Originally Posted by rudypoochris
The air density isn't much of a concern for measuring pressure drops. I'm trying to think of a way to explain it, it's actually a pretty difficult concept. You have lots of physics going on at the same time from Beroulli's principle (pressure vs velocity), temperature changes, gas laws, multiple restrictions and such.
The best way I can go about it is think of it like this. The air is compressed by the turbo. Instead of "pressure", try to think of this as a air mass... how many molecules you were able to get into that pipe post turbo. A turbo is rated mostly in terms of flow, and they will produce that flow (air mass) at various pressures, and those pressures just have to deal with how efficient they are at moving that air mass (the less efficient they are at moving that air, the higher the temperature and thus the higher the pressure). If you think of it that way... yes, there is a pressure component, but it's mostly a side effect of the turbo's efficiency. The airmass, how many molecules of air... is the important number.
Now, picture a system with no intercooler. You will have 100% of that air mass make it to the engine. It will be at whatever pressure... this pressure is dictated not by the turbo (well it is kinda...), but in fact by the engine itself. Picture one of our tiny turbo's mounted to a 500 cubic inch V8, the amount of pressure would be nothing because that engine would suck it all up and need more... pressure, once again is not really important... just air mass. A larger engine or more efficient engine will suck more air, thus causing a lower pressure than a smaller engine with that same turbo/airmass to get it through the motor... I hope that makes sense. Boost pressure is created by an airmass trying to move through an engine if you have no intercooler.
Now for what I'm trying to explain, a pressure drop on an intercooler... it's like you have two systems. The system between the turbo and the IC, and the system between the IC and the engine. The turbo will move a set airmass, say 50lbs/hr to the charge system. Now, say you have a tiiiiiny little intercooler there that blocks 90% of the flow. The turbo will be trying to overcome this, and thus hit 100psi on the hot side to try to move that air. On the cold side, say your engine can move 50lbs/hr with a boost pressure of 10psi... so you will see 10psi on that side. Now say you replace that restrictive IC with a yellow pipe, you will see 10psi between the turbo and the yellow pipe, and 10psi afterwards. "