Start-Up: Intercoolers and You

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Intercoolers for the Mazdaspeed 3. Top Factory top-mount intercooler, bottom CX Racing front-mount intercooler.

One of the topics I am frequently asked about revolves around the debate of which is better: a Top-Mounted Intercooler (referred to as TMIC) or a Front-Mounted Intercooler (a.k.a. FMIC)? The true answer depends on a multitude of factors, which I will delve into later (look for a video about it coming soon). But as I was doing research for the video, I decided this would be a great article for the Start-Up series as well, as I am currently working on my intercooler setup for Project TMB, so the timing couldn’t be better! 

WHAT IS AN INTERCOOLER AND WHAT DOES IT DO?

Inside an tube and fin intercooler. Image courtesy of mishimoto.com

An Intercooler is just another name for a heat exchanger that is used to cool the compressed air from either a supercharger or a turbocharger. It looks like a radiator (which it basically is) and is a network of tubes with cooling fins. There are two types of intercoolers; Air-to-Air and Air-to-Water. For this article we will be mainly talking about Air-to-Air intercoolers, of which there are two types; tube-and-fin and bar-and-plate. In order to understand how an intercooler works, we first have to cover how boost is created, for my example in this article I will use a turbocharged setup.

A image showing how a turbo works. Image courtesy of garrettmotion.com
 

A turbocharger works by using exhaust gases to spin an impeller (turbine) on a shaft that is connected to the intake (compressor) side of the turbo. This spinning motion compresses the incoming air (from the intake) and then the now compressed air exits via the outlet and enters the intake manifold (boost). 

The by-product of compressing the air is heat, and the more boost you run, the hotter that compressed air is going to get. As the air gets hotter, the oxygen content in the air drops (referred to as its density), meaning that it contains less oxygen molecules. The hotter the air, the less power you will be able to make. Hot air can also cause high cylinder temperatures which can cause the fuel to pre-detonate during the combustion cycle (which is known as detonation and is very bad).So how does an intercooler work?

Diagram of how an intercooler works. Image courtesy of haltech.com

The intercooler is routed in between the turbo outlet and the throttle body. As the compressed air is pushed through the intercooler, the heat gets transferred to the tubes and then the cooling fins. With the vehicle at speed, the cooler air passes over the fins, absorbing the heat, and reducing the temperature of the compressed air. This cooler air is now more dense, which means it has more oxygen molecules. The more oxygen molecules in the air, the more fuel you can pack into it and thusly, the more power you can make. Cooler air also helps to lower cylinder temperatures to control detonation.

With all that said, if you have a turbo setup with a low boost pressure ( I am talking in the 6 – 8 PSI range), an intercooler is not a necessity. But if you are planning on going any higher than that, I would highly recommend that you add an intercooler to your turbo setup. An intercooler is not so much a power adding mod as it is a support modification.  By dropping the temperature of the intake air, it is helping the turbocharger become more efficient, which in turn helps the engine. This gives you options during tuning (cooler air is more resistant to detonation, meaning you can run more timing for example) and is very helpful in the chase for more power. 

So now that you understand the basics of how intercoolers work, you are ready to add one to your project, right?!

Not so fast! There are some things you need to keep in mind before you click that buy button!

THE THINGS YOU NEED TO KNOW

Both of these are Mazdaspeed 3 intercoolers. Notice the difference in size. The top one is good for a daily driver/weekend fun car. If you car is going to see any kind of heavy use/track time, you will need to upgrade to something larger. Image courtesy of fivefivegarage.com

INTERCOOLER SIZING

Intercoolers come in all shapes and sizes, the goal is to find the right size for your build. A big intercooler than takes up all of the opening of your bumper could be too much, whereas a smaller core might be too little for your turbo. Allow me to explain.

Heat soak is the bane of all intercooler setups. But what is heat soak? This phenomenon happens when the turbo is pushing so much hot air through the intercooler that it either doesn’t have enough surface area and/or clean airflow to dissipate the heat, resulting in the intercooler being the same temperature as the air that is passing through it. The intercooler becomes soaked with heat and it can not cool effectively and becomes basically useless.

My first front-mount intercooler setup. This is a CX Racing intercooler. It performed fine at stock levels, but had issues of getting heat soaked when I used it for track days/events. Once I cranked up the boost pressure it had trouble keeping cool. Image courtesy of fivefivegarage.com

There are ways to help mitigate heat soaking the intercooler. Positioning the intercooler in the front of the car where it can get all the air it can (this is why front-mount intercooler setups are very popular) and creating ducting where the incoming air is routed to go through the intercooler and only the intercooler. A proper ducting system can increase the efficiency of the intercooler by up to 11%, so it is definitely something to keep in mind. Creating a water sprayer from using an old windshield washer bottle and pump to mist water over the core is another heat soak mitigation solution (Subaru did this on the WRX STi’s at one point).

Even with all of the afore mentioned options available to you, sometimes you just have to step up to a bigger core. But that comes with its own problems as well.

The core I am currently using in my Mazdaspeed 3, the Mishimoto S-Line. This is a bar and plate design, making it very efficient. It has over double the surface area of my previous intercooler. The downside is the installation is totally custom, so there is fabrication work involved, it could also use better ducting. A work in progress. Image courtesy fivefivegarage.com

Look at that big intercooler! You make the switch and are ready to install it and get mad street cred! But hold on, now we get to talk about too much intercooler and some of the problems you need to sort out. The two concerns we are going to talk about is pressure drop and boost lag.

Pressure drop is the change in pressure (boost PSI) when comparing the air coming into the intercooler to the air exiting it. For example lets say that you are tuned for 14 lbs. of boost (post-intercooler), but because of your small, inefficient, heat soaked intercooler (from the previous example), your turbo is actually producing 17 lbs. of boost (pre-intercooler). Pressure drop is a by-product of flowing air and the loss will grow as boost pressure increases. Since the core in our example is inefficient, that pressure drop will be greater, which in turn is making the turbo work harder to reach the required 14 lbs. of boost. 

Now let’s swap to our bigger core.

When stepping up to a larger core, one of the first things to be addressed is going to be its core construction. Since we are discussing air-to-air intercoolers there are going to be two cores we will be talking about; tube-and-fin and bar-and-plate.

TUBE-AND-FIN CORES

Tube-and-fin cores are what is used in OEM intercoolers primarily because they are inexpensive to manufacture and they are lighter in weight. At stock levels they are adequate for keeping intake temps near ambient, but once you increase power levels over stock or are doing repeated pulls (i.e. track days) they become very prone to heat soaking. Because of this, tube-and-fin construction is not used in aftermarket cores.

BAR-AND-PLATE CORES

Bar-and-plate cores are the preferred design for aftermarket intercoolers. The design allows the use of thinner material containing the air flow that does a superior job transferring heat. A bar-and-plate core can operate under high-heat conditions without losing efficiency and is better at handling high-boost applications than a tube-and-fin core. 

Let’s say our example car has a power goal of 350 HP. We purchase a new bar-and-plate intercooler and we want to increase our piping to 2.75″ (which can flow enough air to support 700 HP/1050 cfm). The bar-and-plate intercooler is going to be more efficient than our old tube-and-fin unit. Pressure drop is now around 2 PSI so instead of our turbo having to run 17 lbs. of boost to reach our target, it is now having to make 16 lbs.! The turbo is having to work less to reach our target PSI and as a result the IAT (Intake Air Temperature) will be cooler and result in less pressure drop. Cooler air is more dense, which means more power that can be made, everything is all positive!

Well except for one thing…

Why does it seem like the turbo is taking longer to spool?

Bigger is not always better. You can cause problems for yourself if your piping and/or intercooler are too large. In our example car, the original piping was 2″ in diameter. This is more than sufficient to move enough air to support up to 370 HP, which is larger than our 350 HP target, but leaves us with some head room. When selecting an intercooler, you will want to match your intercooler inlet and outlet to the piping you want to use.

Airflow volume and horsepower output are two specifications that you can use to determine how big your intercooler and pipes need to be. Piping too large will require greater air flow to produce boost, this will cause lag. Piping too small will restrict flow and limit power output. When you step up to a larger intercooler core size remember; the larger the core volume, the more air that is required to fill the cooler.

A chart that show pipe diameter and the hp/cfm it supports. Image courtesy of mishimoto.com

Back to our example car, we went with 2.75″ piping and an intercooler with 3″ inlet/outlet. 2.75″ piping will support up to 700 HP/1050cfm, it can move large amounts of air. But our turbo pumping out 14 PSI is going to have a tough time moving the amount of air necessary (our turbo is moving about 540 cfm of air flow). Until the pressure builds enough to move that air, you are going to experience a lot of lag. The piping is way too large for what our example is running. If we reduce the piping back to our original 2″ size, we can lessen the lag and increase the speed of the air flow and the efficiency of our setup. 

What size you need to use will depend on the horsepower your car is making and the amount of space available for your piping.

Hopefully this gives a good foundation for you to begin your research into a new intercooler! 

See you all on the next one,

-R