In terms of efficiency:  

Air to Ice Water – Most efficient since it offers the potential of getting below ambient. Downside is that the benefit lasts only until the ice melts. 

Air to Air – Next in terms of overall efficiency, best in terms of sustained operation. 

Air to Water with dedicated cooling circuit – Has a short-term advantage over air to air until the cooling circuit reaches a stabilized temperature. Then will have a lower efficiency. This is because for both air to air and air to water, the heat is ultimately being exchange to the ambient air. Except with the air to water cooler, it’s really an air to water to air cooler. In short, the heat is being exchanged twice. The first exchange is from the charge air to the water. The second exchange is from the water to the air. Each exchange represents a loss in efficiency (no perpetual motion machines according to Newton). The extra interface represents a net loss.

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     The merit of one core over another core is its efficiency versus its internal drag characteristics

Laminar flow is the enemy of heat transfer; turbulent flow is its friend. This will require some explanation. Picture a fluid moving between two parallel surfaces, like air flowing between two fins or interior tube walls. During laminar flow, the fluid molecules are moving in parallel streamlines. The fluid is moving the fastest right in the middle. As the fluid gets closer to one surface or the other, it moves slower and slower so that at the surface the fluid is not moving at all. This fluid that is not moving is called a boundary layer. It’s bad for heat transfer because the boundary layer is the only fluid that is directly experiencing the heat transfer. It in turn has to pass heat on (or absorb heat from) each subsequent flow stream, one to another, in order to affect the bulk of the fluid flowing at the center. The boundary layer insulates the moving fluid flow from the surfaces where the heat transfer is taking place. And as we know, air is an excellent insulator.

The interior design of the tube primarily governs Air to air charge air cooler performance. There are three basic types:

Turbulence producing internal fins
Laminar fins
Open tubes

Air resists the transition to turbulent flow. Any turbulence has to be produced by the geometry of the matrix through which it is flowing. Examples of such geometric means are “bumps” or “louvers” in internal fins. The trouble with generating this turbulence is that it also increases pressure drop. An effective compromise, where pressure drop is critical, is to use “laminar enhanced fins”. These are internal fins that have no bumps or louvers, which are brazed or formed into the inside of a tube. They extend the interior heat transfer surface without generating turbulence (and its associated pressure drop). There’s also the option of using a perfectly open tube (think of a smooth wall radiator tube). 

Turbulence producing internal fins produce the highest performance and the highest pressure drop. Generally, think of using this type of tube when the boost pressure is above 15 psi. Why? There are two things happening. First, because of the higher boost pressure, there is more heat being generated, hence more need for heat transfer. Second, pressure drop is a function of airflow velocity. For a given mass flow at high pressure, the molecules are closer together hence the velocity is reduced. Consequently, so is the pressure drop .

Laminar fins produce the good performance with relatively low pressure drop. Generally, think of using this type of tube when the boost pressure is below 15 psi. Why? It’s the flip-side of the above. First, because of the lower boost pressure, there is more less being generated, hence less need for heat transfer. Second, pressure drop is a function of airflow velocity. For a given mass flow at low pressure, the molecules are further apart hence the velocity is increased. Consequently, so is the pressure drop.

Open tubes produce the lowest pressure drop, but produce poor performance. Generally, don’t think of using this tube. If the tube is open enough to produce acceptable pressure drop, the laminar flow problems are huge. It’s not well suited for an air cooling heat exchanger.

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     Frontal area: This is a rapidly decreasing function. If the proper core size is used, then doubling it will definitely not double the efficiency. Remember, we can only approach ambient temperature (with air to air) and at ambient temperature we are 100% efficient (not possible). More likely, doubling the core would raise the efficiency about 5 - 10% and cost twice the necessary amount and add substantially to the weight.
     Increasing the depth or thickness of the core: Thickness, however, is a double-edged sword. With the greater thickness, the total surface area increases but less ambient air can penetrate the thicker core to offer cooling. In addition, each inch of additional depth is picking-up heat from the core depth in from of it. This makes each increment of core depth less effective than the increment in front.
     Interior tube design: (see Does one style core cool better than another? above).
     Ambient air quantity: It is very important to insure that air coming in the snout of the car will actually go through the intercooler.

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     The net result is the reduction of power. It is hugely important because the power required to drive the air thru the system must come from somewhere. Depending on whether the system is turbocharged or supercharged, will determine how much power is lost from the restriction. It causes extra heat of compression, reduces compressor efficiency, it promotes exhaust gas reversion (applicable only to turbos) and increases turbo lag (see below).

* What’s so bad about pressure drop?

With almost all turbo systems, the pressure in the exhaust manifold is higher than the pressure in the intake. When the pressure in the intake is increased the pressure in the exhaust increases at a greater rate. Thus the difference in pressure between the intake and the exhaust increases. Each increase in inlet pressure drop makes this difference even greater. The greater this difference, the less spent charge (already burned air/fuel) wants to leave the combustion chamber during the exhaust stroke. Worse yet, if there is any overlap (intake and exhaust open at the same time) in the cam timing, the spent charge wants to go into the intake instead of out the exhaust. This is called reversion. Reversion heats-up the fresh charge, reducing charge density and since the spent charge won’t burn twice, it further hurts power because it takes-up room in the cylinder that could otherwise be occupied by fresh charge.

All pressure drop in the intake tract is bad news. That’s why Corky Bell in his book, Maximum Boost said: “The single most important aspect of intercooler design is low internal pressure loss”. And in his book, Supercharged! He says: “Clearly, an intercooler must have the least possible restriction to flow. This is so important that one can assemble a plausible argument that the first rule of intercooling is not to lose boost.”

Let’s consider the effect of pressure drop on compressor discharge temperature under the following conditions:

Boost: 10 psi
Ambient temperature: 70 F
70% compressor efficiency
Cooler #1 1.0 psi pressure drop
Cooler #2 2.0 psi pressure drop

I won’t go through the actual math in detail, but under the above conditions the compressor discharge temperatures will be as follows:

Using Cooler #1: 197.0 F
Using Cooler #2: 208.2 F

To get an intake manifold temperature of 100 degrees, cooler #2 has to be 78% efficient. While cooler #1 only has to be 76 % efficient.

If the increased pressure drop causes the compressor efficiency to drop to 65%, the compressor discharge temperature with cooler #2 rises to 218.6 F and the cooler has to be 80% efficient to compensate.

In addition cooler #2 will have more boost lag and more exhaust gas reversion. Back to top