OK... it took a long time to read through this thread over a couple days due to a lot of ahhh...let call it disagreement. But God help us...lets get back to the original question and better yet lets expand on it a wee bit. (If Av-bug and DC-8 could both provide a short simple explanation...and hopefully they'll be in agreement)...I'll copy and paste it into my CFI notes.
#1. During a run-up, why does manifold pressure increases when RPM is dropped...
and (let see if yall agree )
When you do a run-up, you typically dont have the throttle at full power, so the throttle plate is restricting the airflow. So as the ambient pressure is trying to push air into the manifold as the piston goes down it can't get the entire cylinder volume past the throttle plate and you end up with a lower MP reading.
Now when you pull the prop lever back for a prop check you slow the piston down and the ambient pressure has more "time" to push air into the manifold and the MP reading rises.
#2 Why at cruise does manifold pressure rise when you increase the throttle but don't touch the prop lever and...
Same reason as above as to why you have a lower than ambient MP reading while doing a run-up. When you open the throttle you reduce the restriction caused by the throttle plate and ambient pressure can push into the manifold more easily and pressure goes up. Notice that (in non turboed) engines MP is never higher than ambient other wise the air would be flowing out of the manifold. Also try it with the throttle full open and then pull the props back 200 or 300 RPM you wont see any rise in MP, save a small amount that is just recovery from the other restriction in the manifold (filter, friction, bends, etc.)
#3. At crusie power (not low power like in a run-up)... if you reduce rpm by pulling back on the prop lever does manifold pressure still rise?
Yes, as long as the throttle is not full open. If the throttle is full open you are already getting as much "air pressure" from ambient pressure as you can into the manifold.
#4. And I can't help it but I'd like to know if you agree on why during low rpms on the ground does the throttle control RPM and manifold pressure. With props full forward we are able to add the throttle and increase the RPM to approx 17-18 hundred RPM... but at higher throttle settings RPM is controlled by the prop / governor.
Again the governor has high and low pitch stops. So with the prop lever full forward the governor is trying to maintain 2700 RPM (depending on a/c model). When the prop hits the low pitch stops and you keep pulling the throttle back, restricting airflow, reducing fuel flow and thus reducing power, the airload on the prop starts to slow the engine down since the blade can't "flatten out" anymore.
Just the opposite for why you dont go over 2700 RPM (usually) with full throttle, the high pitch stops of the prop are set to maintain 2700 RPM at full throttle at sea level on a standard day plus or minus some fudge
factor.
Chec the above statement, now I cant remember how it is actually set up, since I recall on more than one occaision takeing off with an OAT of negative 20 and Altimeter of 30.??. So that doesnt quiet hold water.
If you could theoretically turbocharge that engine you could get the prop over 2700 RPM because the high pitch stops would not let the prop twist anymore to a higher angle of attack and with the increase in air mass into the engine, it would have enough power to "overspeed" the prop.
Don't confuse that with the slight overspeed(s) you get when you go from low power settings to high power settings, or low RPM settings to high RPM settings, rapidly in flight. That is just the governonr not being able to increase blade angle quick enough to maintain the "set" RPM.
MUCOS GRACIAS
