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Originally Posted by Babi
I think we are expressing the same thing with different approaches. To me it's more intuitive to think that the intake pressure is dependant on "throttle" (throttle butterfly valve position) which causes a pressure drop, and engine RPM because it mechanically drives the supercharger (the relation between supercharger's RPM and its compression ratio may depend on the type of sc, not sure though).
A loaded engine at 2000 rpm achieves more boost (and consumes more fuel) than an unloaded one at 2000 rpm just because in the unloaded one the butterfly valve (throttle) is "more closed" (otherwise RPMs would go up) which causes a bigger pressure drop.
the point is that, ignoring the external pressure dependance, the intake pressure should depend on 2 variables, not three.
Do we agree on this?
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Actually the actual intake pressure of a particular engine will depend on so many variables that I could write a 10 page essay about it. You can use RPM and throttle position as the variables, but without knowing the particular case of which you using the engine, what engine, what percentage of load you are using, it's hard to make any sort of analogy to what's actually going on in a given instance of operation.
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Regarding the game, at constant altitude and throttle, if the prop pitch is coarsed (load increases, RPM decrease) the intake pressure goes up, which is not what one would think since the supercharger spins slower.
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1st you have to understand the principles between the 2 (engine and supercharger) to realize that you will always have positive manifold pressure in that situation.
Think about it this way. Lets say we have a supercharger that creates a nominal output of boost of 8lbs. That means @ full load and full throttle situations you are going to pump out 8lbs of boost / positive manifold pressure of 8lbs. Lets say the operating RPM range of this engine is 600 RPMs to 6000 RPMs. So when you are maintaining 5000 RPMs with 50% throttle and jump the load up significantly to bog the engine down to 4500RPM's (500 revolutions per minute) all while maintaining the same throttle position, you have effectively increased boost because the engine has slowed down it's air intake significantly while the supercharger has only done this marginally. This creates a boost situation. This is exactly why a supercharger is used. Because now, when you jab the throttle, you not only have the fuel to feed it and increase RPMs, you also are exponentially increasing the amount of air the supercharger is positively charging the intake to feed the monster / increase RPMs / go faster / etc.
So the supercharger as you know, is always supplying air, but the engine (say a V12/V16) decreasing 500RPMs with the same throttle position, will significantly decrease the air it draws in (16 pistons/connecting rods all with w/e stroke - basically one big huge vacuum pump (the engine) being slowed down) while that 500 RPMs on little Mister super charger only slows down it's output marginally. The supercharger outlet is probably only slightly bigger in diameter than a single piston of said 16 piston engine. On most supercharged cars it's usually smaller in diameter than a single piston. And with a max output of 8lbs of boost (for instance) slowing down 500 RPMs of that monster vacuum pump is not going to effectively create a 1:1 vacuum pressure with relevance to each other (engine air sucking to supercharger air blowing). It's going to exponentially increase or decrease depending on the situation. That is why at full load and full throttle (say jabbing the throttle at 2000RPMs) you are going to maintain maximum boost and virtually identical boost completely through out the RPM range that you jabbed the throttle at (2000-6000 rpms)
That make sense? I've tried to type it out as best as I can explain it through text lol. I'm running out of analogies!