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BMW Original Parts
Oct 24, 2023
Let’s talk about engine cooling. Or, more precisely, air charge cooling.
In most cars, you probably won’t ever have to worry about engine overheating if everything’s well-maintained. Like when your BMW intercooler is in good condition. However, try to run some additional power or take your ride for more than a spirited drive on a hot day and you could quickly find out that the factory cooling system is not enough.
Especially in the case of forced induction engines, like the good ol’ turbo inline-fours, -sixes, or charged V8s you can find in BMWs. Or MINIs, since it’s a part of the BMW Group and as such shares a lot of their technology with the Bavarians.
But why are turbo- and supercharged engines more prone to overheating than their naturally aspirated counterparts? What is “heat soak”? Dive into this article to find answers to these (and more) questions.
To put it simply – an intercooler is a heat-exchanging device designed to lower the temperature of the air charge from the turbo- or supercharger. To quickly remind you why it’s good to keep it cool: the colder the air, the more dense it is, meaning there is more oxygen per given volume. Obviously, oxygen is necessary for the combustion process to occur and more oxygen allows the engine to burn more fuel. See? Engine physics 101.
Now, BMW intercoolers are usually made of a material characterized by its good thermal conductivity, such as aluminum, which is the most commonly used in this application. As you probably also know, they aren’t just metal slabs. When you take a closer look at their core, you’ll see a kind of grated texture between the end tanks where the piping connects. The texture is very much like the one found on engine radiators – we’ll talk about what sets the two apart in a bit.
These “grates” are actually called fins and they are key for the heat exchange process. Usually, layers of fins are sandwiched between horizontal air ducts that also have fins inside them, although set laterally (don’t worry, we’ll also get to how all that works later).
In terms of design, there are basically two types of intercoolers – bar-and-plate and tube-and-fin. How are they different from one another?
Both of the designs are principally the same, i.e., both of them have two sets of air ducts filled with fins laid out in the same manner. However, they are manufactured differently.
The former are produced by sandwiching the fins surrounded by bars between plates. The latter are made by stacking the lateral tubes filled with fins on top of longitudinal layers of fins – yup, that’s a lot of fins, but stay with us.
What it all means for you as an end-user is that the bar-and-plate design is slightly more robust, which means it can handle higher boost pressures. Since they are also more time-consuming to produce, they are slightly more expensive than comparable tube-and-fin intercoolers.
As to their efficiency, that’s where they differ the most. Bar-and-plate intercoolers will typically take significantly longer to get hot and take equally long to cool down. On the other hand, tube-and-fin designs will absorb less heat over time, but they will also drop the temperature quicker. So the former are more resistant to the heat soak, which is what you call a state where an engine component is no longer able to efficiently get rid of the excess heat.
…Translating it even further from “engineerish” to English:
There, explained as simply as it gets :)
Similarly to an intercooler, the radiator is also a heat-exchanging device that uses metal fins to dissipate the temperature. However, that’s where similarities between the two end. The traditional intercoolers exchange heat air-to-air, meaning that they cool down the charge air that flows laterally through them by using the ambient air.
Radiators work on a similar premise, though instead of air, what flows through them is liquid. In most vehicles that would be the engine coolant (sometimes also called antifreeze), for the heat exchangers installed right behind the front grille. Connected to the metal fins is a series of tubes. When the hot liquid is fed through the tubes, the temperature is dissipated onto the fins and then into the air, which cools down the liquid. Hence the “heat exchange.”
Although intercoolers and radiators are much alike, they are parts of two separate systems. The former are an extension of turbo- or superchargers, working with them in unison to provide the engine with cooler air. The latter, in turn, are meant to keep the engine operating temperature at the optimal level, allowing the water pumps to feed antifreeze through coolant passages. It’s also worth keeping in mind that while you’ll find a radiator on all modern production cars, intercoolers are only found on forced induction engines.
Now that we know what a heat exchanger is, this brings us to another question, that is…
Since we’ve already covered the basics of how a heat exchanger works, it’s just a matter of changing the appropriate variables in the radiator equation to understand BMW intercoolers as well.
In a BMW intercooler, the ambient air goes into the finned ducts you can see from the front, flowing from front to back. Meanwhile, the air charge flows through the lateral channels, so that the lateral fins can absorb its heat and transfer it to the longitudinal ones. What it basically means is that the BMW intercooler takes heat away from the air charge and the ambient air takes away the heat off of the intercooler fins.
As a result, the temperature of the air charge drops in the process, leading to a much-desired increase in its density, as mentioned earlier.
You might’ve already guessed it, but engine intercoolers come in various shapes and sizes.
We already covered two variants of the air-to-air intercoolers, often visible in the front bumper. Since we’re at it, the way an intercooler is mounted also helps differentiate between the designs: on the market you’ll find top and front mount air-to-air intercoolers, each with its own pros and cons.
Additionally, it’s worth mentioning that air isn’t the only medium used for heat exchange. There are also liquid-based intercoolers, commonly known as water-to-air or liquid-to-air intercoolers. Let’s take a quick comparative look at all of them.
This is exactly the type of intercooler described in great detail above. More common than water-to-air setups, air-to-air intercoolers like Awaken Performance Quartz Frost Intercooler rely entirely on airflow through their cores to cool the compressed air charge.
In general, the design is pretty efficient at cooling the incoming air, making them a popular OEM choice. They are cost-effective, lightweight, simple, and offer a strong balance between performance and affordability. However, since they depend so heavily on the airflow at higher speeds, air-to-air intercoolers can be susceptible to heat soak at idle when the vehicle is stationary.
Just like the name implies, water-to-air (or liquid-to-air) intercoolers utilize special coolant for transferring the heat from the air charge passing through them. This means that they not only work on a very similar premise, but they are sometimes also directly connected to a traditional radiator – it basically feeds them with coolant. That being said, the engine coolant usually isn’t mixed with the cooling liquid of the intercooler in such setups.
In this design, air and coolant are kept separate from one another, flowing through their own passages. Coolant is pumped through channels and tubes connected to the fins in the heat exchanger, while air flows through these fins. This enables a more efficient heat exchange when stacked against the simpler air-to-air designs, especially in high-performance applications.
The major disadvantage? It’s a significantly more complex solution, so it obviously raises both production and maintenance costs. Besides the additional piping required to feed the intercooler with coolant and a pump to get it going through the system, the setups often need an additional radiator mounted next to the base one. Oh, and did we mention they are noticeably heavier than air-to-air intercoolers?
Despite the complexities of such setups, many car manufacturers seem to favor the water-to-air design for their more powerful models. Bavarians use intercoolers of this type in their V8 engines, like the N63 and S63, as well as in the more civilized, everyday-friendly BMW B58.
Since the explanation of these terms is actually in their name, we’ll just clarify what they refer to and if there are any noticeable differences between the two. In this case, the “top” and “front” adjectives specify the location of the heat exchanger in relation to the engine. Note that this term can apply to both the air-to-air and air-to-water BMW intercoolers.
When it comes to what sets the two apart, it’s not that easy to say that one will be better than the other. Generally speaking, the mounting method is usually chosen based on some other factors, like the temperature around the intercooler, piping length, and packaging, or how restricted the air charge is going to be on its way to the intake manifold. For the air-to-air intercoolers, the flow of the ambient air also needs to be considered, so the core shouldn’t be obstructed.
In the bimmer world, it isn’t likely you’ll ever need to choose between a top or front mount solution when trying to upgrade your car. Usually, the replacements simply go where the stock one was. In most cases, the air-to-air intercoolers are installed in the front, while water-to-air designs are top-mounted.
The answer to this question largely depends on the status of your car.
If it hasn’t been tuned and is making regular power, the factory-installed intercooler will usually be enough. However, there are some engines that would greatly benefit from an intercooler upgrade even with the stock ECU tune, like the BMW N20, which is notorious for running pretty hot. In this case, a more efficient heat exchanger will significantly prolong the life of the engine’s internal components.
Larger intercoolers will also be a good first upgrade for people who plan on getting an engine remap or want to gradually increase the performance of their car with some subsequent mods. Lastly, an intercooler upgrade will also be worth considering if the one installed on your vehicle gets damaged – since you’re already replacing it, why not choose a better one?
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