Quote:
Originally Posted by MadBlaster
Manifold pressure, I'll take a guess as I'm not a pilot either. I think it's just like a car. Natural aspirated, the engine creates vacume on the piston downstroke and sucks in the fuel/air mix. The higher the rmps, the more sucking power and the higher the manifold pressure. In supercharger setup, pump is belted somehow to the drive shaft and forces air/fuel into the manifold via a pump. I guess what the guys are saying up above to explain your observation about manifold pressure going up when prop pitch is changed to 'course' is that at high rpms, output of the supercharger pump somehow hinders the engine vacume (ala "law of diminishing returns"), causing it to be lower than it would be if the engine was naturally aspirated. So, it's a trade off when you use a supercharger. What you lose in power at the high end of the rpm scale you gain on the low end of the scale (i.e., higher manifold pressure at course pitch setting where the load on the prop in static state (no acceleration or deceleration) is highest).
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Quote:
Originally Posted by Blackdog_kt
The efficiency of direct-drive supercharges is tied to the RPM. This is true.
The above effect is not always linear and probably each supercharger has its own "powerband" so to speak.
There's also another thing to consider. RPMs=amount of combustion cycles per minute. The engine is essentially a vacuum pump, it sucks air, mixes it with fuel and burns it to produce power.
If you lower the RPM you essentially lower the amount of combustion cycles during a given time frame. This means that for the same throttle position, less air is being "used up" by the engine.
Where does this air go then? I guess it stays in the manifold for a while longer because at one end (the intake) air is still being forced in, while at the other end (the actual engine/carbs/pistons) less air is being drawn out of the system. This would easily result in an increase of pressure in the intake manifold system and since this is what the boost/ata/manifold pressure gauges measure (just with different units), it shows up in the instruments.
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Superchargers are Load Sensitive. This means that if the engine is not under load the superchargers will not make boost REGARDLESS of engine RPM. This is something I think many people are missing.
Even though your 1st thoughts are to think that a supercharger is pumping air directly linear with how fast the the supercharger screw(s) are turning (engine RPM), you gotta realize that at even high RPM's unless you are under a load and needing to be heavy on the throttle (aka opening up throttle plate and allowing the carb jets more fuel flow) you are not creating much boost. The engine needs to be working to get ANY boost at any RPM. That same throttle plate limits the amount of air that can enter the manifold to build up positive pressure (boost). It's the same reason why increasing engine load (with the same throttle setting applied) will lower RPMs and increase boost.
It would be like cruising down the interstate at 70mph on a flat road then trying to go up a hill without giving it any more gas (throttle pedal position). You are going to slow down (because of load) unless you increase throttle which in turn, is going to increase fuel consumption, to be able to maintain 70mph. In one instance you are at, say 10% throttle while driving on a flat road. In the other instance you could be at 50% throttle while driving up a hill to maintain the same speed as the 1st instance.