Intercooler Choices

This article was first published in 2005.

So you know you’re going to need an intercooler, but what type should you use? And more to the point, what types are there? When terms like water/air and air/air are thrown around it’s easy to get confused. Aaaaggghhh.

But sit back: here we’ll sort the fact from the fiction and at minimum, give you a better idea of the basic choices.

Intercooling Types

An intercooler is just a heat exchanger - it exchanges the heat in the intake air for heat somewhere else.

In an air/air design, the heat is transferred directly to the atmosphere. In this type of heat exchanger, the intake air flows through small passages which have the outside air flowing around them. The heat is transferred through the walls of the passages. Bar and plate and tube and fin are just different types of air/air intercooler designs.

In a water/air design, the heat is transferred by the primary heat exchanger to water. This hot water is then pumped through a secondary heat exchanger which transfers this heat to the air. A water/air design uses a primary heat exchanger, secondary heat exchanger, reservoir (sometimes integrated into the primary heat exchanger), pump and hoses. The primary heat exchanger usually resembles a box which is mounted between the turbo and the throttle, but in some supercharged cars, the water/air heat exchanger is integrated into the intake manifold.

Thermal Mass

While all intercoolers are basically heat exchangers, they are also heat sinks. That is, in times of boost they store the heat. In other words, the heat is being absorbed at a faster rate than it can be got rid of. If an intercooler has a good ability to store heat before its temp starts to rise, it’s said to have a good thermal mass.

Whether thermal mass or heat exchange ability is more important depends on how the car is used. In a race car, which is on boost a lot of the time, heat exchange ability is much more important than heatsink ability. That’s because so much heat is being continuously removed from the intake air that it simply has to be got rid of fast.

But in a turbo road car, which is typically on boost less than 5 per cent of the time, heatsinking is very important. That’s because if the heat can be absorbed during boost events, there’s plenty of time for it to be dissipated over the 95 per cent of time the car’s not on boost. (Note that positive displacement supercharged road cars are typically on boost a much greater proportion of the time!)

Generally, the heavier the air/air core, the greater its thermal mass. A water/air core has a very high thermal mass – its heatsinking abilities are excellent.

Note that intercoolers mounted under the bonnet can have too much thermal mass. When the car has been running and is then parked, the intercooler will get hot through underbonnet heat transfer. If it takes a long time to cool down once the car is again moving, there will be a longer period than necessary of high intake air temps.

Mounting Locations

Air/air intercoolers have to be mounted where the outside air can flow through them. More specifically, they need to be located so that when the car is moving, there is a higher air pressure generated on one side than the other. That way, the outside cooling air will pass through the core.

Top-mount intercoolers are located above the engine in the engine bay. The high pressure on one side is generated by a bonnet scoop. Side mount intercoolers are mounted ahead of a front wheel, with the pressure generated by a forward-facing opening in the bumper a bit like a brake duct. Front-mount intercoolers are located ahead of the air-con condenser.

In each of the three locations there is a strong possibility of the intercooler acting as a pre-heater. The worse location for this is top-mount, where when the car isn’t moving, hot air from the engine bay naturally chimneys its way out through the intercooler and bonnet vent. This results in high intake temps when the car is boosted off the line. Front-mounts can be radiant heated by the closely mounted air con condenser and radiator, and a side-mount core can be heated by the brakes.

Water/air intercoolers have their primary heat exchanger located under the bonnet, normally directly in the path between the compressor outlet and the throttle body. The heat exchanger often also contains the water reservoir – that is, the water fill cap is located on the heat exchanger itself. The secondary heat exchanger, which cools the circulating water, is almost always located ahead of the air con condenser at the front of the car.

Plumbing

It’s easy to look around an engine bay and say, "Oh yeah, the intercooler will go here." Trouble is, while it’s not hard to find space for the core, getting the plumbing to and from the ‘cooler is often another story!

The diameter of the plumbing depends on how much power the engine is making. Often people fit oversize air/air cores and then run equally oversized plumbing to and from the core. That’s fine, except to fit all this in they need to dementedly take to the car’s bodywork with a pair of metal shears.

If you look at factory turbo cars you’ll see that the size of the core and the size of the plumbing are nearly always much smaller than in the aftermarket. Many people will suggest that this is the case because the factory engineers don’t know what they’re doing, or want to save cost, or both. And sometimes those points are true. But if you’ve got a 200 or 250kW engine, you don’t need to have an intercooler area than can be measured in square metres and huge 75mm plumbing running all over the engine bay. In nearly all road cars, 50 or 63mm plumbing is more than adequate.

However, if it’s impossible to run a front or side-mount air/air core with this size of connecting tube, look towards a top-mount air/air core or water/air intercooling. The difficult spots (eg getting the plumbing past the radiator or headlights) are avoided when the primary heat exchanger is located in the engine bay. (In the case of water/air intercooling, the small diameter water hoses are always easy to route to the front of the car to connect to the secondary heat exchanger.)

If the throttle is to be located ahead of the supercharger (or more rarely, the turbo) you don’t need a blow-off valve. But in the more normal location, where the throttle is after the turbo or blower, you’ll also need to think through the plumbing connections for a blow-off valve.

Intercooling Characteristics

It’s easiest to think of how different designs of intercoolers behave if you take the extremes. In all the following cases we’re talking about a normal street-driven car.

• Low heat exchange and low heatsink

An example of an intercooler that would have these characteristics is a small air/air core mounted at the front of the car. In short spurts of boost it will be fine. But stay on the loud pedal and you’ll overwhelm the core’s ability to exchange heat and, since the core also has poor heatsinking ability, the intake air temp will rapidly rise.

• High heat exchange but low heatsink

An example of this type of intercooler is a large, light-weight intercooler mounted at the front of the car. This type of core will work well, so long as there’s plenty of airflow through the core. Get caught behind a truck up a hill (where the road speed will be slow but the boost high) and the intake air temp will rocket.

• Low heat exchange but high heatsink

This type of intercooler is a lot more common than first thought. That’s because many big air/air cores don’t have much outside airflow through them because of the way they’re stacked in front of the air con condenser and radiator. (It’s common to see easy paths for the air to bypass the intercooler.) With an intercooler with low heat exchange but high heatsinking, the average intake air temp will be higher than ambient but the peaks of temp associated with boost events will be only a little higher again.

• High heat exchange and high heatsink

An example of this type of intercooler is a water/air design or a large, well-mounted air/air core. As you would expect, this design will give very good results in all conditions, although because of its high heatsinking, the average intake temp may well be higher than a design with low heatsinking. (The stored heat is often fed back into the intake airstream when off boost.) However, the peak temps (and they’re what matter!) will be low.

Making Choices

Making a decision about what type of intercooling to go for needs to be based on much more than just theory. There are heaps and heaps of air/air intercoolers available both new and secondhand – but far fewer water/air heat exchangers. This makes air/air intercooler cheaper than water/air designs. Nearly all turbo cars already have air/air intercooling as standard, so if a simple upgrade is needed, it’s usually easy to stick with a larger air/air core and run the plumbing along the same paths.

When considering an intercooler installation, here are some key questions to answer:

• Is there a factory intercooler installation (or an upgrade) already available for the engine?
• Is the car going to be on boost much of the time? (For turbo road cars, the answer is usually ‘no’, which means high thermal mass is important.)
• Can plumbing be easily routed to the front of the car?
• Is heat exchange or heatsinking likely to be more important?
• Where is there space to mount the intercooler?
• With air/air cores, what outside flow will occur through the ‘cooler?
• Is intercooler preheating likely to be a problem?

Think through the answers to those questions and you’ll be much better placed to make a good decision.

We’ve covered intercooling in many articles, including testing the flow and heatsinking characteristics of secondhand intercoolers, discussing placement issues, intercooler water sprays, and other aspects. Do a site search under ‘intercooling’ to find these articles.