Which also brings me to another thing in which they have 3 different options for turbos...
"low rpm range turbo" (according to description lacks boost at high rpms) ... lol
Uh, there are many, many turbos that, while they spool quickly, cannot flow enough air to maintain boost pressures at high RPM. This is why many Volvos, for example, have had a low-pressure turbo engine and a high-pressure turbo engine.
The LPT engine comes on boost faster (generally before 2,000 RPM) but because the compressor wheel is smaller, it loses efficiency at high RPMS and the wastegate starts opening at lower boost pressures to avoid overdriving the turbine and causing excessive heat in the intake charge.
The HPT engines use a turbo with a larger compressor wheel and the turbine takes longer to spool (typically above 2,500 RPM) but the larger compressor side means the turbine does not lose as much efficiency at higher RPMS which results in better peak power.
Also, as engine RPM increases, so does flow volume. Because efficiency for a turbo decreases if it's driven beyond peak, even though it flows more air at higher RPM, this increase is not always enough to keep even with the engine's increased flow, which results in overall boost pressure decreasing.
Basically, when setting up a single turbo, you have trade-offs. You can set up a small turbine that spools quickly but runs into efficiency limitations and peters off well before redline if you want to boost low-end above all else. Or you can set up a medium size turbine that takes longer to spool and leaves the low end a little soft but stays on peak boost longer. Or you can put a huge turbo on that takes ages to spool but can maintain boost right to redline. These differing tradeoffs are perfectly analogous to PD's "Low RPM", "Mid RPM" and "High RPM" turbo setups.
As far as positive displacement superchargers go, the key to understanding them is in their name. They flow a fixed ("positive") amount ("displacement") of air for each revolution of the crank. At high RPM they absolutely do flow more air than at low RPM in absolute terms. However, their gain is strictly linear with RPM which means in practice that the boost level for a properly-matched positive displacement supercharger will remain constant. This is contrasted with a turbocharger (or a centrifugal supercharger) where the flow rate increases exponentially with RPM (up to peak efficiency). A positive displacement supercharger provides the same proportional increase across the whole RPM range while a turbo produces an increasing power increase as RPMs climb (up to the peak efficiency point).
