View Full Version : Intercoolers 101

Si Speed 317
09-07-2013, 02:43 PM
Iím starting from the ground up on this topic Ė no pun intended. Iím going to establish a strong foundation of information and build on it bit by bit, with each aspect of how intercoolers truly work. Then, we can accurately understand this subject.

First, let's talk about how intercoolers work. Thereís always some debate about whether an 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 an intercooler is like a radiator, where the air flow through the intercooler is responsible for extracting heat from the inlet air charge.

The answer is: both are correct.
Iíll explain why.

The air running through the intercooler spends very little time inside the intercooler. 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 of 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.

Make sense so far? Ok, letís move on to some real world practicality. When we start out, the intercooler is cold, and with our first power run, as the compressed (hot) air runs through the intercooler, the heat is transferred to our heat sink (which takes heat away from the air, aka 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 part 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, rendering our intercooler useless!
Some cars have their intercoolers located under the car's hood (like many S13 Sylvia/240sx setups). 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.

Keeping up? Youíre learning! Letís keep this information in mind while we move forward and discuss intercooler dimensions.

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

Cross Sectional Area = Height x Depth
This is the cross section of the intercooler, itís related to how well the intercooler will flow and whether 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 (commonly referred to as length):
Width = the length of the intercooler. This is the distance the air has to travel through the turbulent and complex intercooler core (and if you have a same side inlet/outlet intercooler then your intercooler length is effectively 2*W). 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 wasting turbocharger compression and increasing intercooler pressure drop. It isnít advisable typically advisable to have a same side inlet/outlet intercooler where the air has to travel a long distance in the core. In many of our cases, however, we have limited choices. Using my Si as an example, Iím better off running a single sided intercooler than a normal dual sided intercooler because of both intercooler pipe routing distance and safety. Iíd rather have a single sided setup with a bit more travel than a dual sided with less travel (which is actually more travel in my case, thus the single sided intercooler) and worry of bottoming out some of the intercooler pipes because of how they have to be routed.

Frontal Area = Width x Height (remember, length is width)
The frontal area of the intercooler is the side that 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 all day road racing events, for example).

Depth = the depth of the intercooler (distance from front to back). 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), you may slow down the incoming ambient air enough to cause your radiator to start 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. Thereís a limit to how much you can safely increase your D. Story of life! :biggrin:
Last but not least:
Total Volume = Height x Width x Depth
This is an indirect measurement of the internal surface area of the intercooler. The larger the volume, the larger the heat exchange surface area and thus 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). And at this point, we can safely say: the bigger the volume, the better the cooling - and the worse for pressure drop. Again, this number needs to be controlled.

Some of you may wonder: how do I know if the intercooler I have now is adequate?
Well, intercooler efficiency can be tested in two ways:

1- Thermal performance
a. (Our easiest method); Track the temperature of your intercooler in a prolonged power run, or on back to back power runs. This figure can easily be seen by your IAT values in Kpro/Flashpro/other tuning software programs. 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, 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.

b. (More of a PITA); 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. Though not many intercooler companies will post this data, you can try to find this information from reviews by others with the intercooler youíre looking at. The best intercoolers out there can drop air temperature by over 100*F and get you within 20* of ambient air temperatures.

2- Flow performance
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 for your personal setup will have less than 1psi of pressure drop (typically 0.5psi to 0.9psi) at peak boost and horsepower. If your intercooler is within these power figures then youíre in the green.

Whatís the best method to use? For nearly all of us, taking the data from 1a is more than efficient enough. 1b and 2 are simply unrealistic for 99% of you reading this article. So, based on this (1a), we can accurately decide if the intercooler we have is more than efficient, efficient enough, or upgrade time.

Here is another solution. Please only read this for informational purposes, this isnít necessary to go through to find the best intercooler for your setup:

Comparing flow tests (CFM) and Dyno tests (HP). Since we know that it takes roughly 1.5 CFM of air to produce 1 HP (depending on density), we can combine both sets of data to produce the following (the following are based on a comparison of 30 intercoolerís data found online, both OEM and aftermarket):
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 the authorís '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.

All in all, this is meant to be an intercooler informational article. I hope this, at the least, has helped you understand intercoolers more thoroughly. Hopefully, this can also help you decide which intercooler is best for you and why.
If you have any questions, please feel free to post them here!

Article Credit: http://www.articlesbase.com/automotive-articles/intercoolers-explained-986620.html

09-07-2013, 03:22 PM
Good info. This is why I'm swapping IC.

09-07-2013, 04:03 PM
good info

09-07-2013, 04:41 PM
Stickied and posted in the General Supercharging section as well, for reference. Great information!

Si Speed 317
09-08-2013, 04:10 PM
Thanks everyone. If you have any questions or IC DIYs, feel free to post up!

09-10-2013, 08:14 AM
Good info here!

09-11-2013, 01:13 AM
This is... AWESOME! Repped! Great info man, thank you for this. It's actually perfect for me since I'm swapping front mounts soon

Ninja edit: You must spread some Reputation around before giving it to Si Speed 317 again.