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.
Air-To-Water
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.
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.
