For quite some time now, I've been hearing about people's turbo setups and there are a lot of misconceptions on Intercooling. The purpose of this thread is to cover the basics of intercoolers in relationship to turbo charged vehicles.
To start off, a LOT of people think that intercoolers HAVE to be put on a vehicle if it is turbo'd. This all started because someone would say something along the lines of:
John: "Wow, that car is Turbo'd!"
Joe: "How do you know?"
John: "Because it has an intercooler in the front!"
Joe: "Wow! I know nothing about Turbos, but I will spread this information among my friends!"
Another misconception is intercooler sizing. Again, I imagine a conversation going somewhat like this:
Jane: "Daaaaaamn, look at that huge intercooler on that Silvia!"
Jill: "Yeah! That thing is HUGE! This car MUST be making power!!"
Jane: "Yes, I know, and think of how efficent that must be, even though I don't know the effect that proper design has on it!"
Basics (How they work):
So WHY would one need an intercooler? What is the purpose of having one? Well, that is a very good question.
We learned in our highschool science class that any compressed fluid will increase in temperature, but because hot air is less dense (meaning less oxygen per unit of volume), in an automotive application, hot air is our enemy. Less oxygen means less power, and lower temperatures reduce the risk of detonation.
This is why intercoolers are used. Like a radiator, an intercooler is a simple heat exchanger for the incoming air charge, the intercooler cools the intake air charge to temperatures closer to ambient, condensing it and allowing you to fit more into each combustion stroke. This also helps in reducing combustion temperatures (bad) and again, helps prevent detonation.
Construction (Part I):
Intercoolers are made in two primary parts. The core and the end-tanks. The core is a collection of fins, each one passing a small part of the air charge by a small part of the cooling medium (outside air in an air/air unit, or water in an air/water unit), who's low temperature and high flow around the core absorb the heat of the air charge. The endtanks simply take the air from the pipe and distributes it to the fins of the core, or the other way around.
The curved pieces of sheet metal that you see on both the inside and outside of intercooler cores, between the fins are called Turbulators. They do much to increase drag, but more over, they increase heat transfer. Their job is to separate the air charge (as well as the cooling medium) into even smaller units, forcing virtually all of the two fluids to do their part in heat exchange. The more turbulators there are, the better the heat exchange, but the greater the flow restriction as well. Therefore, in an application with lots of internal flow area, more turbulators can be reasonably employed, while in an application with less internal flow area the reverse is the case.
What makes a good intercooler?
There are several things to take into consideration when selecting the proper intercooler.
Internal Volume:
While more volume will provide more space for heat transfer, just as overly large piping will result in more lag and less throttle response, a larger intercooler will take more time to pressurize and have the same effect. The goal of designing/selecting the proper intercooler for each application is to find the balance between maximizing the ability to remove heat from the system while minimizing flow restriction and pressure loss. Therefore, depending on your application, bigger is not always better.
Internal Flow
In the design of intercooler cores, there is always a balancing act between too much and not enough drag. If the air charge has a hard time passing through the core and spends more time in it, it will consequently have more time to give up its heat to the cooling medium and cool down. However, if it spends too much time in the core, it will experience large pressure drop, forcing the compressor to do extra work to achieve the same boost level, and thereby reducing efficiency.
Core Size
As the cooling medium passes through the core and absorbs the heat of the air charge it heats up, by the end of the core the efficiency of the heat transfer decreases significantly; so much that the second half of the intercooler only does one half of the work. Increasing depth also increases the air for the passing medium, which can be a considerable issue for air/air intercoolering. If the outside air can more easily pass around the intercooler then through it, then it will do just that, thereby decreasing the amount of heat exchange that can possibly occur. Fortunately, proper positioning of the intercooler and ducting to it can be used to counter this problem, as will be discussed later.
Core Material
Generally speaking, aluminum seems to be the material of choice for all automotive heat exchange, but there are other materials that offer distinct advantages and disadvantages. Silver, for example has a lower coefficient of heat than aluminum, and will support more heat exchange in an otherwise identical situation. The price increase is surprisingly small as well. For air to water cores, copper offers even better heat properties at a fraction of the cost of both aluminum and silver. Unfortunately, due to copper's corrosive properties, it generally isn't appropriate for street use in an air to air situation.
Air to Air or Air to Water?
The main draw of an air/air intercooler is its low cost and simplicity. The outside air offers a virtually limitless supply of cooling medium, and offers excellent efficiency at high speeds. Unfortunately, an air/air intercooler can never lower charge temperatures below ambient air temperature, and in some applications (such as the top mount unit found in WRX's) at lower speeds can function more as an interheating then an intercooler (although this isn't a huge problem, as at low speeds lower charge temperatures generally aren't as important, if important at all).
The Air/water intercooler's biggest performance draw is its ability to lower air charge temperature below ambient air below ambient air temperature. Water can transfer heat 14 times more easily then air. The obvious drawback is the need for a much more complex. For a setup like this, a pre-cooled cooling medium boost be employed, such as an ice water tank. However, such as setup can only be used for a short time, as after the medium absorbs all the heat of the air charge, no further heat exchange can occur. It would also prove to be quite costly.
Intercooler Basics
Linear Mode
