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So a PD blower provides a set amount of air per revolution, where as a centrifugal blower is progressive, as rpm rises so does the amount of airflow.
Boost is a measure of restriction of the engine. With centrifugal type blowers and turbocharger compressors, they have lesser or greater efficiency depending on the level of restriction of the engine and the rpm. Most producers will run tests and provide compressor maps of their products. This will give you an efficiency level based on the pressure ratio and the rpm of the turbo.
You can determine what efficiency your combination will likely be at by understanding where your combination will fall on the compressor map.
Now, with either type of compressor a lot of top end versus low end power and how it comes on will depend on the N/A power curve. This is because the amount of air the engine can ingest at different RPMs will affect how efficient the compressor is at providing air and what boost the car will be running.
IOW, there will be combinations where a PD will provide higher boost at higher rpm and combinations where it won't. Regardless, with a PD blower, you will know EXACTLY how much air it's providing at any given rpm.
My setup provides 1000cc's/revolution. So everytime it spins over it provides 1000cc's of air.
I was able to use this knowledge to estimate exactly how much boost I would get. This allowed me to choose the right pulley for the superchargers to get where I wanted. With FMIC losses, I max out at about 8 psi. Which is right where I calculated it would be.
So, if your car ingests a disproportional higher amount of air at the top end versus the lower end, then it will have less boost at the top end with a PD blower, if it ingests less at the top end proportionally, you will have more boost, but for every revolution you are getting the same amount of air, it is fixed.
Jaden
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