Basic Turbocharging Components & Theory - Turbocharging 101
An introduction to basic turbocharging components and theory
From the May, 2011 issue of GM High-Tech Performance
By Justin Cesler
Photography by Courtesy of the Manufacturers, Justin Cesler
Look, all technical mumbo-jumbo aside, turbocharging is actually a pretty simple concept. The objective here is to convert the energy contained in your exhaust stream, which would normally go to waste, into positive pressure within the intake manifold, forcing air into the engine and thus producing more power. Now, we understand this is a lot to cover--enough to write a book on--but the goal of this particular article is to get everyone, including readers who haven’t ever seen a turbo before, up to speed on the concepts involved. To put it bluntly, this is Turbochargers 101-A and covers the very tip of the iceberg, from 1,000 feet away. In this first article, we hope to establish a baseline vocabulary and a working knowledge with which to build off in the future, so if you’re an advanced turbo guru who is looking for tips reading compressor maps or tweaking turbine housings for your exact application, fear not--those stories are yet to come. For now, we’re going to cover the basics of turbocharging by looking at each component, defining its purpose, and explaining the theory behind its operation.
At the most basic level, a turbocharger consists of just three major components: the turbine, the compressor, and the bearing system that supports the turbine shaft, connecting the turbine and compressor wheels together. Understanding how all three parts work together is crucial, and even a basic understanding of the component’s relationships to one another will make selecting a turbo for your project much easier.
The turbine wheel is responsible for converting heat and pressure into rotational force. To understand how this process occurs, we would need to delve into some of the basic laws of thermodynamics, but within the scope of this article, understand that high pressure (from the exhaust manifold) will always seek low pressure and, within this process, the turbine wheel converts kinetic energy into rotation. As the turbine wheel rotates, it spins the turbine shaft, which in turn spins the compressor wheel. Often overlooked, turbine wheel selection is critical to a properly built turbocharger system, as having too small of a turbine wheel will induce excessive backpressure and can choke the engine, making it lose power. On the other hand, selecting too large of a turbine will result in increased lag and can make it difficult to achieve specific target boost numbers.
Of course, the turbine wheel doesn’t act alone. It is part of the turbine housing, which is that giant, sometimes rusty looking piece of iron or steel you always see bolted to an exhaust manifold or merge collector on a turbo car. Because of the tremendous heat involved in collecting and moving pressurized exhaust gasses, the turbine housing is constructed from thick iron or steel and always consists of a turbine foot (the flange which connects to the exhaust manifold piping), an outlet connection (the large opening that connects to the downpipe), and a volute, which is the path the hot exhaust travels to get across the turbine wheel, from the turbine foot to the outlet. When someone calls a turbo a "T4 turbo," they are talking about this flange. Exhaust enters from the flange, rotates around the wheel inside the volute, and exits across the outlet connection, into a piece of exhaust that enthusiasts call the downpipe.
Like the turbine, the compressor section is made up of two primary components: the compressor wheel and the compressor cover. The compressor’s job is to, quite literally, compress fresh air and funnel it towards the throttle body. Since it is connected directly to the turbine wheel via the turbine shaft, the compressor wheel rotates at the same RPM as the turbine wheel and, as the engine and turbine wheel accelerate, so does the compressor wheel. This process creates pressure in the intake tract, which we call "boost" and is the reason anyone would install a turbocharger in the first place. Again, to fully understand this process, we would need to explain several laws of thermodynamics, including the ideal gas law, but for our purpose, understand that a compressor wheel’s job is to gather fresh air and compress it--simple as that. As the wheel spins, it takes ambient air, rotates it 90 degrees along the blade of the wheel, and forces it into the compressor cover, where it is collected and then forced into the intake tube.
Compressor wheels are one of the most commonly talked about parts of a turbocharger. Even if you haven’t ever seen a turbo before, you have probably heard someone say "it’s an 88mm turbo" or "I can’t believe they outlawed the 116." What we are talking about is the compressor wheel diameter, measured at the tip or, more accurately, measured at the tip of the inducer. The compressor wheel and cover are also the most photogenic parts of a turbocharger since they are made of shiny aluminum, and, consequently, people enjoy taking pictures of them with dollar bills, Coke cans, or various other items, to show how large the compressor wheel actually is. Now, all fun aside, it is important to understand the compressor is the money maker in this system and it is the one part of the turbocharger that does all of the pumping, so it is important to size it correctly for your application.
Center Housing / Rotating Assembly (CHRA)
The CHRA may not get a lot of ink time, but it is one of the most critical parts of any turbocharger assembly. Practically, the CHRA serves as the mounting point for both housings and must be made of substantial material to handle the heat and stress of the turbine. Of course, holding the housings together is child’s play compared to the real job of the CHRA, which is to support and lubricate the turbocharger’s bearings. With turbine shaft speeds in excess of 100,000 rpm, the bearing’s job is much, much more difficult than that of a traditional camshaft bearing, and as such turbo manufacturers have spent a lot of time and money building serious bearings to handle these jobs. If you have ever heard of someone "rebuilding a turbo," they are most likely talking about replacing the bearings, which can start to wear based on a variety of factors including oil condition, axial loads, or shaft movement. Traditionally, a CHRA will house two bronze full-floater bearings and a separate bronze thrust bearing. Today, many quality manufacturers offer upgraded bearing systems, including the Turbonetics ceramic ball-bearing unit, which eliminates the traditional thrust bearing allowing the turbo to withstand "up to 50 times the thrust load capacity, compared to a conventional unit." Many other manufacturers have also upgraded to ball bearings systems, including Garrett, to help reduce drag and increase a turbocharger’s life.
Understanding that a turbocharger works by compressing air, it is easy to see why an intercooler is important. Without jumping into too much math (we’re talking about the ideal gas law again...), let’s just say that as pressure increases within a fixed volume, heat is created. This is a law of thermodynamics and, no matter what anyone may argue, is present in any turbocharged engine application, even on "low boost" settings. Anyway, knowing that heat is present, we need a way to cool the incoming air charge before allowing it to enter the intake manifold, and for that we commonly use an intercooler. In reality, an intercooler is nothing more than a heat exchanger, and its job is to remove heat from the intake charge that we created by compressing it. If you understand how a radiator works, you understand how an intercooler works--it really is that easy!
In today’s performance market, two types of intercoolers prevail: air-to-air and air-to-water. An air-to-air intercooler is probably the most common on street cars, and you have probably seen them hanging out behind the bumper of some of your favorite GMHTP feature cars. Just like a radiator, an air-to-air intercooler works by passing hot air from the compressor through a series of tubes, which are physically connected to a series of thin aluminum fins. As ambient air passes over the intercooler surface and the thin fins as it draws heat away from the compressed air, which provides a cooling effect. On typical street cars, which drive for extended periods of time, an air-to-air intercooler is one of the most efficient ways to keep charge temperatures under control. An air-to-water intercooler, on the other hand, uses similar principals to an air-to-air unit, although instead of ambient air passing over the surface, it uses chilled water, which allows for an incredible amount of cooling capacity. However, what an air-to-water system gains in temperature drop and efficiency, it gives up in time, as the water will eventually heat up and provide much less cooling.
A wastegate is simply a device that bleeds off exhaust gas before it reaches the inlet of the turbine housing. To fully understand the concept, let’s look at a turbo system without a wastegate. As exhaust fills the manifolds, it is directed toward the turbocharger and enters the turbine housing before expanding across the turbine wheel and exiting through the downpipe. In a closed system, the turbine would see all of the exhaust throughout the engine’s operating range and boost would continue to rise, uncontrollably, until either the throttle was shut or the turbine wheel reached its choke point. For most engines, this would create an excessive amount of boost/airflow and destroy parts, leaving you with a couple of melted pistons at best or a giant hole in the block (much more likely). To control boost and overall engine power, turbocharger systems rely on wastegates, which are mounted before the turbine housing (or inside of it in the case of an internally-gated turbo), and act as a controlled bypass for a percentage of exhaust gas to regulate turbine speed and, thus, overall boost.
Wastegate design varies, but in the most simplistic terms, every wastegate features an inlet and outlet port to which exhaust gas may enter, a valve that regulates the flow of exhaust gas through the inlet port and a spring/diaphragm actuator, which controls when the valve opens and closes. Under normal driving conditions, the wastegate valve remains closed, and all exhaust gas is sent directly to the turbine housing. As boost pressure rises, pressure acts upon the spring assembly and begins to lift the valve, diverting exhaust flow away from the turbine and controlling turbine speed to regulate boost pressure. In order to adjust target boost levels, wastegates rely on different springs, which can be swapped in order to increase or decrease target boost pressure.
A blow-off valve is essentially a pressure relief valve, which is mounted on the compressor side of a turbo system. Its job, quite literally, is to blow off excess boost pressure trapped in the system when the throttle blade closes. Imagine a turbo producing 10 psi, with piping connecting the outlet of the compressor cover directly to the throttle body. With the throttle blade wide open and the engine under a full load, the compressed air has a straight path into the intake manifold and can fill the cylinders easily. As the driver releases (lifts) the gas pedal and closes the throttle blade, the turbo is still spinning and producing boost (remember, the compressor wheel can spin at upwards of 150,000 rpm!), which creates an undesirable condition within the system. The turbo is moving a lot of air, but since the throttle is closed, the air has nowhere to go, except back towards the compressor wheel, which can result in compressor surge. Compressor surge can damage the turbocharger by placing excessive load on the bearing surfaces and, in extreme cases, can even stall the compressor wheel.
A blow-off valve is, in construction, similar to a wastegate, although normally smaller and built with much less tolerance to high heat, since it is mounted on the compressor side of the turbo piping. Under normal operating conditions, the actual valve is closed against the seat and air is trapped in the compressor charge piping. When the throttle is closed, the blow-off valve spring/diaphragm sees a change in pressure (from above atmospheric to vacuum) and the valve opens, releasing the compressed air out of the charge pipe and into the atmosphere. Unlike a wastegate, most blow-off valves ship with one preset spring and tuning of the valve’s opening speed is done via small adjustments to the spring’s preload. Note that a blow-off valve’s boost reference source must be located after the throttle body, within the intake manifold, so that it can accurately read vacuum when the throttle blade is shut.
Piping may be the last thing most enthusiasts consider when building a turbo system, but proper application and sizing are essential to ensure optimal performance. On a typical turbocharger system, piping can be broken into three distinct sections: the manifolds, the hot side, and the cold side.
Turbo manifolds live incredibly difficult lives. Extreme temperature changes, incredible backpressure, and high stress make these one of the most likely areas in a turbo system to develop issues. Understanding the extremes that a manifold must endure, day in and day out, it is best to develop a manifold based on longevity and strength, even if it means giving up a tiny bit of performance. Also, knowing that the turbine wheel works off of heat and velocity, one must build a manifold to carry heat efficiently and quickly, keeping as much heat inside as possible without developing cracks or slowing the exhaust gas’ momentum. As such, cast-iron manifolds should be considered if available and as the LSX racers have seen, even stock units like a pair of GM truck manifolds can produce well over 2,000 hp in stock form. If such a manifold doesn’t exist for your application or you are working in a specific space that can’t accommodate them, fabricating a pair of manifolds will be your best option and you can turn to many excellent fabricators to complete this job.
Any piping related to moving actual exhaust gas, whether to the turbocharger or away from it, is typically referred to as hot side piping. Because of the extreme heat involved in transferring exhaust to the turbine housing, it is critical to use a strong material here, and for many fabricators stainless steel is the material of choice. As far as diameter is concerned, that really depends on a variety of factors including cubic inches, turbine wheel design, RPM range, backpressure, etc., but as a rule of thumb 2.5-inch inner diameter (I.D.) tubing from the exhaust manifolds to the turbine housing works very well. Of note, some builders are now going to smaller pipe, if possible, to increase velocity to the turbine, which should work well, although the results will vary depending on the specific application. As air exits the turbine wheel, it enters into a section of exhaust known as the downpipe, and here, the bigger the better. You can’t really oversize the downpipe, which means if you have room for a 4- or 5-inch downpipe, go for it!
The "cold side" of a turbo kit refers to any piping related to moving compressed air from the turbocharger to the throttle body. If you are installing an intercooler, it is also part of the cold side and will need to be plumbed correctly to make everything work. Because heat isn’t as much of a concern, aluminum tubing is generally regarded as the optimal choice, as it is easy to work with, lightweight, and strong enough to withstand the relatively mild temperatures associated with the cold side. Piping diameter varies by turbo, intercooler, and throttle body sizing, although most GM enthusiasts will find 3-inch I.D aluminum tubing works perfectly. Any area where a semi-permanent connection must be made, such as connecting a section of 3-inch tube to an intercooler end tank, can be done using high quality silicone couplers and traditional clamps, which work well for a majority of applications. For those of you looking to make a large amount of boost, companies like Vibrant Performance offer double O-ringed quick disconnect clamps, which can handle 100-plus pounds of boost without blowing off or leaking.
What else do I need to know?
A lot. Seriously, understanding turbo systems isn’t something that can be done overnight and, like engine building or suspension setup, it can take years to properly understand all of the nuances of turbo design. But that doesn’t mean you shouldn’t start learning and exploring this fascinating form of forced induction right now! If you’re interested in learning more today, we recommend you check out two excellent books that we keep on hand at all times. The first is a classic by Corky Bell, titled Maximum Boost and it covers system design from theory to real world application, without becoming overly tech heavy or scientific. The second book we recommend is Turbo: Real World High-Performance Turbocharger Systems by Jay K. Miller. Turbo has an excellent section on turbocharger anatomy and delves into topics such as compressor maps and turbocharger rebuilding, for those of you looking to really expand your working knowledge. Last but not least, we invite you to follow along with us in the upcoming months, as we team up with some of the best in the industry to fabricate and install a single turbo system on our newest project car
Bell, Corky. Maximum Boost.
Cambridge, MA.: Bentley Publishers, 1997
Miller, Jay. Turbo.
North Branch, MN.: Cartech Books, 2008