Explain The Flow Of Coolant Through The Engine And Radiator Intercoolers – Explained

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Intercoolers – Explained

Engine performance parts enhance supercharger performance…

I am putting together a guide with information on how to select the correct engine parts to match your target power requirements. Basically, I want to take all the guesswork out of tuning and save you some money from repeating things over and over again.

While researching to ‘buy the right intercooler’ I got lost, honestly. There are two types of information you will find there:

1-One class of articles are written by engineers who talk about pressure differentials, thermal efficiency, enthalpy, and multi-variable equations that are very distantly related to flow, horsepower, torque, compressor speed, or other things we KNOW we can use as input data to our equations. (This science basically needs to be translated into layman’s terms)

2-The second class is a bunch of random trial and error enthusiast advice, press releases, and other stuff you can find on the internet.

Here’s what we know:

First let’s talk about how intercoolers work. There is some debate as to whether an intercooler is like a radiator whose function is to absorb heat energy from the intake air to prevent heat from reaching the engine, or whether an intercooler is like a radiator, where the airflow across the intercooler is responsible for extracting heat from the intake air charge.

The correct answer is that both are correct…

Air passing through the intercooler spends very little time inside the intercooler and slowing it down for more heat exchange (like we would coolant in a radiator) would mean preventing air from reaching the engine, which is a power limitation. Since the air spends little time in the intercooler, the intercooler usually has multiple passages, internal fins and fins inside it to increase the surface contact between the intercooler aluminum and the compressed air molecules. In this sense, the total volume of the intercooler and the total area of ​​its internal surfaces are like a refrigerator that absorbs thermal energy from compressed air. From this aspect, it makes sense that the bigger the intercooler, the better. Furthermore, it also makes sense that the more complex and convoluted the internal passages of our core are, the more heat we will be able to extract from the charge air. Of course, the flip side 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, the intercooler is cold, and with our first start, as the hot compressed air passes through the intercooler, the heat is transferred to our intercooler (which is the intercooler) and nice cool air is left to enter the engine. After the first ride, the intercooler is warm; and if we were to make another current, the intercooler will not be able to SINK much heat because it is already somewhat heated. This is where the intercooler comes in as a radiator, the heat that has been transferred from the air to the intercooler core, needs to be removed either by cross-flow air in the air-to-air intercooler, or coolant to the air-to-water intercooler, or even an ice water bath for drag racing applications. Without collecting the heat that the intercooler has absorbed from the compressed air, the intercooler will heat up cycle after run until its temperature is the same as the compressed air that is 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 intercoolers located under the hood (such as the Mazda Sentia / 626). In this type of installation, the intercooler is mostly a radiator and will be used for several passes until it is soaked, and once soaked it needs to be allowed to cool down to under-hood temperature before it can be effective as an intercooler again. From this we conclude that any intercooler, no matter how small or poorly placed, is better than no intercooler because at least for that first run it will potentially increase horsepower.

I would like you to keep this information in mind while talking about intercooler dimensions…

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

Cross section of the area:

Height x Depth = cross section of the intercooler and refers to how well the intercooler will flow and whether or not it is a restriction to intake flow. This is the area of ​​the surface facing the compressed air traveling through the intercooler. Just like free intakes, throttle bodies, and exhaust systems, if this area is too small, it will restrict flow and reduce performance.

Width:

Width = intercooler length and if you have the same side inlet/outlet intercooler then your intercooler length is effectively 2*W. This is the distance the air must travel through the turbulent and complex core of the intercooler. The longer this length, the greater the pressure drop in the intercooler, so it is not advisable to have an intercooler that is too wide because we would lose compression of the turbocharger in the pressure drop of the intercooler, nor is it advisable to have the same side inlet/outlet intercooler where the air has to travel a long distance in the core.

Frontal area:

Width x Height = the front area of ​​the intercooler that faces the incoming ambient air, a good sized front area is needed to ensure that the intercooler does not heat up and that the current airflow can effectively cool the intercooler (like a radiator) so you can run sequentially. As we increase this range, we expect the intercooler to have better control of its peak operating temperature and better repeatability regardless of how long we stay in boost (good for mile races for example or all-day road races).

Depth:

Depth = depth of the intercooler, usually the intercooler is placed at the front in front of the radiator… if you increase the depth too much (and especially without adequate air drainage to the intercooler and airfoil between the intercooler and the radiator) then you can slow down the incoming ambient air enough that your radiator starts to overheat. So, increasing D gives us better intercooler performance and higher flow capacity (H*D is the cross-sectional area mentioned above), but reduces engine cooling efficiency, so that too must be controlled.

Last but not least:

Total volume:

Height x width x depth = total volume of the intercooler, which is an indirect measure of the internal surface of the intercooler. The larger the volume, the larger the surface for heat exchange, the more heat we can remove from the air in an extremely short period of time (100 milliseconds or as much as the air spends inside the core). It is obvious that the larger the volume, the better the cooling and the worse the pressure drop. Again this number needs to be controlled.

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

The efficiency of the intercooler can be tested in two ways:

1-Thermal performance

a. Measure the temperature difference between the intercooler inlet air and the intercooler outlet air and use this delta T to compare the intercoolers available to you. The best intercoolers can lower the air temperature by more than 100*F and bring you within 20* of the ambient air temperature. If your factory intercooler can already achieve similar results, there may be no need to upgrade.

b. Monitor the intercooler temperature during extended operation or continuous operation. The design and placement of the intercooler should be adequate so that the temperature rise over time (with say 60+mph air hitting the intercooler) should be controlled, if the temperature rise is too steep you may need a better ‘radiating’ core with more front end, better guides air and airfoils, and a better setup with high pressure air in front and low pressure air behind it… we’ll explain more about that later.

Performance of 2 flows

a. Measure the flow through the intercooler core at 28″ of water (standard for most flowmeters) or measure the total pressure drop of the intercooler at the flow rate required by your target horsepower. If the intercooler is on an automobile, measure the differential pressure across your intercooler at peak hp values .

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

Now going back to choosing the best sized intercooler for your application, I would find it very difficult to figure out the exact math of how to optimize the size of your intercooler, and then I would have to translate that math into ‘car conditions’ of power, intake air temps, supercharger outlet temps, pressure ratios and precharge pressure…etc

Here is another solution; one thing engineers like to do when dealing with this type of problem is plot the statistics on a graph and look for some trends…

I found about 30 different intercoolers online with flow tests (CFM), or Dyno tests (HP) or both, and since we know it takes roughly 1.5 CFM of air to produce 1 HP (depending on density), I combined both data sets for both flow-tested OEM intercoolers and 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) versus cross-sectional area (square inches) for the 30 cores I had data for. As you can see, there is a linear relationship between flow rate and surface area that is expected. So we can use this as a guideline to determine (for a given depth D) the available cores, which must be the minimum height of our intercooler to get good flow performance.

It should be noted here that these flow measurements were taken at 28″ or 1 psi water pressure. As we know from supercharger theory, the higher the boost pressure (and the higher the pressure ratio), the more compressed the air. Air at 15 psi boost is actually half its volume compared to 0 psi (or 1 psi). So making 700 hp (1050 CFM) @ 15 psi (on a 3.5 liter 6 cylinder for example) may only require 42 square inches of cross-sectional area (because is air at half its original size), while making 700 hp (1050 CFM) @ 3 psi (on a 7.0 liter 8 cylinder, for example) may require a larger cross-sectional area than 91 square inches. So be sure to factor in your ratio pressure before selecting the cross-sectional area.

Here is my second trend:

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

This is a plot of horsepower (vertical) versus total core volume (cubic inches) for the 30 cores 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 produce, the more air we need to bring in. The more air mass; the more energy that mass can carry (at the same temperature compared to a smaller mass) and therefore the more intercooler core we need to sink that energy into our intercooler.

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

I wanted to peak at 320hp at 20psi. This equates to 480 CFM at a pressure ratio of 2.36.

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

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

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 reasonable size for the intercooler on the front.

Now, 28″ is a reasonable intercooler width for pressure drop. If this figure was too big, I’d go back and use, say, a 3.5″ deep core. Likewise, if my 6″ intercooler height wouldn’t fit behind my bumper, I could go back and increase the depth a bit and redo the calculations.

Intercooler pressure drop is really important to monitor for a supercharged car because unlike a turbocharger we can’t just increase boost pressure with a supercharger regulator, with superchargers we are limited to the gear we have available in our supercharger pulley. So wasting any of this boost is really bad for performance. This is the reason why it is really important that the intercooler is not too small to choke the engine, nor oversized to create a large pressure drop.

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