supercharger inlet size?

[jstein]

Greetings! How does increasing or decreasing the size of the inlet effect the boost curve of a roots type supercharger? I realize that being a pos. displacement pump its going to pump the same air every rev. regardless of rpm but at higher rotor rpm with a small inlet would the vacume become so high that it chokes any further air delivery? I would like to increase the pumping eff. at higher rotor speeds even if there is a trade-off of a loss at lower rotor speed.-thanks

 

[Rentan]

I'm far from being an expert on the use of superchargers, but I don't think a larger (assuming your talking about the inlet into the blower itself) inlet size would create any sort of negative impact on low end torque, or be detrimental to the supercharger boost curve, which like you said is basically set, and is mainly the product of internal volume and lobe size/shape. Logically, it seems a larger inlet would just allow more air to be compressed at a time, which in turn would create more boost all the time, thus squeezing more compressed air into the engine at all rpm's, and finally creating more power all across the rpm range, plus, it seems like the actual inlets on superchargers like turbos are about as big as they can be while still retaining the integrity and functionality of the structure around it.
But, maybe we should first ask, "what would the possible benefits be of using a smaller inlet to essentially restrict the air going into the compressor?"
ANSWER: see next question
"Have you ever heard of someone purposely making the air inlet of a turbo smaller, in order to increase low end torque?"
ANSWER: I don't think so, turbos have huge inlets, like superchargers, they want air as fast and as abundant as they can get it, because they are strictly air compressors, and their sole purpose is to cram very dense air into the engine's cylinders so it can burn more fuel and make more power and torque, so the more air they cram in regardless of rpm, the more power and torque right?

So, I guess my point is that, in thinking about this, one has to remember that it is the engine that creates the actual power and torque and does the work, and that torque curves and horsepower curves are mostly dependent on factors within the engine itself, all the compressor does is create greater-than-atmospheric pressure in the engine intake, and in complete honesty there is no concern of low- or high-end torque in direct regard to the supercharger's inlet size ( like ther would be with a normally aspirated engine's intake size) because the supercharger doesn't make any torque at all...ever...period, that takes place in the engine, remember the engine is also spinning this thing. I don't mean to make all these points to teach you what you probably already know, but just to illustrate my thought process, and finally I was also wondering what reasons you have for thinking that a larger inlet (greater/ smoother flow of air into the compressor) might sacrifice low end torque made by the engine...which would be receiving greater quantities of compressed air from the supercharger, for making power and torque.

Finally, remember that my main point is just that the supercharger has no torque or horsepower curves, because it doesn't make any torque or horsepower. It doesn't motivate anything...it doesn't motivate even itself, it simply compresses air via engine drive, and when you start to talk about torque curves the question is really, "How does my particular engine respond to varying amounts of boost across it's rev range?" another question is "will making the inlet to the supercharger bigger even affect the amount of compressed air going into the engine at all, or is it already as big an opening as the fixed volume of the compressor could ever require for air to be drawn in most effectively?" and thirdly, "Is there ever a situation where having LESS boost will give me MORE torque or horsepower at a given point in the rpm range?"