Intercoolers

An intercooler is a radiator for the air a turbo or supercharger has squeezed. Why forced induction makes more power showed why it’s needed: squeezing air heats it, hot air is thin, and an intercooler gets most of the gain back by cooling it down again. It also introduced the two kinds, air-to-air and air-to-water.

This lesson goes further: how well an intercooler cools and what changes that, why the same car can feel strong one minute and flat the next, and the trade-offs in where it goes and how big it is.

The demo follows the same 2.0-litre engine at 1 bar (15 psi) of boost through a drive on a warm day. The front of the car is on the left. The air leaves the compressor at about 108°C (226°F), and the readouts show how hot it is going into the engine, and how much power it would make on full boost with air at that temperature. Use the stage buttons to jump through the drive, and the buttons to change the intercooler.

It starts with an air-to-air intercooler, behind the front bumper:

1. In traffic. Crawling along, the intercooler sits in hot air from the radiator and engine bay, and heats up to about 55°C (131°F). That’s called heat soak.
2. Floor it. The hot core can only cool the air to about 59°C (138°F), so the engine makes about 226 kW (303 bhp). As the car speeds up, more and cooler air rushes through the core, and it starts to cool down.
3. Flat out. At 112 mph (180 km/h) the intercooler is at its best. The air goes in at about 37°C (99°F), and the engine makes about 242 kW (325 bhp): 16 more than when you first floored it.
4. Ease off. With little boost, the core cools back down, ready for next time.

Now press Chargecooler. Its core is right by the engine, with short pipes, and water carries the heat to a small radiator of its own at the front. A pump keeps the water moving even in traffic, so it starts cooler, at about 35°C (95°F). Floor it and the air goes in at about 47°C (117°F) straight away, for 234 kW (314 bhp). But keep it flat out and the water slowly warms up, to about 45°C (113°F), and the air with it, to about 55°C (131°F). On a long, fast run, the air-to-air intercooler wins.

No intercooler sends the air in at 108°C (226°F) all the time, for about 197 kW (264 bhp).

The model is simplified, and the drive is much shorter than a real one, but the numbers show the right kind of difference.

An intercooler’s effectiveness is the share of the compressor’s extra heat it takes back out. If the compressor heats the air from 20°C (68°F) to 108°C (226°F), and the intercooler cools it to 42°C (108°F), it has taken out 66 of the 88 degrees: 75%.

Four things set it:

• The air passing through the core. An air-to-air intercooler can only give its heat to the air rushing through it, so it depends on road speed, how well the air is ducted to it, and whether that air is fresh or has come through the radiator first. In the demo’s model, a core that’s 75% effective at 62 mph (100 km/h) manages under half that at 12 mph (20 km/h).
• How much charge air there is. The harder the engine works, the more air there is to cool, and the less time each bit spends in the core. A bigger turbo and more boost need a bigger intercooler.
• The core’s size. A bigger face catches more cooling air, and a thicker core gives the charge more time to lose heat, but the back of a thick core sits in air that the front has already warmed.
• How hot the core already is. A heat-soaked core has to cool down before it can cool the charge properly.

The ECU measures the temperature of the air going in, with an intake air temperature sensor after the intercooler. As that air gets hotter, it fires the spark later to avoid knock, and some engines also cut boost, so a hot intercooler costs more power than the thinner air alone.

Cooling isn’t free. The air has to squeeze through hundreds of narrow passages in the core, and it loses some pressure on the way: often around 0.1 bar (1.5 psi) at full power. The turbo has to make that much more boost to make up for it, which heats the air a little more.

The pipes and core also hold a lot of air. When you floor it, the turbo has to fill all of it before the pressure reaches the engine, so long pipes and a huge core can make boost build a little slower. On a heavily tuned car, the cooler air is worth it. On a standard car, a much bigger intercooler often makes little difference, because the original was sized for the power it makes.

• Front-mounted. Behind the front bumper, in the coolest, fastest air. It needs long pipes forward and back, and it’s the first thing hit by stones and minor bumps. Most turbo cars use one, sometimes in front of the radiator, which then gets air the intercooler has warmed.
• Side-mounted. Tucked into a corner of the front bumper, as on many older VW Group cars such as the Mk4 Golf GTI. The pipes are shorter, but the core is small and can only get the air one corner of the bumper lets in.
• Top-mounted. On top of the engine, fed by a scoop in the bonnet, as on the Subaru Impreza WRX and the supercharged Mini Cooper S of the 2000s. The pipes are very short, but sitting on a hot engine it heat-soaks badly in traffic. Some Imprezas had a button that sprayed water over it to cool it.
• In the intake manifold. Many modern small turbo engines, such as VW Group’s 1.0- and 1.5-litre TSI, use a chargecooler built into the intake manifold. The path from turbo to cylinders is tiny, so boost builds quickly.

A chargecooler has its own small cooling system: an electric pump, a radiator at the front of the car and coolant kept separate from the engine’s, because engine coolant at about 90°C (194°F) would heat the charge, not cool it. Drag racers sometimes fill a chargecooler’s tank with ice for a few seconds of very cold air.

Most cores are tube-and-fin: flat tubes carry the charge air, with thin wavy fins between them for the outside air. They’re light and cheap, and fitted to most cars from the factory.

Many aftermarket cores are bar-and-plate: stacked plates and bars brazed together, with fins inside the charge passages as well as outside. They’re heavier and tougher, and their extra metal soaks up heat in short bursts, at the cost of warming up and cooling down more slowly.

At each end of the core is a tank that spreads the air across the tubes. On many cars the tanks are plastic, crimped on to an aluminium core.

• Cracked tanks and split pipes. Plastic end tanks and rubber or plastic pipes harden with heat and age, and boost finally splits them. The result is a boost leak: a hiss or whoosh under load, low power, and often fault code P0299 (underboost). On an engine with an airflow meter, the air it measured escapes, so it also runs rich, and a diesel may puff black smoke.
• Oil inside. A light film of oil in the pipes is normal: it comes from the crankcase breather. Pools of it mean the turbo’s oil seals are worn or the breather is blocked, and oil coating the inside of the core makes it cool less well.
• Condensation. Cooling damp air makes water condense out of it, as on a cold glass. On a humid day, water can collect in the intercooler while you cruise, then get sucked into the engine in one gulp when you floor it, causing a misfire. Early Ford EcoBoost V6 engines in the F-150 pickup were known for this, and Ford changed the parts in front of the intercooler and the engine’s software to cure it. In very cold weather that water can freeze.
• Blocked or bent fins. Leaves, insects, mud and stones clog or flatten the fins on a front-mounted intercooler, so less air gets through. Rinse them gently from behind, never with a pressure washer up close.
• A failed chargecooler pump. With the water no longer moving, the chargecooler soon heat-soaks. The air going in gets very hot, and the ECU cuts the power to protect the engine.

• An uprated intercooler is one of the most popular upgrades for a tuned turbo car. On a standard car it rarely adds much power, but on one running more boost it keeps the air cool lap after lap, or pull after pull, so the power stays consistent, and it lets the ECU run more spark advance without knock. A big front-mount can mean cutting the bumper, and its long pipes add a little lag.
• Uprated pipes in silicone or aluminium replace rubber and plastic pipes that swell under boost or split with age. They’re often fitted at the same time as a remap.
• A bigger chargecooler radiator or a stronger pump helps a tuned car with a chargecooler cope with long, hard runs, where its water would otherwise heat up.
• Water or water-methanol injection sprays a fine mist into the intake. As it evaporates it cools the charge further, and methanol also raises the fuel’s resistance to knock. It needs a tank, pump and controller, and running out mid-pull on a map that relies on it can cause knock.