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Here is a good read on the benefits of a good IC in a turbo diesel....


Another component in a good turbo setup is the intercooler. After intake air passes through the turbo, it heats up partly because of higher pressure. The ideal gas law states that when all other variables are constant, if pressure is increased, so will temperature. An intercooler lowers air temps before passing the air into the engine. (Some other sources of heat are the intake piping soaking heat from a hot engine bay, because the turbo is so close to the exhaust with hot exhaust gasses passing through the exhaust side of the turbo, and mechanical agitation of the air by the turbine wheel.) Without an intercooler, hot air increases the likelihood of uncontrolled detonation and engine damage.

An intercooler is basically a heat sink that takes away the heat of the intake charge. Here is a picture of an intercooler in a Jetta TDI. Cooling ambient air moves through the front bumper, through the intercooler, and through the wheel well in the direction of the arrow. More air moves through the intercooler as the car moves faster.

You don't see intercoolers on non-turbo cars because the intake air is already at ambient temperature. An air intake directly connected to an intercooler or anywhere not after the turbo would actually decrease performance by restricting airflow. Below is a funny picture of an "interfooler", someone who put an intercooler on a non turbo car. It's there because they want to look cool and are ignorant of what its function is. Even worse, the air filter is exposed and low enough to suck up water and damage the engine.


The goal of intercooling is to produce the least pressure drop (so the turbo doesn't have to work as hard) and remove the most heat. Depending on the exact setup, the average well designed intercooler in a car may have .5-2.0 psi pressure drop. There is always some pressure differential between the turbo and the engine to get air moving from one spot to another. An intercooler acts more like a heat sink and less like a radiator when boosting because boosting heats up the intake air. This heat is transferred into the intercooler like a heat sink. Then the intercooler releases the heat into the ambient air or coolant. Most of the heat leaves with the ambient air flow (while the car is moving, air is passing through the air ducts) but a little heat can go back into the intake air once air temps have dropped (heat moves from hot to cold).

A good air-air intercooler can cool the air to within 20 degrees of ambient temperature if it has steady airflow to take away the heat. The advantage of a good air-water intercooler is more consistent intake air temperatures because water is a better heat sink. Water (coolant) is not as quickly affected by rapid changes in ambient air temperatures and car speed. But once water is hot, some heat goes out a radiator and some goes back into the air-water intercooler's intake air. Some cars don't have the routing or space for a good air-air intercooler so they must use an air-water intercooler.

An air-air intercooler is preferred for diesels because they are normally front engine so there's plenty of space for plumbing. An air-air intercooler is also easier to fabricate with less chance for leaks. If there is a major water leak into the intercooler core, it's possible that this could hydrolock the high compression diesel engine. A air-water intercooler is more appropriate on a mid engine car due to difficulty of intercooler packaging or a car with more peaky temperatures.

In a gasoline engine, the engine is operating at vacuum or low boost most of the time. Low boost doesn't heat the intake air as much as hard boosting and as a result, doesn't transfer as much heat to the intercooler. In other words, a larger intercooler is not needed unless you need the extra heat sink capability! Most modified gasoline cars would benefit a little from a larger intercooler due to higher than stock boost levels. However, how much it's needed in only lightly modified cars is debatable due to variations between cars, ambient outside temperatures, intended use (street vs. track), desired safety margin and fuel octane, etc.. For example, a large front mount intercooler will cool better than a small intercooler but it may not fit, may be blocked by the bumper, cause overheating problems due to blocking the radiator, etc.. Also check for leaves or dirt blocking the face of the intercooler.

A diesel engine has a greater need for an effective heat sink vs. a similar gasoline engine because of higher sustained boost levels. Turbos are also smaller for a number of reasons, for example, the smaller rpm range. I think that even lightly modified VW TDI could benefit from more efficient intercooling for maximum peak power. The best way to determine the need is to log pressure and temperature at the turbo and at the intake manifold. Especially for a front engine TDI, an air-air intercooler (which you already have) is the best option. The VW TDI naturally puts an oily mist on the inside of the intercooler from the crank case ventilation (CCV) system but trying to keep the inside clean is like trying to keep the oil dipstick clean. Gasoline cars shouldn't have any oil inside the intercooler.

If you must paint the intercooler to help hide it, use 1-2 light sprays of radiator paint or even better, a heat shedding coating like Swaintech's "BBE heat emitting coating". I don't know how well it works since bare Al is already very good at shedding heat. My guess is that because it sells well and measuring before-after intake air temperatures is so easy (assuming equal ambient test conditions), that it probably works.

Spraying coolant onto the outside of the intercooler is very effective because it can lower the temperature of the intake air below ambient air temps. CO2 (compressed carbon dioxide gas), N2O (nitrous), and just regular water all work very well at increasing intercooler effectiveness but only work until your coolant runs out. If you are preparing a short race, placing bags of ice on an air-air intercooler or chilling the coolant in a water-air intercooler works well too.

Keep in mind that in most modern turbo cars, turbo pressure is regulated by how much pressure is seen at the intake manifold, not at the turbo! Some also measure the air temp at or near the manifold. Regardless of intercooler efficiency, pressure at the intake manifold should drop only a little. As an example, assume an engine that limits boost to 15 psi at the intake manifold. If you have two turbo setups, one with an efficient intercooler with only 1 psi pressure drop and the other with than an inefficient intercooler with 4 psi pressure drop, the turbo with the efficient intercooler only has to make 16 psi at the turbo whereas the inefficient setup has to make 19 psi at the turbo. The turbo making 19 psi is mechanically more stressed and is creating more heat than the turbo that has to make only 16 psi, everything else being equal. If the turbo is pushed beyond the optimum area of efficiency, it will create exponentially greater amounts of heat and pressure. Again, pressure does not equal density, you are still creating the same amount of pressure seen at the intake manifold that regulates the turbo but the air is less dense and hotter, which creates less engine power and efficiency. This could also happen with an air leak. A common issue with the VW TDI is the sudden loss of power known as limp mode. The VW TDI ECU has pressure and air temp sensors and if the ECU senses a problem, it cuts power to prevent damage to the turbo and engine, preventing damage to the turbo from an overspeed.