This is a good time to learn about the Engineering Toolbox website [LINK]

"Corrected Air Flow" (numbers across the bottom) is the air flow in pounds of air per minute, with the air density as a correction factor. In "children's book" lingo - it's a ton of math to wrest this number to the ground. There's no easy way to sort that out, unless you go on the Garret website and use their very basic formula to color inside the lines when sizing a turbo. If you want to be precise - break out the slide rule, spread sheet, reference charts, and Ouija board.

The Pressure Ratio (numbers up the left) is more straight-forward. Take that number and multiply it by the air pressure of your altitude. Sea level is 14.7 PSI, so adding 14.7 PSI boost gives you a Manifold Absolute Pressure of 29.4 PSI - Pressure Ratio of 2.0.

That hard line on the left is a cliff, almost quite literally. That is the stall line. If you try to cross that line to the left, the turbo will stall, loose speed, then try again - over and over. This is where the warble ogre named Surge lives, and he's an annoying bugger. He likes to sing like a rabid PacMan, and he beats the boost needle down like it's his own personal drum set.

The curved line on the right is the fence between you and the fire-breathing dragons. This is where a combination of exhaust heat and flow pushes the turbine so hard that it exceeds the ability of the compressor wheel. Crossing that line makes the EGT needle spin right and incites the Exhaust Back Pressure to climb in an effort to flee the Manifold Absolute Pressure.

Boost and power. More boost is good - right up to the point where it's not. Too much air "thins the mix" and is just adding backpressure and heat. Now don't anybody get their panties in a wad and start a big discussion on stoichiometry - I think we can all agree that adding a phonebooth of air to one injector shot will not liven up the combustion compared to a breadbox of air.

Rule of thumb: More air to the cylinder increases the O2 molecules available for the fuel to meet. Increased boost definitely heats the air more during compression - making a more energetic combustion. More air in the cylinder also means more expansion when flash-heated by combustion. Too much air can stall the cylinder from reaching TDC, thus stalling the truck like a hydrolock. Somewhere in the middle is that Goldilocks boost pressure. The Goldilocks boost has a visual indicator - clean exhaust. This is the point where more boost won't do much for you.