level, atmospheric pressure approximately 25 inches with full

Fig. 50