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An Intercooler Increases Air Density and Power Potential
Probably the simplest way to describe an intercooler is just to say that it is just another radiator. Instead of being a radiator for engine coolant, intercoolers act as radiators to cool air that is going into the engine. More specifically, intercoolers are used to cool the compressed, heated air that comes out of a turbocharger or supercharger. Because of the way things are (some physics here), when a gas is pressurized and compressed, it heats up. The molecules in the air get more "excited" and become more active when air is compressed, and this results in an increase in the temperature of the compressed air. A typical forced induction engine will see boost pressure anywhere from 5-15 psi, with high boost forced induction engines running upwards of 20-30 psi. On my own engines, I've run boost pressure in the low 20's on pump gas. On my Powerstroke turbodiesel engine, I've seen boost over 30psi. That's nothing compared to some of the diesel engines used in drag racing and tractor pulling competitions. These ultra-high boost engines can run well OVER 100 PSI! These type of engines usually have multistage turbocharging where more than one turbo (typically 2) are configured so that the compressor outlet of the first turbo feeds into the inlet of the second turbocharger inlet. While this used to be an exotic setup found on race engines, dual stage turbocharging has also found it's way into some mainstream production vehicles. In order to see how an intercooler works, the picture below is an excellent diagram from a company that makes intercoolers showing how an IC works to cool the air going into the engine. By the way, sometimes the abbreviation "IC" is used to describe an intercooler, so you will also see that designation used as well in this article.
An intercooler is necessary in order to extract the maximum performance from a forced induction engine. Without an IC, you are almost defeating the purpose of forced induction. A turbo or supercharger is designed to increase the pressure of the air being fed into an engine. This increase in pressure leads to an increase in density which leads to an increase in the oxygen content of the air. This additional oxygen can be combined with additional fuel to produce more power. That's the theory of how forced induction works. However, as has already been mentioned, when you increase the pressure of the air, you also increase the temperature of the air. This increase in temperature reduces the density of the air and reduces the amount of oxygen contained in any given amount of air. This heating effect negates some of the benefits of forced induction, and that's why intercoolers are an important part of any high performance forced induction engine. There are 2 main categories of intercoolers: Air/Water IC and Air/Air IC. The air to air type are much more common. First, we'll take a look at the air to water type intercooler.
Air-Water intercoolers use water as the cooling fluid to carry the heat away from the heated air in a forced induction engine. My first experience with an IC was with an air/water IC. The first car that I owned that had a turbo did not come with an intercooler installed from the factory. It was a Merkur XR4Ti and it was set up to run up to 15psi boost from the factory with no IC. This was not ideal in terms of performance or reliability. Not only does an intercooler help to increase performance, but it can also help to improve the reliability of an engine. The reason this is true is because hot air that enters the combustion chamber of an engine is more likely to trigger destructive detonation (uncontrolled explosion of the air/fuel mixture). Detonation can lead to blown head gaskets, damaged pistons, and even bent rods under certain conditions. Intercooling helps to reduce the likelihood of detonation on a properly tuned engine. I say "properly tuned engine", because no intercooler will make up for an improperly tuned engine that is running too lean of a mixture (not enough fuel for the amount of air) which can also lead to detonation. Back to my experience with air-water intercooling... I decided to add an intercooler but I didn't want to have to fabricate a long run of pipe going to/from a front mount IC. Instead, I decided to install a Spearco air-water intercooler as seen below.
Air/Water Intercooler on a Ford 2.3 Turbo Engine
In an air-water intercooler, there is water flowing through the IC core that removes the heat from the pressurized air coming out of the turbocharger (or supercharger). In the picture above, you can see the piping going out of the turbo and into the intercooler (on the side with the black connector). On the outlet of the intercooler (with the orange connector), you can see a short pipe that curves into the throttle body. Using this air-water intercooler made if much simpler to install the IC itself, but then there is the added complexity of having to run the water lines along with separate heat exchangers (radiators), and also a pump to circulate the water through the system. I decided to go this route because I figured it would be easier to route rubber water hose than it would be to deal with a long run of piping for an IC. The picture below shows the front mounted coolers that I used on my project.
Front Mounted Coolers for Air/Water IC
These particular coolers were actually stacked plate transmission coolers. I added a rubber air dam along the bottom of the front plastic bumper cover to help force more air through the coolers. It all actually worked pretty well and during testing with a thermocouple in the air piping, I found this set up to reduce the air temperature by around 150F. This is a significant drop in air temperature which would lead to a significant increase in air density and performance potential. To experiment further, I even picked up a big Igloo water cooler at a thrift store. I put a small submersible bilge pump at the bottom of the cooler and ran temporary hoses to my air-water intercooler. I would put some water in to cover the pump and then I would fill the cooler with ice. This is not practical for everyday use, but I was just experimenting. The only place that this is probably practical is for short runs at a drag strip. I never tried it at the drag strip. I just did some informal testing on some back roads.
Submersible Bilge Pump for Temporary Air/Water Ice Water Use
Air-water intercoolers are used on some production vehicles like some of the supercharged Ford Lightning trucks, but you will find that most IC's will be air-air intercoolers. The reason for this is that air-air IC systems are generally simpler and less expensive. An air-water IC system requires the use of a pump which is prone to wear and failure. This is something that I discovered in my air-water system. I originally used a Jabsco positive displacement pump that had a rubber impeller. The impeller continually rubbed inside the pump housing. This type of pump design is fine for short term use, but it is not well suited for continuous duty. In my case, the impellers kept wearing out and falling apart. I did find another pump that had a bronze impeller that did not rub in the housing and it worked very well. It was much quieter too. My advice for anyone wishing to put together an air-water intercooler system is to look for an OEM pump from a supercharged Ford Lightning. That way, you can be sure that the pump will last as long as possible, since the Ford engineers would have specified a pump that will probably be the best choice for an air-water IC. In my case, I eventually sold the air-water intercooler and went with an air-air intercooler that I got from a wrecked supercharged Thunderbird (see picture below).
Air-air intercoolers are the most common type of IC. They are proven effective and efficient and their simplicity and reduced cost makes them a natural choice for a forced induction engine. Unlike an air-water IC system with a pump, there are no moving parts to wear out. The only exception to the "no moving parts" statement is if there is a anti-surge valve (blow off valve), but this is something that is typically mounted to the intercooler piping and is not a part of the IC itself. It's not typically a high wear item anyway.
Thunderbird Supercoupe Air-Air IC with Inlet & Outlet Pipes Added
In case you didn't know, a blow off valve releases the pressure inside the intercooler piping in the event that the driver abruptly releases the gas pedal under high boost. What happens is when the foot is quickly removed from the gas pedal, the throttle plate inside the throttle body quickly slams shut. Suddenly, the column of fast moving, pressurized air that was traveling through the piping slams into the closed throttle plate. This creates a pressure surge/spike that travels back through the piping and back to the fast spinning turbo compressor. This surge can create a shock load on the compressor and over time this can damage the turbocharger bearings and even damage the compressor wheel itself. So, the blow off valve just simply acts as a pressure relief valve in the IC piping. In effect, it's like a side door that is quickly opened to provide an alternate path for the pressurized air to exit, so that there isn't a harmful surge that slams back into the fast spinning turbocharger compressor wheel.
On my system, I gained 50 horsepower with the addition of this air-air intercooler. I was able to safely run higher boost and this intercooler was able to effectively cool the charge air. It was designed for use on a supercharged 3.8L V6 engine, and it was more than adequate for my smaller turbocharged 2.3L 4 cylinder engine. The reason that I chose to use this junkyard intercooler was primarily because of cost savings. Back at that time, intercoolers were quite expensive. A good quality Spearco intercooler was expensive - especially for a larger size IC. These days, things have drastically changed. There is an abundance of inexpensive made in China intercoolers available on the market. If you haven't seen them, then just do a search online and you'll find a multitude of inexpensive choices. What used to cost $1000, now you can find for $200-300 (or less). Admittedly, I don't know about the quality of these Chinese made intercoolers, but generally if it's made properly and doesn't leak, then it will probably be OK. If you are involved in high stakes racing, then you're probably still going to be getting a Spearco or some other well known brand of top quality intercoolers. However, if you are just having fun building a casual street car, then you might be able to use one of these inexpensive intercoolers.
My Low Cost "Redneck" Intercooler Piping
Another thing that you will need to consider is how to plumb the intercooler piping. Fortunately, there is also a variety of intercooler piping and connectors available. You can find intercooler pipe in straight sections, with 90 degree bends, 45 degree bends, and other angles. These pipe sections can be attached with silicone connectors and clamped securely. Again, these days these items can be purchased for a much more reasonable price. In the past when I was working on this stuff, I was always on a relatively tight budget and I always improvised in ways that made my projects more affordable. For example, for my intercooler piping, I used to buy mandrel bend exhaust piping because it was less expensive. Also, for connectors I would use truck cooling hose that can be purchased in bulk sticks of 3 feet in length. I cut these in sections to make my redneck intercooler piping connectors. It wasn't the prettiest sight (as you can see from the picture above), but it did work quite well. In fact, with my XR4Ti, I drove it for years under high boost of over 20psi without any problems. The one occasion where I had problems was when I was at an open night at the drag strip. I had just run the 1/4 mile in 13.1 seconds at 104 mph. I had a passenger that needed to get out because of the track rules. I also had to rent a helmet to meet the track rules once you exceed a certain performance level in the 1/4 mile. With nearly 200 less pound less weight, I was ready to knock off 0.2 seconds off my 1/4 mile time and break into the 12 second range. Not bad for a clunky old 4 cylinder powered street car. This is where a problem arose. As I launched at the start, I could feel that the tires hooked up well and I was headed for a good run. So I thought... then it happened. POP! I lost almost all power and the engine sputtered badly while I tried to limp the car off the track. My first thought was that I blew the engine. After I was safely off the track, I popped the hood to see what happened. There I found one of my intercooler pipes popped out of one of the connectors that I made from truck coolant hose. For years, I drove the car on the road and this never happened. When I was on my way to make a decent run at the drag strip, then it pops off! Murphy's Law! The moral to this whole story is simply that with proper intercooler piping and connectors, this would have never happened. On my sections of exhaust pipe, there was no retaining bead at the ends of the pipe. On real intercooler tubing, you will find a bead formed along the ends of all the pipes. When you clamp a connector onto the end of this type of pipe with the retaining bead, then it is very difficult for the pipe to pull out of the connector.
Well, there you have a brief overview of intercoolers, what their purpose is, and some of my own experience with both air-water and air-air intercoolers. If you have a turbocharged or supercharged vehicle that does not have an IC, or has a grossly undersized IC, then upgrading to a good intercooler can give you a nice upgrade in power as well. Intercooling - the power of physics at work to increase the horsepower of your forced induction engine. School textbook science was never this much fun!