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With respect to automotive performance, the turbocharger is one of humankind's great inventions. When properly sized for the application and used with the correct supporting components, a turbocharger provides a massive increase in power without overly complex changes to packaging. Since air quantity determines engine power (the physical amount of air the engine can ingest), using a turbocharger to pack more air into the engine's inlet tract is one method for increasing power without increasing engine displacement and corresponding physical dimensions.
So let's talk relevant turbocharger terms, what they mean, how each component and dimension affects the others, and more.
Turbochargers are relatively simple devices. A turbocharger harnesses otherwise unused exhaust energy from the engine to spin a turbine wheel contained within a turbine housing.
The turbocharger's configuration differs from a centrifugal supercharger; a centrifugal supercharger uses only half of the turbocharger — the compressor stage — driven from the crankshaft through a belt or gear drive.
A turbocharged engine's exhaust gases exit the engine and feed into the large opening of the turbine housing. As the gases move through the housing — which gets progressively smaller to keep velocity up — the energy spins the turbine wheel. It exits through the turbine housing outlet and into the vehicle's exhaust system.
On the opposite side of the turbocharger, the compressor wheel connects to the turbine wheel through a common shaft. The compressor wheel is contained in the compressor housing and sources its air supply externally to feed cool, fresh air into the engine. Air enters the large hole in the center of the compressor housing and is sucked through the compressor wheel's curved blades, exiting at the wheel's outer edge into the compressor housing. The housing's cross-section starts smaller and becomes progressively larger as it nears the housing outlet to help compress the air and increase its density. The compressed air is then typically routed through an intercooler to remove heat from the air before entering the engine.
Add the appropriate amount of fuel to support the extra air forced into the engine, and power increases substantially.
You can think of a turbocharger as a pair of opposed yet connected waterwheels. One side is the driven side, and the other side is the drive side, and the two sides never intermingle. To recap turbocharger operation, its compressor wheel draws air in through an air filter. Air pressure is increased as it passes through the compressor wheel and housing, then through the intercooler to remove heat and increase the air density, and then into the intake manifold and engine cylinders. The cylinders move through a fixed swept volume, but the engine is ingesting a higher mass flow rate of air due to the turbocharger's influence.
That's the simple explanation for how a turbocharger operates.
Now that we have the basics out of the way concerning how a turbocharger functions, let's dig a bit deeper into the components and determining factors of turbocharger choice. This high-level explanation provides enough information to understand the various parts of a turbocharger better, how they function, and how they affect component operation in different situations. A complete explanation of turbocharger selection criteria could fill a healthy-sized book.
Turbochargers are rated by airflow capability; the larger the turbine housing, the more air the turbocharger can supply the engine when under full boost. But that increased airflow capability comes with a cost — reduced backpressure (exhaust pressure) against the turbine wheel since the air can pass through the wheel more easily. Larger turbines are also heavier, which means they take more exhaust flow to build up to full boost.
Turbocharger flange sizes are a critical component of the sizing process. The traditional turbocharger flange sizes used with V8 automotive applications are T3, T4, T5, and T6. These "T" size classifications offer a rough estimate for turbocharger performance. The larger the number, the bigger the turbo. More importantly, the T designation specifies the turbine inlet flange dimension.
In general, T3 turbochargers are used on smaller engines — 200 to 300 horsepower — but there are many T3 turbochargers rated for various horsepower outputs, and 300 horsepower is certainly not a top-side limit.
Additionally, there is a T3/T4 hybrid turbocharger designation. It uses the exhaust turbine and housing of a T3 turbocharger (to work well with small-displacement engines, 2.0 to 3.0-liters) and the compressor wheel and housing of a T4 turbocharger to provide more airflow and support somewhere around 300-550 horsepower.
T4-style turbochargers fall into the 400 to 800 horsepower range and have a larger turbine housing flange to support the increased airflow. Like with T3 turbos, 800 horsepower is not a guaranteed top-side limit as there are many racing programs using T4 turbos and making substantially more horsepower than that figure.
T5 and T6 turbos are yet larger than T4 turbos, with corresponding performance capabilities well into the thousands of horsepower per turbocharger.
Ball-bearing center-housing turbos do away with the traditional journal bearing and help massively reduce friction and spool time while improving maximum turbocharger RPM capability under full boost. Ball-bearing turbochargers spin approximately 30-percent faster than a journal-bearing turbo and offer a corresponding increase in airflow — which means that a small turbocharger can be used. A turbocharger with a ball-bearing CHRA (center housing rotating assembly) compares favorably to a T4-style journal bearing CHRA when used in the same application, as it offers reduced spool time with comparable airflow.
Two slightly confusing terms are inducer and exducer. On the compressor wheel, the inducer is the circumference of the blade where air enters the turbocharger inlet. The exducer is the circumference of the blade where it "throws" the air out of the compressor wheel into the compressor housing after compressing it.
But because the air flows in the opposite fashion on the turbocharger's exhaust side, the inducer is the larger diameter of the turbine wheel, where the air enters the wheel from the exhaust manifolds. The exducer is the smaller part of the wheel, where the air exits the exhaust housing into the exhaust pipe after its energy is used to provide drive pressure.
The A/R (or area/radius) is the last critical term to understand when it comes to turbocharger selection. According to Garrett Motion, the technical definition is: "the inlet (or, for compressor housings, the discharge) cross-sectional area divided by the radius from the turbo centerline to the centroid of that area."
What this means to the average turbocharger enthusiast: exhaust housing A/R matters big-time when it comes to optimized turbocharger performance. A smaller A/R will increase exhaust gas velocity into the turbine, and a larger A/R will reduce air velocity. Smaller A/R housings help drive the turbine more quickly at lower engine speed, reduce the time needed to achieve full boost from a given compressor, reduce ultimate flow capacity, increase backpressure, and drop peak engine power. The smaller A/R housing will improve low-end torque and provide a vehicle with better performance on the street. In contrast, a housing with a larger A/R will typically be more effective under racing conditions where the engine is operated high in its powerband.
A/R adjustment does not have nearly as much of an effect on compressor housing performance, and as a result, is not typically considered. The compressor housing is generally optimized for A/R during the design phase.
This is an example of a turbocharger compressor map. Image courtesy: BorgWarner
The compressor map is one of the essential pieces of information a user can have related to a turbocharger. The compressor map details the compressor's performance, which helps the buyer to choose a particular turbocharger for an engine combination.
Every compressor map has two axis. The bottom axis is the corrected mass flow, which shows how much air the turbocharger flows in units of time, which could be pounds per minute or kilograms per second, depending on the manufacturer. A good rule of thumb is that every pound of air generated equals around 10 horsepower at the flywheel.
The vertical axis is the pressure ratio of the compressor, which is calculated by dividing the absolute outlet pressure by the absolute inlet pressure; put in layman's terms, it's the amount of boost you intend to run.
The third key points on the compressor map are the efficiency islands; the smallest one, in the center, represents where a turbo is most efficient, and the graph moves outward, the efficiency drops. Think of the efficiency islands as the uppermost point on a topographical map, and as you work outward from that point, the efficiency falls off as a hill would slope downward.
These measurements vary from turbo to turbo and compressor wheel to compressor wheel, so it is helpful to work hand-in-hand with your turbo provider to understand which turbocharger is best for you and why.
At certain points, turbocharged engines need to relieve pressure in the intake tract to prevent compressor surge. Surge happens when boost pressure is high, and airflow demand is low — such as when the driver lets off the throttle after a burnout on the dragstrip. Put simply, compressor surge is bad for a turbocharger as the air tries to back up through the compressor housing and can damage the turbocharger's internals if it is severe enough. A blowoff valve, sometimes called a bypass valve, will prevent surge as it relieves the excess air pressure and vents it to the atmosphere (or in some cases recirculates) before it affects the turbocharger.
Similarly, a wastegate offers the ability to control boost on the exhaust side of the turbocharger. Wastegates exist are used to control the exhaust flow to the turbocharger to achieve the desired boost level.
There are two different styles of wastegate: internal and external. An internal wastegate is built into the turbine housing and utilizes just a couple of simple components — a valve, crank arm, rod, and pneumatic actuator. The actuator, which includes a spring inside a sealed chamber, is connected to boost pressure. The spring rate is specified to work with the turbocharger to ensure proper operation. As boost overcomes the spring rate, the wastegate opens and vents the excess exhaust pressure to slow the turbo down and reduce boost pressure.
An external wastegate is plumbed into the exhaust system and is self-contained. Racing applications often require external wastegates; as such, they have larger springs, more robust actuator diaphragms, and larger inlets and outlets to handle the increased boost pressures these engines regularly see. External wastegates offer many advantages to internal-wastegate turbochargers, such as improved heat management, much higher flow, adjustability, liquid cooling, electronic actuation, and more. Typically, an external wastegate references to manifold pressure, but this has drawbacks.
An electronic boost controller can also be used in conjunction with a solenoid or solenoid pack. When all of the boost is allowed to progress through the solenoid to the wastegate, this is considered running "spring pressure," or the amount of boost that it takes to overcome the spring inside the wastegate. For example a "10lb spring" would, in theory generate 10psi of boost. Springs are offered in many rates and should be tailored to the combination.
Electronic boost controllers can also be used to direct boost pressure to the opposite side of a wategates diaphram to allow the turbo to produce more boost pressure than the wastegate spring would typically allow. This amount usually maxes out at about double the rated spring pressure.
In racing applications where even more boost pressure and precise control is required, CO2 is the solution. By adding an onboard tank, the user can monitor the engine's manifold pressure and control it by holding the wastegate closed with CO2 pressure to decrease spool time and add boost pressure. To open the wastegate the CO2 pressure in the wastegate is vented.
The science of turbochargers could fill thousands of pages. The topics covered here should provide a basic understanding of the individual pieces and terms needed to grasp what a turbocharger does and how.
Ultimately, the goal of an appropriately sized turbocharger is to raise an engine's efficiency without creating too many drawbacks in the form of unwanted heat or packaging complexity. Selecting the proper turbocharger for an application has roots based in science and mathematics and is often best left to the experts. While similar to another, an engines may have specific operating parameters or other minuscule differences that would require a different compressor or turbine housing, or compressor or turbine wheel — or both.
Compressor map: The compressor map is a document that provides a window into the turbocharger's characteristics, including boost range and airflow potential. It also shows whether the wheel is too big for the application. To verify whether a compressor will work properly on a specific engine, the user needs basic information such as expected engine power and the desired boost pressure. The left axis details the pressure ratio, which shows how much pressure the compressor can generate at a given speed relative to atmospheric pressure. The bottom axis demonstrates how much air the turbocharger can flow per unit of time. Think of the compressor map in the same way as you would a spec card for a camshaft.
Compressor wheel: The compressor wheel's curvature helps to scoop up the incoming air and force it into the compressor housing. There are several different types of compressor wheels and materials, from cast wheels to billet. Different wheels may have a different number of blades and blade shapes depending upon the application.
Compressor Housing: The compressor housing can be outfitted with different inlets, from a standard hose inlet secured with a T-bolt-style clamp to a V-band-style or other type of positive locking connection designed to keep the pipe firmly attached under boost.
Turbine Wheel: The turbine wheel's shape maximizes velocity without choking the exhaust from the engine and not being too hard or soft when it comes to a smooth, drivable powerband. It is a delicate balance for the engineer to get the wheel just right.
Turbine Housing: Turbine housings can be swapped on the turbocharger to affect its characteristics. For example, a housing with a smaller area/ratio (A/R) helps boost low-speed performance, while a housing with a larger A/R provides top-end power at the cost of slower low-speed engine responsiveness.
Turbo flanges: Several different sizes of turbocharger flanges exist. All dimensions feature rounded corners.
• T3: 58.49mm x 45.25mm
• T3 twin-scroll: 29.38mm x 47.5mm (x2)
• T4: 75.2mm x 49.2mm
• T4 twin-scroll: 50mm x 34mm (x2)
• T5 twin-scroll: 51mm x 34mm (x2)
• T6: 89mm x 59mm
Divided turbo flange: The twin-scroll flange must be paired with a twin-scroll exhaust housing to maximize performance. These systems use an exhaust manifold designed to provide even exhaust pulses into the turbocharger to maximize the energy delivered to the turbine wheel. For example, a typical four-cylinder engine would fire 1-3-4-2, and pulses 1 and 3 would be directed into one scroll with cylinders 4 and 2 run into the other scroll. Twin-scroll turbocharger systems are typically used on smaller-displacement engines to provide quicker boost response and low-end performance.
Center housing rotating assembly: Typically, a rotating assembly uses one of two different types of bearings: a journal bearing or a ball-bearing system. The ball-bearing system is far more efficient but also more costly.
Wastegate: A wastegate is used to control boost pressure on the hot side (turbocharger side) of the system. Whether internal (inside the turbo housing) or externally plumbed, the wastegate is a critical component that keeps the turbo running in its efficiency range without going into an overboost or underboost situation.
Blowoff valve: Similarly, a blowoff valve is used to vent the compressor side of the system. If the driver lets up on the throttle, the blowoff valve will open, so the unused boost pressure does not flow back into the turbocharger and cause compressor surge.
