Turbocharging Explained

[Russ]

I realized in another thread that I've had a bunch of links that I have referred people to over the years to totally pretty much answer just about every question that could be asked regarding turbocharging, so I wanted to post all that information here for you all to read and get educated. It's a very long read, but hopefully very informative.

Turbocharging 101

How Turbochargers Work
by Karim Nice from: http://auto.howstuffworks.com/turbo.htm



When people talk about race cars or high-performance sports cars, the topic of turbochargers usually comes up. Turbochargers also appear on large diesel engines. A turbo can significantly boost an engine's horsepower without significantly increasing its weight, which is the huge benefit that makes turbos so popular!


Photo courtesy Garrett Caption

In this article, we'll learn how a turbocharger increases the power output of an engine while surviving extreme operating conditions. We'll also learn how wastegates, ceramic turbine blades and ball bearings help turbochargers do their job even better.

Turbochargers are a type of forced induction system. They compress the air flowing into the engine (see How Car Engines Work for a description of airflow in a normal engine). The advantage of compressing the air is that it lets the engine squeeze more air into a cylinder, and more air means that more fuel can be added. Therefore, you get more power from each explosion in each cylinder. A turbocharged engine produces more power overall than the same engine without the charging. This can significantly improve the power-to-weight ratio for the engine (see How Horsepower Works for details).


In order to achieve this boost, the turbocharger uses the exhaust flow from the engine to spin a turbine, which in turn spins an air pump. The turbine in the turbocharger spins at speeds of up to 150,000 rotations per minute (rpm) -- that's about 30 times faster than most car engines can go. And since it is hooked up to the exhaust, the temperatures in the turbine are also very high.

The Basics
One of the surest ways to get more power out of an engine is to increase the amount of air and fuel that it can burn. One way to do this is to add cylinders or make the current cylinders bigger. Sometimes these changes may not be feasible -- a turbo can be a simpler, more compact way to add power, especially for an aftermarket accessory.

Turbochargers allow an engine to burn more fuel and air by packing more into the existing cylinders. The typical boost provided by a turbocharger is 6 to 8 pounds per square inch (psi). Since normal atmospheric pressure is 14.7 psi at sea level, you can see that you are getting about 50 percent more air into the engine. Therefore, you would expect to get 50 percent more power. It's not perfectly efficient, so you might get a 30- to 40-percent improvement instead.

One cause of the inefficiency comes from the fact that the power to spin the turbine is not free. Having a turbine in the exhaust flow increases the restriction in the exhaust. This means that on the exhaust stroke, the engine has to push against a higher back-pressure. This subtracts a little bit of power from the cylinders that are firing at the same time.

High Altitudes
A turbocharger helps at high altitudes, where the air is less dense. Normal engines will experience reduced power at high altitudes because for each stroke of the piston, the engine will get a smaller mass of air. A turbocharged engine may also have reduced power, but the reduction will be less dramatic because the thinner air is easier for the turbocharger to pump.

Older cars with carburetors automatically increase the fuel rate to match the increased airflow going into the cylinders. Modern cars with fuel injection will also do this to a point. The fuel-injection system relies on oxygen sensors in the exhaust to determine if the air-to-fuel ratio is correct, so these systems will automatically increase the fuel flow if a turbo is added.

If a turbocharger with too much boost is added to a fuel-injected car, the system may not provide enough fuel -- either the software programmed into the controller will not allow it, or the pump and injectors are not capable of supplying it. In this case, other modifications will have to be made to get the maximum benefit from the turbocharger.

How It Works
The turbocharger is bolted to the exhaust manifold of the engine. The exhaust from the cylinders spins the turbine, which works like a gas turbine engine. The turbine is connected by a shaft to the compressor, which is located between the air filter and the intake manifold. The compressor pressurizes the air going into the pistons.


Image courtesy Garrett
How a turbocharger is plumbed in a car

The exhaust from the cylinders passes through the turbine blades, causing the turbine to spin. The more exhaust that goes through the blades, the faster they spin.


Image courtesy Garrett
Inside a turbocharger

On the other end of the shaft that the turbine is attached to, the compressor pumps air into the cylinders. The compressor is a type of centrifugal pump -- it draws air in at the center of its blades and flings it outward as it spins.


Photo courtesy Garrett
Turbo compressor blades
In order to handle speeds of up to 150,000 rpm, the turbine shaft has to be supported very carefully. Most bearings would explode at speeds like this, so most turbochargers use a fluid bearing. This type of bearing supports the shaft on a thin layer of oil that is constantly pumped around the shaft. This serves two purposes: It cools the shaft and some of the other turbocharger parts, and it allows the shaft to spin without much friction.

There are many tradeoffs involved in designing a turbocharger for an engine. In the next section, we'll look at some of these compromises and see how they affect performance.





Design Considerations
One of the main problems with turbochargers is that they do not provide an immediate power boost when you step on the gas. It takes a second for the turbine to get up to speed before boost is produced. This results in a feeling of lag when you step on the gas, and then the car lunges ahead when the turbo gets moving.

One way to decrease turbo lag is to reduce the inertia of the rotating parts, mainly by reducing their weight. This allows the turbine and compressor to accelerate quickly, and start providing boost earlier. One sure way to reduce the inertia of the turbine and compressor is to make the turbocharger smaller. A small turbocharger will provide boost more quickly and at lower engine speeds, but may not be able to provide much boost at higher engine speeds when a really large volume of air is going into the engine. It is also in danger of spinning too quickly at higher engine speeds, when lots of exhaust is passing through the turbine.

A large turbocharger can provide lots of boost at high engine speeds, but may have bad turbo lag because of how long it takes to accelerate its heavier turbine and compressor. Luckily, there are some tricks used to overcome these challenges.

Most automotive turbochargers have a wastegate, which allows the use of a smaller turbocharger to reduce lag while preventing it from spinning too quickly at high engine speeds. The wastegate is a valve that allows the exhaust to bypass the turbine blades. The wastegate senses the boost pressure. If the pressure gets too high, it could be an indicator that the turbine is spinning too quickly, so the wastegate bypasses some of the exhaust around the turbine blades, allowing the blades to slow down.

Some turbochargers use ball bearings instead of fluid bearings to support the turbine shaft. But these are not your regular ball bearings -- they are super-precise bearings made of advanced materials to handle the speeds and temperatures of the turbocharger. They allow the turbine shaft to spin with less friction than the fluid bearings used in most turbochargers. They also allow a slightly smaller, lighter shaft to be used. This helps the turbocharger accelerate more quickly, further reducing turbo lag.

Ceramic turbine blades are lighter than the steel blades used in most turbochargers. Again, this allows the turbine to spin up to speed faster, which reduces turbo lag.

Some engines use two turbochargers of different sizes. The smaller one spins up to speed very quickly, reducing lag, while the bigger one takes over at higher engine speeds to provide more boost.

When air is compressed, it heats up; and when air heats up, it expands. So some of the pressure increase from a turbocharger is the result of heating the air before it goes into the engine. In order to increase the power of the engine, the goal is to get more air molecules into the cylinder, not necessarily more air pressure.

An intercooler or charge air cooler is an additional component that looks something like a radiator, except air passes through the inside as well as the outside of the intercooler. The intake air passes through sealed passageways inside the cooler, while cooler air from outside is blown across fins by the engine cooling fan.

The intercooler further increases the power of the engine by cooling the pressurized air coming out of the compressor before it goes into the engine. This means that if the turbocharger is operating at a boost of 7 psi, the intercooled system will put in 7 psi of cooler air, which is denser and contains more air molecules than warmer air.

[Russ]

Article Provided by http://www.turbominivan.com

This is an excellent read and I finally got a good solid understanding of turbo trims, wheel sizes and whatnot.

Why a bigger turbo?
This one is easy: for more power! When it all comes down to it, any engine is little more than an air pump. The amount of air moving through the engine is what ultimately determines total power production, and more air = more power. This concept is critical to everything we discuss below.
Now then, let's look at the stock Mitsubishi TE04H-13C turbo. Putting it mildly, this turbo is horribly undersized for serious power production on our engine. Why is it too small, exactly? There are two reasons (and one is far more important than the other): the compressor and the turbine. Any given compressor has a certain range of operational airflow, of course, and ours is designed to only flow so much air. The larger problem, though, is the turbine--it absolutely chokes our engine! Imagine running a marathon where you can inhale freely through your open mouth but you're forced to exhale through a soda straw--this is the equivalent of our engine trying to breathe through the tiny TE04H turbo!


2.5L exhaling through a soda straw:



What do I look for in a turbo upgrade?
This is a complicated question which, naturally, has a complicated answer. The reason is because there is no one turbo which is the best one for all situations. How much total power do you want your van to make? Where in the rev range do you want your power to be? Where will you drive the van, and how will you use it day-to-day (if at all)? You need to know the answers to these questions in order to form a plan of attack. Research will be in order, so be ready to do some homework!

Sooner or later you'll come to a general conclusion about the sort of turbo you'll need. Now you have to sit down and spec it out. This is nothing new; racers have been doing it for decades. In my opinion, though, there is one thing which people constantly get wrong when choosing a turbo: they go about it backwards. When designing your new turbo, move from the general to the specific: begin with the turbine, not the compressor. Why is this? It all goes back to the whole air pump thing--if the air cannot get out of the engine then you will never make very much top-end horsepower. Once you know how much power you want to make and where in the rev range it will reside, you must choose a turbine to support this airflow requirement. Now that the turbine has been specified--which also dictates the shaft speed--select a compressor which will supply the proper amount of air to match.


Tips on choosing a turbine
Turbines are actually a very simple science. The turbine powers the compressor because it is a physical restriction in the exhaust flow. The more it restricts (ie: the smaller the turbine) the faster it spins the shaft... but the more it chokes the engine and robs you of top-end horsepower. The less it restricts (ie: the larger the turbine) the slower it spins the shaft... but the less it chokes the engine and the more top-end horsepower you can make. That's the key to understanding a turbine.

So now let's talk sizing. When outlining the details of your new turbo, move from the general to the specific. Thus your first decision will have to do with the A/R ratio. Quite frankly, there are only two choices for a Chrysler-style housing: .48 (stock Garrett) or .63. Unless you want to keep your van's power output very close to stock--and you wouldn't be reading this page if you did--you can safely assume you want to use a larger .63 A/R housing. But what does this number mean, exactly? Look at this picture:

Select the point where the turbine housing begins and measure the cross-sectional area A at that one point. Now measure the distance between the center of this area and the center of the turbine wheel--that's the radius R. Do some division and you come up with a measurement. Now move to a different point in the turbine housing and do it again--the calculated ratio remains constant because the housing constantly gets smaller in diameter the closer it gets to the turbine wheel. When upgrading from the .48 to the .63 A/R, it's the area that changes; the radius is essentially identical. This is precisely why the .63 housing flows more air--the passage is larger!

Now you've decided your turbine's A/R ratio. Next, choose your exact turbine wheel. Turbine wheels are typically referred to in stages: StageI, StageII, StageIII, etc. One very important fact is that these stages are not universal! A Turbonetics StageII wheel is far different from a Garrett StageII wheel, for example. Make sure you know what you're getting when you ask for it.

What's the big deal about stages? This is how turbo manufacturers refer to the differences from one wheel to another. What changes, exactly? The shape, curvature, pitch and "overlap" of the wheel's blades, primarily. For a great example, look at the picture below. See how the stock wheel's blades "fold over" one another, preventing you from seeing through them? By contrast, check out all the open area between the blades of the aftermarket wheel.


Factory Garrett turbine wheel verses aftermarket Garrett StageII turbine wheel:

All those open areas on the aftermarket wheel result in far less turbine backpressure, which paves the way for lots more top-end horsepower... but remember: this aggressive turbine will spin more slowly than the stock one. The slower shaft speed means the compressor spins more slowly, also. When the compressor speed slows down, your boost output falls off as well. This is why large turbines need large compressors to match!

[Russ]

I suggest checking with other racers and seeking their advice for choosing your exact turbine wheel. Find out which stage they have, what the resulting boost threshold is, and how well it pulls on the top end. Their real-world experience can help you fine tune your exact turbine wheel selection to meet your desires.


Tips on selecting a compressor
Now that you've selected a turbine, it's time to choose a compressor to match it. You will have a variety of options. However, it won't be a case where only one exact wheel will work--instead, it's a matter of one (or two) wheels which will work best. This is where you'll need to do some math, come to grips with a few technical terms, and so on... but it still isn't outrageously difficult to do, so don't sweat it.

Speaking of technical terms, let's get right to a few of them. To understand why and how one compressor wheel flows differently than another, you need to understand the anatomy of the wheel itself. Let's take a look at the following picture:


Compressor wheel terminology:

Two key parts of a compressor are the inducer and the exducer. The inducer (sometimes called the minor diameter) is the part of the wheel that first takes a "bite" of ambient air. The exducer (sometimes called the major diameter) is the part of the wheel that "shoots" the air--now compressed--out of the turbo. Just remember that the inducer is where the air comes in and the exducer is where the air exits. Got it? Good.

You need to understand those two terms in order to grasp the concept of trim, a bizarre bit of tech-speak which is often thrown about. Trim is simply a term to describe the size of a specific compressor within a family of wheels. It can be expressed in abstract ways (such as when Turbonetics says they have P-trims, Q-trims, etc) or you can use the actual numeric measurement (50 trim, 57 trim, etc). Here's how you calculate the measurement:


Trim = (minor diameter / major diameter) ^2 * 100
So now we have a way to perform some math and get a number. What does it all mean? Generally speaking, the larger the trim the more flow the wheel will have. Nevertheless, one should not rely solely on a trim measurement when selecting a compressor wheel! Find out specific wheel measurements (inducer and exducer), understand how subtle differences will affect airflow and response, and then choose a wheel accordingly.

Speaking of subtle differences, let's take a look at them. First, the inducers:


Compressors with different inducers and identical exducers

What happens when you upgrade to a larger inducer while retaining the same exducer? The most notable change is more airflow capability; since the turbo is taking a bigger "bite" of air in every revolution, it can obviously "spit out" more air as well. Gee, more airflow aounds great... so why not go to the biggest inducer you can find? Because that creates two main problems, one much more important than the other. The smaller problem--really it's just a nuisance--is the turbo will now have a little more lag during spoolup (because the bigger wheel weighs more, plus it has to do more work with each revolution, etc). While this extra lag might not be noticed on a dyno--all the bystanders will be oohing and ahhing at the huge top-end horsepower such a turbo would produce--it would make for dissatisfaction in your day-to-day drive and could even cause you to lose a drag race to a car with less peak horsepower but more area "under the curve" due to his turbo that spools sooner. The real trouble with a large inducer increase but no exducer increase, though, is it makes the turbo much more likely to surge. Surge is the situation when the compressor "spits out" more air than the engine can swallow, which causes a backup of air at the intake and it actually creates reverse-flowing pressure waves that can be very damaging to the turbo. You want to avoid surge at all costs.

Okay, so maybe we won't go hog nuts wild with the inducer. How 'bout the exducer? Let's take a look:


Compressors with identical inducers and different exducers

When you upsize the exducer without modifying the inducer, the exact opposite effect happens: your spoolup time is reduced. Why does this happen? Remember that a compressor "spits out" the air in a radial fashion. The larger exducer gives a higher wheel edge speed for a given shaft speed, and that higher edge speed means the compressed air exits at a higher speed than before... and thus it builds boost faster. Another effect of this upgrade is an increase of the compressor's pressure ratio capability without a significant increase in its maximum flow rate; we'll discuss these more later on.

So now let's tie it all together. If you want more power with similar response, look for an upgrade of both diameters. The larger inducer will net you more airflow and thus greater power capability, while the larger exducer keeps boost response within reason and lessens the chance of surge.


(Stop and take a deep breath--you've digested a lot of info.)
Now that we've covered all that, it's time to mention compressor flow maps. These are simply charts which give you a feel for how one wheel's flow compares to another; in fact, they give you the entire relationship of airflow, compressor efficiency, pressure ratio and shaft speed. Don't overlook the importance of that last varible--this information will be crucial when matching up to free-flowing turbines which are spinning quite a bit slower than stock. Here's an example of a flow map:


Flow map for a Turbonetics T04E-50

Compressor maps always use X:Y graphs where the horizontal axis represents airflow (typically expressed in lb/min of air) and the vertical axis indicates the pressure ratio (manifold pressure divided by ambient pressure). The oval lines represent "islands" of efficiency, and the percentage figures for each one tell you the thermal efficiency of the compressor wheel at that combination of airflow and pressure. The lines which cross the islands indicate the shaft speed required to generate that amount of air flow; for example, the top line indicates a shaft speed of 126,077 RPM. These numbers are very important since a 'larger' turbine will spin slower than a smaller one.

[Russ]

From here: http://www.turbosaturns.net/articles/wastegate101.htm

Wastegate 101
by:Titan
A turbo by itself does not know how to regulate boost levels. Basically, a turbo system is a positive feedback loop meaning that the engine's exhaust spins the turbo which, forces more air into the intake making more exhaust which, in turn spins the turbo even faster. Without a way to regulate boost levels the turbo would keep producing higher pressures until the engine exploded. This is where the wastegate comes into play. The wastegate attaches onto the turbo header before the turbo. When you begin accelerating exhaust gas pressure builds inside the manifold and is forced through the turbo. This pressure continues to increase as the turbo spins faster (remember the positive feedback loop). When the desired boost level is reached the wastegate opens and vents pressure from inside the manifold so the turbo won't spin any faster.

So how does the wastegate work exactly?




Above is a diagram of a typical external wastegate. Inside the wastegate is a diaphragm which creates a seal, and a spring which holds the wastegate closed. Spring rates vary depending on the amount of boost you want to run, typically they are given in a "bar" value for example 1 bar would be 14.7psi. This would mean that in order to open the wastegate you would need to excerpt a greater pressure than the 14.7psi spring holding the wastegate closed. In order for the wastegate to work you must have the compressor reference port hooked up to the compressor side of the turbo, if you don't have this vacuum line attached than the boost pressure will not be limited to the set spring pressure; it will build unlimited boost pressure until your engine is destroyed.

Normally pressure from a spooling turbo pushes against the diaphragm (though the vacuum line attached to the compressor reference port) which in turn pushes against the wastegate spring. When the pressure from the spooling turbo exceeds the spring pressure the wastegate's plunger opens releasing the excess pressure through the dump tube into the exhaust after the turbo or to open atmosphere. Typically, if you use the wastegate to control your boost levels you will experience a decrease in power and spool times. Why? Although the spring fully opens at its set spring pressure it tends to begin opening before reaching the set spring pressure. This "pre-opening" leaks boost pressure through the dump tube before max boost pressure is reached resulting in a decrease in power mostly toward the top end. This can be corrected by using a boost controller.


Boost controllers serve two functions; increase boost levels beyond the set wastegate spring pressure and reduce the "pre-opening" of the wastegate-controlled boost pressure.

A manual boost controller will allow you to increase boost levels beyond what the wastegate spring is set. How does it work? Below is a diagram of a manual wastegate.




In order to run a manual boost controller we need to tee off of the vacuum line which runs from the turbo compressor housing to the compressor reference port. The manual boost controller works using a spring and check ball, by screwing the adjusting screw into the boost controller you put more pressure on the spring which reduces the amount of airflow through the boost controller and into the boost controller port. Less airflow means less pressure will be assisting the spring to keep the wastegate plunger shut. The pressure in the vacuum line going to the compressor reference port will equal the pressure the turbo is producing. A boost controller will allow you to direct some of that pressure to the top of the wastegate diaphragm creating two opposable forces. By adjusting the spring pressure of the boost controller you can vary the amount of boost that the turbo will make before opening the wastegate's plunger. If you want to run a higher boost level than the wastegate spring allows you will need a boost controller.


The manual boost controller is a very simple device that can help you make more power from your turbo setup. Here are three additional things to keep in mind about wastegates:

Without a line running from the compressor housing to the wastegate's compressor reference port boost pressures will keep increasing forever. This will quickly destroy your engine!

Run a wastegate as close to the desired boost pressure as possible this will help the boost controller handle the pressure better.

You can't reduce your desired boost pressure lower than the spring rate.

[Russ]

Intercooling 101 provided by Swift Performance Technology: http://davidenglish.com/swift/Tech/Interco...ooling_101.html

Intercooling 101
The Basics:

Any compressed fluid will increase in temperature. Because hot air is less dense (meaning less oxygen per unit of volume), however, in an automotive application, hot air is our enemy. Less oxygen means less power, and lower temperatures reduce the risk of engine-destroying detonation.

Components:

Just like a radiator, an intercooler is a simple heat exchanger, built in two primary parts: the endtanks and the core. 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 vise versa.

Turbulators are curved pieces of sheet metal seen on both the inside and outside of intercooler cores between the fins. They do much to increase drag, but much more too 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.

Design considerations
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.

Internal flow area

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 sizing
Because 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
Although aluminum tends to be the material of choice for core construction with generally all automotive heat exchanged, several other materials offer distinct advantages and disadvantages. Silver has a lower coefficient of heat then aluminum, and will thus support more heat exchange over aluminum in an otherwise identical situation, and the increase in price is surprisingly small. The core manufacture Blackstone, used by Porsche, Ferrari, Saab and Johnisenglish uses all silver cores, as well as several other foreign companies. For air/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/air situation.

Air/Air or Air/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. To do this, as 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.

To construct an air/water intercooler capable of cooling the air charge in a street application, a separate heat exchanger (along with a pump to power it) must be employed to cool the water itself. The benefit of such as setup is the ease of piping in applications where such is a consideration, and the desirable heat properties of water (water can transfer heat 14 times more easily then air). The obvious drawback is the need for a much more complex, usually much more expensive setup.

[Russ]

Air/Air Intercooling Design Considerations:

Core streamlining
As mentioned earlier, air will travel the path with the least resistance. More drag there is between the fins of a core, leads to less ambient air will flow between them and the less heat exchange. The simplest method of increasing air flow is to increase the internal flow area, but doing so lowers the amount of heat exchange, so alternate means of doing so should be employed first.

Core Flow Direction
Because greater internal flow area will increase the amount of heat transfer over a unit with the same volume but less internal flow area, the orientation of endtanks with respect to the core should always be designed to maximize internal the flow area. However, because of space considerations in our cars, its very difficult to employ an intercooler of this sort in the front bumper.

Fin shape
The two primary fin designs of intercoolers on the market today are bar and plate and round edged extruded. Because it is easier for air to flow around a smooth curve, round edged finds will always provide for better air flow over bar and plate designs.

Placement
The placement of the intercooler plays a huge role in the amount of heat exchange that can take place, but is almost always limited by available space. Front air dams, fender wells and practically anywhere that has consistent access to a large amount of ambient airflow will work, but for Integra purposes the front air dam tends to be the place of choice, both for reasons for success and necessity. Top mount units, such as those used in the WRX and FC RX-7 usually enjoy sufficient airflow at high speeds when paired with a hood scoop and proper ducting for the air to leave, but will always be subject to the high temperatures of the engine bay; it gets the job done, but by no means is it ideal.

Ducting
Proper ducting of ambient air to and away from the core can increase airflow by up to 20%. However, contrary to what common sense might imply an opening larger then the internal flow area of the core is not the ideal size. Instead, the opening should be between 60 and 25% of the core area. This number is both a result of the fact that in a completely open situation, less then one forth of the area going towards the core would flow between its fins, and a smaller inlet tapering open towards the core will produce a low pressure area, sucking in more air and maximizing air flow. An inlet tapering down towards the core will produce a high-pressure area, and consequently hinder airflow towards the core. After leaving the intercooler, the outside air should be given an unrestricted path away from the core, thus allowing cooler air to come and take its place. However, in most Integra front mount applications, the radiator sits behind the intercooler, causing it to both restrict airflow and receive already heated air for cooling. Fortunately, the affect of this on the cooling system is generally acceptably small.

Endtank Design
An ideal endtank will take the air charge from the piping inlet and evenly distribute it among each fin, or vise versa. However, many intercoolers on the market today employ endtanks that unevenly distribute the air, putting more work on one section of the intercooler then the other, and as a result decreasing efficiency. Un-ideal endtank design can be compensated for by using internal baffles to even out the airflow, or simply altering the shape of the endtank itself.

Intercooler Piping
Piping Diameter
Although intercooler piping generally follows the best (if not only) path it can take, there are several factors to take into account. Larger diameter piping will allow for less restriction in a high pressure, high flow application, but will also require more time to fill and hinder throttle response. 2.25” outer-diameter piping tends to be the size of choice for low/mid boost street Honda applications, while 2.5” piping is more appropriate for high boost race applications.

Piping Material
Although there are limitless possible materials to construct intercooler piping out off (they’ve got carbon fiber intakes these days…), the cost and performance effective material is hands down aluminized steel. Cheap, common and easy to work with, it can quickly be fabricated to whatever shapes are necessary. The only drawback to aluminized steel is its heavy weight when compared to aluminum. Unfortunately, aluminum can easily cost over three times as much as aluminized steel, but fortunately, a full set of aluminized steel intercooler piping only weighs about 15-20 pounds.

Due to the high flow velocity of the air charge and the relatively low surface area of a simple pipe (when compared to that of an intercooler core), heat soak from the engine bay is generally unimportant. Insulating wrap, chrome plating, ceramic coating or different piping materials can all be used to reduce it further, but for virtually any street application the gain is power doesn’t even come close to validating the effort.

Connector Material
Rubber and Silicone are the two most common and readily available base materials for piping connectors. While silicone by far offers the best heat resistance (and therefore the greatest ability to hold the piping together under the high temperatures of the engine bay), many blends of rubber exist, some being better then others.

Beading
Piping blow off under boost can be a huge problem (not to mention embarrassment) if not prevented. Pressing or welding a bead around the edge of each pipe will provide the piping connectors with something to hold on to, and in most cases eliminate the problem altogether. However, piping connectors will always be the weak link in the system, and wherever possible (with easy removal in mind) pipes should be bent as one piece or welded together.

Conclusion
Intercooling is an integral part of an Turbocharged air induction system. Lower air charge temperatures produce more power per psi and a larger safety margin of detonation resistance. Because of this, boost levels can be safely increased by 3-4psi, increasing power output that much more.

Proper intercooler design should aim for the least possible pressure drop and the highest possible efficiency of heat exchange.

[Russ]

Complete Guide to Intercooling brought to you by AutoSpeed: http://www.autospeed.com/cms/article.html?&A=0084&P=1

When a turbo or supercharger compresses air, the air is heated up. While this hot air can be fed straight into the intake of the engine (and often is), there are two disadvantages in taking this approach.

Firstly, warm air has less density than cool air - this means that it weighs less. It's important to know that it's the mass of air breathed by the engine that determines power, not the volume. So if the engine is being fed warm, high pressure air, the maximum power possible is significantly lower than if it is inhaling cold, high pressure air. The second problem with an engine breathing warm air is that the likelihood of detonation is increased. Detonation is a process of unstable combustion, where the flame front does not move progressively through the combustion chamber. Instead, the air/fuel mixture explodes into action. When this occurs, damage to the pistons, rings or head can very quickly happen.

If the temperature of the air can be reduced following the turbo or supercharger, the engine will have the potential to safely develop a higher power output. Intercoolers are used to cause this temperature drop.

Temperature Increase
There are a number of factors that affect the temperature increase that occurs when the air is compressed. Firstly, the higher the boost pressure, the greater will be the temperature increase. As a rule of thumb, if you are using a boost pressure level of more than about 0.5 Bar (~ 7 psi), an intercooler is generally a worthwhile investment.

Secondly, the lower the efficiency of the compressor, the higher the outlet air temp. However, it is difficult to accurately estimate the efficiency of the compressor and even if such a figure is available, it doesn't necessarily apply to all the different airflows that the compressor is capable of producing. In other words, there will be some combinations of airflow and boost pressure where the compressor is working at peak efficiency - and other areas where it isn't. While a well-matched compressor should be at peak efficiency most of the time, in some situations it will be working at less than optimum efficiency. This will change the outlet air temperature, usually for the worse.

Thirdly, the turbo- or supercharged car engine is not working in steady-state conditions. A typical forced induction road car might be on boost for only 5 per cent of the time, and even when it is on boost, it is perhaps for only 20 seconds at a stretch. Any decent forced induction road car will be travelling at well over 160 km/h if given 20 seconds of full boost from a standstill, meaning that longer periods of high boost occur only when hill-climbing, towing or driving at maximum speed. While all of the engine systems should be designed with the maximum full load capability in mind, in reality very few cars will ever experience this. This factor means that the heat-sink ability of the intake system must be considered.

If the inlet air temperature of the engine in cruise condition is 20°C above ambient, then on a 25° day the inlet air temp will be 45°C. After 30 minutes or so of running, all of the different components of the intake system will also have stabilised at around this temperature. If the engine then comes on boost and there is a sudden rise in the temp of the air being introduced to this system, the temperature of the turbo compressor cover (or blower housing), inlet duct, throttle body, plenum chamber, and inlet runners will all increase. These components increase in temp because they are removing heat from the intake air, limiting the magnitude of the initial rise in the actual intake air temperature. As a result, the infrequent short bursts of boost used in a typical road-driven forced-induction car often produce a lower initial intake air temperature than expected. This doesn't mean that intercooling is not worthwhile - it certainly is - but that the theory of the temperature increase doesn't always match reality.

Water/air intercooling is used less frequently than the air/air approach. However, it has several benefits, especially in cramped engine bays. A water/air intercooler uses a compact heat exchanger located under the bonnet and normally placed in-line with the compressor-to-throttle body path. The heat is transferred to water which is then pumped through a dedicated front-mounted radiator cooled by the airflow generated by the car's movement. A water/air intercooler system consists of these major parts: the heat exchanger, radiator, pump, control system, and plumbing.

Technically, a water/air intercooler has some distinct cooling advantages on road cars. Water has a much higher specific heat value than air. The 'specific heat value' figure shows how much energy a substance can absorb for each degree temp it rises by. A substance good at absorbing energy has a high specific heat value, while one that gets hot quickly has a low specific heat. Something with a high specific heat value can obviously absorb (and then later get rid of) lots of energy - good for cooling down the air.

Air has a specific heat value of 1.01 (at a constant pressure), while the figure for water is 4.18. In other words, for each increase in temp by one degree, the same mass of water can absorb some four times more energy than air. Or, there can be vastly less flow of water than air to get the same job done. Incidentally, note that pure water is best - its specific heat value is actually degraded by 6 per cent when 23 per cent anti-freeze is added! Other commonly-available fluids don't even come close to water's specific heat value.

The high specific heat value of water has a real advantage in its heat sinking affect. An air/water heat exchanger designed so that it has a reasonable volume of water within it can absorb a great deal of heat during a boost spike. Even before the water pump has a chance to transfer in cool water, the heat exchanger has absorbed considerable heat from the intake airstream. It's this characteristic that makes a water/air intercooling system as efficient in normal urban driving with the pump stopped as it is with it running! To explain, the water in the heat exchanger absorbs the heat from the boosted air, feeding it back into the airstream once the car is off boost and the intake air is cooler. I am not suggesting that you don't worry about fitting a water pump, but it is a reminder that in normal driving the intercooler works in a quite different way to how it needs to perform during sustained full throttle. However, the downside of this is once the water in the system has got hot (for example, after you've been driving and then parked for a while), it takes some time for the water to cool down once you again drive off.

[Russ]

The Heat Exchanger
Off the shelf water/air heat exchangers are much rarer than air/air types. Water/air intercooling has been used in cars produced by Lotus, Subaru and Toyota. A few aftermarket manufacturers also produce them. If you want to make your own, the easiest way to go about it is to jacket an air/air core. Pick an air/air intercooler that uses a fairly compact core that still flows well. If it uses cast alloy end tanks (as opposed to pressed sheet aluminium) then so much the better. (Plastic end tank types need not apply!) The core is then enclosed in 3mm aluminium sheet, TIG welded into place. Water attachment points can be made by welding alloy blocks to the sheet metal, with these blocks then drilled and tapped to take barbed hose fittings. Pressure-test the water jacket to make sure that it actually does seal, and make sure that the water flow from one hose fitting to the other can't bypass the core. Small baffles can be used to ensure that the water does fully circulate before exiting.

Another type of water/air heat exchanger can be made using a copper tube stack. These small heat exchangers are normally used to cool boat engine oil, exchanging the heat with engine coolant or river or seawater. While the complete unit uses a cast iron enclosure and so is too heavy and large for car applications, the core piece itself can be enclosed to make a very efficient heat exchanger. Comprising a whole series of small-bore copper tubes joining two endplates, the core is cylindrical in shape and relatively easy to package. The induction air flows through the tubes while a water-tight sheet metal jacket can be soldered around the cylinder. The resulting heat exchanger is a little like a steam engine boiler, with induction air instead of fire passing down the boiler tubes! The one here is shown installed on a car undergoing fuel pump testing.

As with air/air designs, the more efficient that you can make the heat exchanger, the better is the potential system performance. If you plan to use an off-the-shelf heat exchanger that has specifications available for it, you will be interested to know that the 150kW turbo Subaru Liberty (Legacy) RS uses a factory-fitted water/air exchanger that has a 4kW capacity. This heat exchanger also works quite effectively when power is increased to about 210kW. Remember in your design considerations that you want a reasonable store of water in the actual heat exchanger (2 or 3 litres at least) to help absorb the temperature spikes.

Radiator and Pump
The front-mounted radiator for the water/air intercooler should be completely separate to the engine cooling radiator. Some turbo trucks use the engine coolant to cool the water/air intercooler, but their efficiency is much reduced by taking this approach. Suitable radiators that can be used include large oil coolers, car air conditioning condenser cores, and scrap domestic air conditioning condensers. If you use a car airconditioning condenser there is likely to be available a small dedicated electric fan that attaches to the core easily. This fan can be triggered to aid cooling when the vehicle is stationary. The radiator should at least match (and preferably) exceed the cooling capacity of the heat exchanger, but again finding proper specifications is often difficult. The Subaru Liberty (Legacy) RS with the 4kW heat exchanger uses quite a small radiator, only 45 x 35 x 3cm.

An electric pump is the simplest way of circulating the water, with the type of pump chosen influenced by how the pump is to be operated. Some factory systems have the pump running at low speed continuously, switching to high speed at certain combinations of throttle position and engine airflow. If you follow a similar approach, the pump that is chosen must be capable of continuous operation. Another approach is to trigger the pump only when on boost, or to trigger a timing circuit that keeps the pump running for another (say) 30 seconds after the engine is off-boost. The latter type of operation will mean that the pump operating time is drastically reduced over continuous running.

Twelve volt water pumps fall into two basic types - impeller and diaphragm. An impeller pump is of the low pressure, high flow type. In operation it is quiet with low vibration levels. A diaphragm pump can develop much higher pressures but generally with lower flows. A diaphragm pump is noisy and must be rubber-mounted in a car.

Suitable impeller type pumps are used in boats as bilge pumps and for deck washing. They are relatively cheap and have very high flows - 30 litres a minute is common. However, they are not designed for continuous operation and generally don't have service kits available for the repair of any worn out parts. Diaphragm pumps are used to spray agricultural chemicals and to supply the pressurised water for use in boat and caravan showers and sinks. They are available in very durable designs suitable for continuous running and have repair kits available. Flows of up to 20 litres a minute are common and they develop enough pressure (45 psi) to push the water through the front mounted radiator and heat exchanger without any problems.

The factory water/air intercooler system in the Subaru Liberty RS uses an impeller-type pump rated at 15 litres a minute (all flow figures are open-flow). It is automatically switched from low to high speed as required. This is an ideal pump because it was designed by Subaru to circulate the water in a water/air intercooling system! However, it is a very expensive to buy new, but if one can be sourced secondhand it is ideal.

A cheap and simple impeller pump is the Whale GP99 electric pump. It is so small that the in-line pump can be supported by the hoses that connect to it. It flows 11 litres a minute and has 12mm hose fittings. It is 136 x 36mm in size and is suitable for discontinuous operation. This pump is available from marine and caravan suppliers.

The Flojet 4100-143 4000 is a diaphragm pump suitable for water/air intercooler use. The US-manufactured pump uses a permanent magnet brush-type fan-cooled motor with ball-bearings and is fully rebuildable. The pumping head uses four diaphragms which are flexed by a wobble plate attached to the motor's shaft. The 19 litre/minute pump uses ¾ inch fittings and is 230mm long and 86mm in diameter. It is available from companies supplying agricultural spray equipment.

The Flojet pump needs to be mounted either vertically with the pump head at the bottom, or horizontally with the vent slots in the head facing downwards. This is to stop any fluid draining into the motor if there are any sealing problems in the pump head. At its peak pressure of 280 kPa (40 psi), the pump can draw up to 14 amps; however, in intercooler operation the pressure is vastly less and so the pump draws only about 5.5 amps at 12 volts. The pump is noisy (as all diaphragm pumps are) but mounting it on a rubber gearbox crossmember mount effectively quietens it. Note that these pumps are much louder when mounted to the car's bodywork than they are when sitting on the bench!

[Russ]

Control Systems
As already indicated, there are a number of ways of controlling the pump operation. The simplest is to switch the pump on and off with a boost pressure switch. This means that whenever there is positive manifold pressure, the pump circulates the water from the heat exchanger through the radiator and back to the heat exchanger. If boost is used frequently and for only short periods, this approach works well. However, it is better if a timer circuit is used so that the pump continues to operate for a short period after boost is finished.

A suitable pressure switch is an adjustable Hobbs unit (pictured), available from auto instrument suppliers. However, this switch is relatively expensive and a cheaper unit is easily found. Spa bath suppliers stock a pressure-operated switch that is ideal for forced aspirated car use. The pressure switch is designed to work as part of the air-actuated switching system which is used in a spa bath so that bathers don't have to directly operate high voltage switches. The switch triggers at around 1 psi and costs about half that of a traditional automotive pressure switch. If a switching pressure above 1 psi is required, simply tee a variable bleed into the pressure line leading to the switch. Adjusting the amount of bleed will change the switch-on point.

Another approach to triggering pump operation is to use a throttle switch. A micro switch (available cheaply from electronics stores) can be used to turn on the pump whenever a throttle position over (say) half is reached. A cam can be cut from aluminium sheet and attached to the end of the throttle shaft. If shaped with care, it will turn on the switch gently and then keep it switched on at throttle positions greater than the switch-on opening throttle angle.

If a two-speed pump operation is required, the pump can be fed current through a dropping resistor to provide the slow speed. When full speed is required, the dropping resistor can be bypassed. Suitable dropping resistors are the ballast resistors used in older ignition systems or the resistor pack used in series with some injectors. The value of the resistor that is used will depend on the pump current and its other operating characteristics. In all cases, the resistor will need to dissipate quite a lot of power and so will need to