It’s the age old debate of which is better, air or water? Both are required for our biological survival as human beings, and both are used as a cooling medium for compressed intake charges in automotive applications. While there are benefits and drawbacks to both air-to-air and air-to-water charge cooling, what is “best” will vary greatly by application and the debate will rage on for a very, very long time.
However, before you can get in on the debate, you really need to understand how each type of charge cooling system works. For that, we turn to Jason Fenske of Engineering Explained. In his latest video, he goes over the basics of each type of system, and their pros and cons in a production application.
It wouldn’t be a Jason Fenske video without a whiteboard. This shows a simplified view of the two types of systems. On the left, the system shows an air-to-water system plumbed into the intake manifold, as often seen on positive-displacement supercharged engines, but are now being used by manufacturers on factory turbocharged applications. On the right is a turbocharged (or centrifugally supercharged) application using a more traditional (in a production application) air-to-air intercooler.
The air-to-air intercooler system is relatively simple. It uses airflow through the intercooler to remove heat from the compressed charge air. Heat is transferred from the charge (air) to the atmosphere (air) – hence the name “air-to-air”. “You have air coming in through the air intake, through the compressor, then to the front of the vehicle through the heat exchanger and then into the intake manifold,” Fenske explains of the air-to-air system.
In an air-to-water system, the heat from the intake charge isn’t removed by external airflow (at least not directly), but rather by a liquid coolant. “The air to water system is a little more complicated. The air again comes in through the intake and through the compressor,” Fenske says. “The compressed air then feeds into the intake manifold with its integrated intercooler.”
While in the production example Fenske is using – a BMW X3 M40i with the B58 engine, which uses a manifold-mounted air-to-water intercooler, much like the venerable lineup of supercharged Ford Four-Valve Modular engines, and aftermarket Kenne Bell and Whipple supercharget kits – the science and design of all air-to-water intercoolers are similar across the board, regardless of charge cooler mounting location.
In addition to the actual charge cooler, air-to-water systems have a secondary cooling system, much like a standard engine cooling system, but dedicated specifically to the intercooler. “You have a coolant which passes through the intercooler core and is then pumped through the system to a radiator in the front of the car to have the heat removed,” Fenske says.
Front-mounted air-to-air intercoolers like this one from Full Race (with the OEM intercooler in the rear) act much the same way a radiator does, except instead of cooling down your engine coolant, it cools down your intake charge, once it has been compressed. By mounting the intercooler in the front of the car, it is ensured a clear, cool supply of air.
Pros and Cons
Trying to ask which method of charge cooling is better is like asking what the best power-adder is. The answer is simply, “It depends.”
“The air-to-air system is a much simpler system. You don’t have to worry about fluid leaks; you don’t have the additional heat exchanger and the [associated] fluid plumbing. You have less weight with an air to air system as well,” explains Fenske
In an air-to-water system, once the coolant has pulled the heat out of the charge air, the heat must then be pulled out of the coolant itself. “Another big advantage to the air-to-air system is that you are only relying on heat being exchanged once. With the air-to-water, you’re relying on ambient air to get the temperature of the coolant as low as possible.”
Fenske does point out that air-to-air coolers do have drawbacks, saying, “However, you must mount an air-to-air where there is airflow, and ideally that would be in front of the engine, although you can mount it on top of the engine as well. You won’t get as much airflow, and potentially be susceptible to heat soak from the engine.”
Moving to the air-to-water system, Fenske continues, “Air to water intercoolers [in production applications] reduce the volume of space between the compressor and the intake valves, because the air-to-water charge cooler can be mounted anywhere under the hood, and not have to be routed up front into the airstream This reduces the distance the compressed charge has to travel.”
In theory, that reduction in volume and distance traveled by the compressed intake charge will not only increase engine responsiveness (reducing lag) but also reduce the potential of further heat soak by reducing the amount of time the charge is exposed to underhood heat.
Here you can see aftermarket examples of air-to-water intercoolers. On the left, a Vortech Power Cooler, which does keep the path from the compressor outlet to the intake manifold about as short as you can get with an intercooler, highlights Fenske's argument. However, on the right, you can see a popular setup for high-power drag racing vehicles, in which the air-to-water intercooler is located in the rear seat, requiring the intake charge to travel quite a distance, and increasing the volume of tubing between the compressor outlet and the intake manifold substantially.
Up to this point, Fenske has been looking at production applications. However, once you get into aftermarket forced induction and competition settings, not only is it a whole new ballgame, thanks to specific rulebooks, but also the specific form of racing can change what you are asking of the system.
For example, in drag racing, remote-mounted air-to-water intercoolers increase the intake charge system volume significantly – the opposite of what is discussed here – and since the duration of the performance window is so much shorter, the second heat exchanger can be eliminated, and ice water used, to significantly increase the charge cooling capabilities of the system.
Conversely, for a form of motorsport that gives a lot of airflow due to sustained high-speeds, like road racing, the lighter weight and simplicity of an air-to-air system may be preferable. Again, it all comes down to application, rulebook, and ultimately, personal preference. Step one to any of that is understanding how each system works, along with its strengths and weaknesses.