Turbos and belt driven blowers are both superchargers, which allow more than atmosphereic pressure to shove air and fuel into an engine. Most of the time, a supercharged engine behaves like a normally aspirated engine running under vaccuum. But when you step on it, and allow more air into the unit, the superchager acts as a pump and goes beyond atmoshpheric pressure (14.7 psi or thereabouts) and pressurizes the intake charge, shoving the mixture into the engine. This allows a smaller engine to burn more mixture and act like a bigger engine. Turbos are exhust-driven centrifugal pumps. Belt-driven pumps, commonly called blowers, can be of the centrifugal (Paxton), the rootes (Magnussen, GMC, Marshall, etc.) or one of several others, like the vane-type (Schorrock)or screw-type.
Turbos will often release pressure, via a wastegate, sooner than a similar blower setup, due to the incresed temperature of the boost charge. Since most blower setups are "wet" meaning that fuel flows through the blower, you can get away with higher boost, since the fuel cools the charge and gets atomized very finely to boot. Adding an intercooler to a turbo setup to shed the extra heat will allow higher turbo pressures. Turbos also have what is called "turbo-lag". After stomping on the pedal, the impeller takes a moment to spool up before it generates pressure. This is usually offset in modern engines by very high-pressure impeller designs coupled with a wastegate. This gives better driveability at the cost of wasted efficiency whenever the gate bleeds pressure. Blowers have no lag. Since they are belt-driven, thay always have the ability to make boost, but normally are kept from doing so by a mostly-closed throttle. When you stomp on a blown car, air is allowed right into the already over-driven pump, the misture is pushed into the engine, and off it goes. Boost is controlled by pulley sizing so that the blower's cfm is at some percentage over the engine's normally aspirated requirements, say from 20% to 70%, with around 40% over-driven being average.
6-9 is what I would consider a normal range for an engine in the 8:1 or 8.5:1 range. Lower boosts are required for higher compression stock engines. (Say, a Chevy running 9.2:1 or so, you want to go for around 4-5 pounds.) I built a special blower engine for my Toyota Celica with 7.8:1 compression and regularly ran 13-14 pounds of boost. (Of course I had fuel injection and an ignition retarding setup that kept things right on the bleeding edge of detonation...)
And there is the rub. You can get away with quite a lot assuming that the lower end is built to be very strong, especially our long stroke tractors, and detonation is controlled. No hot spots, keep it cool, a careful timing curve, and a camshaft ground for a blower which will be more sedate than a stock grind. The idea is to run the engine at its safe limits AND NO MORE! Just on the edge of detonation is great, but start pinging a blower engine and you will do lots of damage in a hurry. Blower Engines, like nitrous oxide injection, are great fun and very reliable (your average fuel consupmtion will improve due to increased efficency) but if you get greedy, you will pay for it. My old Toyota college project is still running 14 years later and still screams. Wish I never sold it.
But to get right down to it, there are no rules except, "Don't melt it!!" Depending upon lots of different variables, one can use a little or a lot of boost, as long as the bottom end can take it and you don't detenate. To get to specifics, I personally wouldn't bolt a blower right on a stock XPAG engine. It would probably work fine, but not at its peak efficiency. Blueprint the bottom end, since they are a bit flimsy to start with, compression at 8:1 or so, a good camshaft designed for blower use, and some porting and polishing to the head. Carefully fiddle with the timing curve to get the most advance without pinging. Now you are cooking! depending upon variables, you might get up to 10 PSI safely, but 6-8 would still be a kick in the pants!
So, to quote Bill Nye the Science Guy, "Now you know!!!", probably more than the question needed, but I do like blowers.
email me with any questions