Originally Posted by RealStreet

A few things come into play but basically think of it in the terms of pressure and mass @ temperature. Different turbochargers can move different air masses at different efficiencies (easier understood as temperatures). If a turbocharger can move say 40lb/min of air (about 400hp) but do it more efficiently - at the given pressure, the air mass will be more dense. If it is more dense, a larger mass of air is getting in the cylinder versus the same volume.

It may be hard to understand but think of it as if you had a 2 liter of soda at room temperature you had cracked and poured a glass out of. If you reseal it, and put it in the fridge, the next time you pull it out you may notice that the container as almost 'sucked in'. When you take off the cap it will then fill and regain it's normal shape. Then you take that cold 2 liter that is now "filled back up." Reseal it and go put that in the sun. When you come back to it in 2 hours, you'll feel how tight the container is as the air and soda expanded due to the temperature change. It's not that any less air or soda is actually in the container when it gets cold or that any more is in it when the container is hot, but a substance has a certain density at a certain pressure. As the temperature goes down, it becomes more dense and the inverse is true for when the temperature goes up.

If a compressor wheel can move a certain amount of air at a lower temperature and higher density, than this turbocharger will be able to make more power at a lower 'boost level' since boost is simply a measure of the pressure of the air NOT getting into the engine...something hotter with a higher volume will create this pressure earlier (at lower horsepower) than a more efficient system.

Not only that but there is also something to be said for turbine wheels. If you have a higher flowing turbine wheel/housing combination, then it may also help an engine make more power at a given boost level. As exhaust pressure increases before the turbine wheel, it can cause flow reversion in the cylinder during camshaft overlap. I'm just using these numbers as an example, but lets say a given turbochager has 25lbs of exhaust pressure to move 400hp of air/fuel through it. Another turbocharger can get the same 400hp through it with only 15psi of exhaust pressure. This means that the clean air mass coming into the cylinder is fighting against 10 less pounds of pressure while entering the cylinder. High exhaust pressure during camshaft overlap can essentially shrink the cylinder (reduce dynamic compression ratio) since it's slowing the air mass moving into the cylinder on the intake stroke. When you look at it in these terms it's easy to see why the exhaust pressure compounded with compressor efficiency can change the power that a given turbocharger will make at a certain 'boost level'. You can see the exhaust pressure problem really start to happen when you see people with small turbine wheels or small housings dynoing on big boost, the precipitous drop in VE / horsepower is many times due to exhaust pressure stacking up and reducing the engine's ability to breathe. If you can't get air in and out, you can't make power. Generally the larger the engine, the sooner this is an issue on a given turbocharger since it's able to make a certain amount of power at a lower boost level since it's a larger engine. Obviously the turbine doesn't know what size engine it is working with, it just knows how much air and fuel is going through it. The ratio of boost pressure to exhaust pressure is what's important here.

There is a lot more to it but that should give you the basics.