Could you say that when the RPM is reduced there is less flow of the air/fuel mixture through the induction manifold resulting in less dynamic pressure which would mean an increase in static pressure hence an increase in MP?
There might be an element of that (trying to think about it now...
), but it is not the main effect. You are confusing two concepts. This is all about total energy loss due to friction, so Bernoulli's theorem does not apply. Remember the very important part of Bernoulli's theorem that mentions "a streamlined ideal fluid" - airflow in the induction manifold is not streamlined, and therefore suffers from real energy loss. Dynamic + static is not constant.
The harder the engine sucks, the faster the air will flow, and the more q (dynamic pressure) you will get inside the manifold. But friction causes you to lose static pressure - the more q you have, the more pressure is lost to friction, and the greater the energy loss. When bringing the air to rest again inside the cylinder head, you will end up with a much lower static pressure because so much energy has been lost due to friction.
You asked for an explanation with physical concepts. I wonder if an example might clarify?
Atmospheric pressure (static) 100
Air accelerated into an induction system running at
high rpm: static 50, dynamic 50.
Downstream of this point, past some kinks and corners in the tube: static 25, dynamic still 50 (air is still moving just as fast). We've lost half our static pressure due to friction.
In the cylinder head (air brought to rest): dynamic converted to static pressure, total now 75. We've lost 25% of our original pressure.
Atmospheric pressure (static) 100
Air accelerated into an induction system running at
low rpm: static 70, dynamic 30.
Downstream of this point: static 60, dynamic 30. Because the air is flowing more slowly, it can get past the obstructions with less turbulence and less loss in static pressure.
In the cylinder head (air brought to rest): dynamic converted to static pressure, total now 90. We've only lost 10% of our original pressure, and you the pilot will see a higher manifold pressure.
Barit1
But the vacuum-powered wipers were in effect a "manifold vacuum gage", driven by the pressure drop across the carb venturi. If the throttle was wide open, especially at low RPM, there is no pressure drop available to drive the wipers.
Again, and hopefully without causing offence, no, no, no. Manifold pressure (even that which drives windscreen wipers) is
not related to the carburettor venturi. A fuel injected engine, which has no venturi, still has a variable manifold pressure as a function of throttle and rpm. Windscreen wipers etc might be driven by the pressure difference between
the atmosphere and
the induction manifold. Carburettor doesn't come into it.
Older light aircraft sometimes have a little double venturi tube beside the pilot's window; the static pressure drop is used to drive the vacuum instruments. However, the carburettor venturi is used to supply fuel to the induction airflow, and no-one is going to mess with that very critical part by putting other services in the way!