Intercoolers explained

Engine Performance Parts improve supercharger performance...

I am compiling a guide on information on how to pick the exact engine performance parts to fit your target power requirements. Basically I want to eliminate all the guess work out of tuning and save you some money from having to do things over and over again.

While I was doing research for 'buying the right intercooler' I got lost, honestly. There are two types of information you will find out there:

1-One class of articles is written by engineers talking about pressure differentials, thermal efficiencies, enthalpy and multi variable equations that are very remotely related to flow, horsepower, torque, supercharger rpm or other things that we KNOW that we can use as an input to our equations. (Basically this science needs to be translated to layman's terms)

2-The other class is a group of random trial and error advice by enthusiasts, press releases and other materials that you find online.

Here's what we do know:

First let's talk about how intercoolers work. There is some debate about whether the intercooler is like a heat sink whose function is to absorb thermal energy from the incoming air to prevent the heat from reaching the engine, or whether the intercooler is like a radiator, where the air flow over the intercooler is responsible for extracting heat from the inlet air charge.

The true answer is both are correct...

The air running through the intercooler spends very little time inside the intercooler and slowing it down for more thermal exchange (like we would coolant in the radiator) would mean preventing air from reaching the engine which is a restriction on power. Because the air spends little time in the intercooler, the intercooler usually has multiple passages, internal ribs, and fins inside of it to maximize the surface area contact between the intercooler aluminum and the compressed air molecules. In this sense, the overall volume of the intercooler, and the overall surface area of its internal surfaces are like a heat sink that absorbs the heat energy out of the compressed air. In this aspect it makes sense that the larger our intercooler, the better. Furthermore it also makes sense, that the more complex and intricate the internal passages of our core, the more heat we will be able to extract out of the charge air. Of course the flipside of this is that very complex internal passages can create turbulence and restrict airflow so ultimately there is a balance in good design between internal complexity and flow capacity.

When we start out, the intercooler is cold, and with our first power run, as the hot compressed air runs through the intercooler, the heat is transferred to our heat sink (which is the intercooler) and nice cool air is left to enter the engine. After the first run, the intercooler is warm; and if we were to make a second power run back to back, the intercooler will not be able to SINK much heat because it is already somewhat heated. This is where the intercooler as a radiator comes in, the heat that was transferred from the air to the intercooler core, needs to be taken away either by cross flowing air in an air to air intercooler, or by cooling fluid in an air to water intercooler, or even by an ice-water bath for drag racing applications. Without harvesting the heat that the intercooler has absorbed out of the compressed air, the intercooler will heat up run after run until its temperature is the same as the compressed air heating it. At this point there is no temperature difference between the air and the intercooler core and we can no longer SINK any heat.

Some cars have their intercoolers located under the car's hood (like the Mazda Sentia / 626). In this kind of installation the intercooler is mostly a heat sink and will be used for a few passes till it soaks, once it soaks it needs to be left to cool till it returns to under hood temperatures before it can be effective again as an intercooler. From this we gather, that any intercooler no matter how small, or poorly placed is better than no intercooler because at least for that first power run it will potentially increase horsepower.

Now I'd like you to keep this information in mind while we talk about intercooler dimensions...

There are three main dimensions to the intercooler, the height (H), width (W) and (D) depth and based on that there are some physical concepts that we want to think about:

Cross Sectional Area:

Height x Depth = the cross section of the intercooler and is related to how well the intercooler will flow and whether or not it poses a restriction to the intake flow. This is the area of the surface facing the compressed air as it travels through the intercooler. Just like free flowing intakes, throttle bodies, and exhausts, if this area is undersized it will pose a flow restriction and reduce performance.

Width:

Width = the length of the intercooler and if you have a same side inlet/outlet intercooler then your intercooler length is effectively 2*W. This is the distance the air has to travel through the turbulent and complex intercooler core. The longer this length is, the more pressure drop there is in the intercooler so it's not advisable to have too wide an intercooler because we'd be waste turbocharger compression in intercooler pressure drop, neither is it advisable to have a same side inlet/outlet intercooler where the air has to travel a long distance in the core.

Frontal Area:

Width x Height = frontal area of the intercooler which faces the incoming ambient air, a good sized frontal area is required to ensure that the intercooler doesn't heat soak and that the rushing air stream is able to cool the intercooler efficiently (like a radiator) for you to be able to make back to back power runs. As we increase this area, we expect the intercooler to have better control over its peak operating temperature and have better repeatability no matter how long we stay in boost (good for standing mile races for example or all day road racing events).

Depth:

Depth = the depth of the intercooler, usually the intercooler is front mounted in front of the radiator... if you increase the depth too much (and especially without proper air ducting to the intercooler and airfoils between the intercooler and radiator) then you may slow down the incoming ambient air enough that your radiator starts overheating. So increasing D gives us better intercooler performance and more flow capacity (H*D is the cross sectional area mentioned above) but it reduces engine cooling efficiency so it must also be controlled.

Last but not least:

Total Volume:

Height x Width x Depth = the total volume of the intercooler, which is an indirect measure of the internal surface area of the intercooler. The larger the volume, the larger the heat exchange surface area, the more heat we can sink out of the air in an extremely short period of time (the 100 milliseconds or so that the air spends inside the core). Obviously the bigger the volume, the better the cooling and the worse for pressure drop. Again this number needs to be controlled.

How do I know if the intercooler I have now is adequate?

Intercooler efficiency can be tested in two ways:

1-Thermal performance

a.Measure the temperature difference between the intercooler inlet air and intercooler outlet air and use this delta T to compare between the intercoolers you have available to you. The best intercoolers out there can drop air temperature by over 100*F and get you within 20* of ambient air temperatures. If your factory intercooler can already accomplish similar results then there may be no need to upgrade.

b.Track the temperature of your intercooler in a prolonged power run, or on back to back power runs. The design and placement of the intercooler should be adequate that the temperature rise over time (with say 60+mph air hitting the intercooler) should be controlled, if the temperature rise is too steep then you may need a better 'radiating' core with more frontal area, better air guides and air foils, and better placement with high pressure air in front and low pressure air behind it... we'll explain more about this later.

2-Flow performance

a.Measure the flow through the intercooler core at 28" of water (standard for most flow meters), or measure the overall intercooler pressure drop at the flow rate required by your target horsepower. If the intercooler is on the car, measure the differential pressure across your intercooler at peak hp figures.

The best intercoolers will have less than 1psi of pressure drop (typically 0.5 to 0.9psi) at peak boost and horsepower. If your intercooler is within these power figures then there may not be any need to upgrade.

Now going back to selecting the best sized intercooler for your application, it would be very tough for me to figure out the exact math of how to optimize your intercooler size, and then I would have to translate that math to 'car terms' of power, inlet air temps, supercharger outlet temps, pressure ratios and boost pressures...etc

Here is another solution; one thing engineers like to do in dealing with this kind of a problem plotting statistical data on a chart and looking for some trends...

I found some 30 different intercoolers online with either flow tests (CFM), or Dyno tests (HP) or both, and since we know that it takes roughly 1.5 CFM of air to produce 1 HP (depending on density) then I combined both sets of data both for flow tested OEM intercoolers and for aftermarket 'engineered' intercoolers to produce the following graphs:

Flow in CFM vs. Cross Sectional Area trend:

Flow (CFM) = 11.63 * Cross sectional area (square inches) - 12.84

This is a plot of flow in CFM (vertical) vs. cross sectional area (squared inches) for the 30 cores that I had data for. As you can see there is a linear relationship between flow and area which is expected. So we can use this as a guideline to figure out (for a given depth D) of available cores, what the minimum height of our intercooler must be to get good flow performance.

One thing to note here is that these flow measurements were taken at 28" of water pressure or 1psi. As we know from supercharger theory, the more boost pressure (and the higher the pressure ratio) the more compressed the air is. Air at 15psi of boost is actually half of its volume compared to 0psi (or 1psi). So making 700hp (1050 CFM) @ 15psi (on a 3.5 liter 6 cylinder for example) may require only 42 squared inches of cross sectional area (because the air is at half its original size) whereas making 700hp (1050 CFM) @ 3psi (on a 7.0 liter 8 cylinder for example) may need a larger 91 squared inches of cross sectional area. So make sure you factor in your pressure ratio before choosing your cross sectional area.

Here's my second trend:

Horsepower (hp) = 0.533 * intercooler volume (cubic inches) + 50.17

This is a plot of horsepower (vertical) vs. total core volume (cubic inches) for the 30 cores that I had data for. As you can see there is a linear relationship between horsepower and volume which is expected. The more horsepower we want to make, the more air we need to ingest. The more air mass there is; the more energy that mass can carry (at the same temperature compared to a smaller mass) and thus the more intercooler core we need to sink that energy into our intercooler.

I think between these two charts it becomes now possible to go back to my 'twin-charged' Toyota Celica and say:

I wanted to make a peak of 320hp @ 20 psi. That equates to 480 CFM @ 2.36 Pressure ratio.

Starting with a standard 3" deep intercooler core, let me figure out my other 2 dimensions:

Minimum cross area = ((480/2.36) + 12.84) /11.63 = 18 square inches = D*H

Intercooler height = 18 / 3 = 6"

Total volume = (320 - 50.17)/0.533 = 506 cubic inches.

Intercooler width = 506/18 = 28"

So my ideal core size seems to be 28" X 6" X 3" which is a pretty reasonably sized front mount intercooler.

Now 28" is a reasonable intercooler width for pressure drop. If this figure were too large I would go back and use a 3.5" deep core for example. Likewise, if my intercooler height of 6" would not fit behind my bumper I could go back and increase depth slightly and redo the calculations.

Pressure drop across the intercooler is really important to track for a supercharged car because unlike a turbocharger, we can't just increase boost pressure with a boost controller, we are limited with superchargers to the gearing we have available in our supercharger pulley. So wasting any of this boost is really bad for performance. This is why it's really essential to neither undersize the intercooler to choke off the engine, nor to oversize it as to create a big pressure drop.