There are two main factors in what makes an intercooler work effectively.
1. Pressure Drop
2. Heat Rejection
#1 Pressure Drop
Is exactly as it sounds. If you put 13psi into the intercooler and receive 10psi out of it.
You have a 3psi pressure drop across the intercooler.
Clearly the lower this drop is, the better the intercooler is allowing the air to flow through it.
Somewhere between .5psi and 2psi is a good level of drop to have. With zero drop at all, the intercooler probably doesn’t have enough internal surface area to cool the air passing through it effectively.
#2 Heat Rejection
How much heat can the intercooler pull out of the air and how fast can the intercooler pull it out.
Think of dissolving aspirin in a glass of water. Drop the whole tablet in it and it will take 2-3 minutes to dissolve completely. The surface are of the aspirin tablet we will say is 3cm2 (the whole outside of it)
Crush the aspirin tablet up to a powder, drop it in and watch it dissolve in a matter of seconds.
What we have effectively done is increase the surface are of it to say 30cm2. By having smaller pieces, more of it is in contact with the water at the same time allowing better mixing.
Same rule applies with intercooling. Having fins or tabulators inside the tube allows a lot more of the air inside to be in contact with the intercooler at the same time vastly reducing the time taken to cool, and allowing more cooling to be done for a given size intercooler.
Water to Air Intercoolers VS Air to Air Intercoolers
The first thing to say in this matter is that neither is perfect or best for every situation.
Both have pro’s and con’s which I will attempt to outline here, but do not use this as a definitive guide as to what is going to work for your particular setup
Water to Air
Air to Air
It is clear to see, many considerations must be taken when choosing what is going to benefit your own vehicle.