Forced induction is the only way to get something like a 2.0L, four-cylinder engine to puff in enough air to behave more like a V-8 twice its size or to get that V-8 to bang out the sort of horsepower legends are made of. Out of the three distinct methods of forced induction, though—turbocharging, supercharging, and nitrous oxide injection—none are as capable as the turbocharger. Its components are many, its tuning demands strict, and its ability to play well with the rest of your engine is vital to whether or not power is quadrupled, for example, or one of your connecting rods decides to take a hike through the front of your block.
VOLUMETRIC EFFICIENCY AND AIR DENSITY
Understand complicated engine dynamics like volumetric efficiency and air density just a little bit and you'll see why turbos work. Naturally aspirated engines rely on how well their internal pumping capabilities are up to snuff as their rotating assemblies spin about, inhaling air equal to whatever Earth's atmospheric pressure says it can. In between all of those air particles, though, is all kinds of wasted space. Forced induction systems like turbos compress all of that nonsense together, filling those voids with even more air. So long as you've figured out a way to add more fuel, you're on you're way to making more power.
TURBOCHARGING VS. SUPERCHARGING
Like superchargers, turbos make more power by compressing air, allowing more of it inside an engine's cylinders at any given time. They do all of this in entirely different ways, though. A turbo relies on an engine's exhaust gases to spin its turbine, which, through a common shaft, spins an opposing wheel that compresses incoming air and does everything you care about. Superchargers, however, aren't driven by exhaust gases at all, but instead by a belt that's linked to the crankshaft pulley. Here, there are all sorts of shortcomings, like the parasitic power losses that are associated with asking the crank to drive another device as well as the fact that, unlike turbos, manipulating boost on a supercharged engine requires pulley swaps and, even then, can be limiting. But turbos aren't perfect, either. Strapping one onto your engine will generate a whole lot more heat than anything else, which means detonation is more likely to occur, and tuning just got a whole lot more complicated.
As far as efficiency goes, no other form of forced induction compares to the turbo, though—and that includes those blown Top Fuel dragsters you're thinking about that generate more than 8,000 hp. Take mid-1980s, turbocharged Formula One engines, for instance, that produced a whole lot less power but from smaller powerplants that resulted in some of the most impressive specific outputs in automotive lore. Your Audi will never experience anything close to those same sort of power-to-cubic-inch ratios, but the advantages of a proper turbo system mean the same thing for your 2.0L as it does to guys with the multimillion-dollar race car program: more power, more efficiently than anything else.
HOW A TURBO SYSTEM WORKS
Turbocharging any engine requires more than just a turbo. Here, the number of additional components are many and their jobs complex. It starts with the exhaust manifold, which interfaces between the engine's exhaust ports and the turbo, giving it a place to hang from. If you've got a V-type engine, you'll need two of them. Every turbo system also needs some sort of wastegate or bypass valve, which redirects exhaust gases away from the turbo to regulate boost pressure. These can be externally mounted, which is typical of high-performance applications that've got to bypass a lot of exhaust gases, or internally, and remain a part of the turbo itself. Piping also needs to span from the turbo to the throttle body and, in most cases, will include some sort of intercooling system in between to help prevent detonation. A blow-off valve is another component of any well-thought-out turbo system, which, similar to the wastegate, relieves pressure, only here it does so on the high-pressure intake side, preventing damage to the turbo and improving throttle response. Most turbo systems also require some sort of boost management system as well as upgrades to the engine and fuel system to account for all of that additional cylinder pressure.
Sometimes, a single turbo just isn't enough. Multiple turbos can be integrated in two ways: in parallel or sequentially. Parallel turbo layouts are the simpler of the two and make use of two equally sized turbos that deliver the same amount of air at the same time. Sequential turbo systems, on the other hand, allow one turbo to do its job first and the other to join in at higher engine speeds. Generally, a smaller turbo precedes a larger one, however, some applications make use of two equal-sized compressors and turbines. Sequential turbo systems are also able to yield a slightly broader power range when compared to parallel or even single-turbo configurations. That's mostly because of the smaller turbo that typically comes online first and, through a series of wastegates and bypass valves, the second, larger unit joins in later, resulting in the kind of mid-range torque and high-end power output good stories are made of.
THE AIRFLOW PATH
Every turbo is made up of a compressor housing and compressor wheel on one side and a turbine housing and turbine wheel on the other. A center cartridge sits in between both housings along with a common shaft located inside that's supported by a system of bearings that connects to both wheels. The two snail-shaped housings haven't changed much over time and are really just a place to store the pair of wheels, made up of inlets and outlets that draw in and direct air to the right places.
Fire up your engine and air enters the turbo's compressor-side inlet where the already-spinning exhaust wheel on the other side allows the compressor wheel to do the same thanks to that common shaft they share. It'll take you applying your foot to the throttle for anything beyond near-atmospheric pressure to come out of the compressor's outlet and into your engine, but the airflow path won't change.
After making its way out of the compressor, the air charge moves through a series of pipes, through an intercooler if you're smart, past the throttle plate(s), and into the intake manifold. In the meantime, whatever exhaust gases the cylinder head's been spitting out continue to fill the turbine housing, creating a whole lot of backpressure, all kinds of heat, and, because of the housing's unique shape, the wherewithal to keep that turbine wheel spinning and repeat the cycle. Stab the gas pedal, watch the tach's needle rise, and expect the compressor's ability to generate more airflow to go up.
This all sounds like a dream come true, where otherwise useless exhaust gases are all of a sudden put to work to make a whole lot more power, but it isn't entirely efficient. The turbine assembly itself, for example, sits right in the middle of the exhaust path, restricting airflow and increasing backpressure on the exhaust side. Consider the effects of cranking up the boost, though, and whatever trade-offs you can imagine you'll soon forget about.
THE CENTER CARTRIDGE
In between both housings you'll find the center cartridge, which carries the load of the common shaft and supports its bearing assembly. An oil stream directed from the engine block lubricates all of this, making sure temperatures and friction remain at bay as the whole assembly approaches rotational speeds as high as 300,000 rpm. Some center cartridges also include inlets and outlets that may be integrated into the engine's cooling system, which can help regulate internal temperatures.
Like your engine's crank, a turbo's common shaft has to be supported by a series of bearings. Here, journal bearings like you'd find inside your engine block are most common, however, higher-end applications are often made up of more sophisticated and longer-lasting ball bearing assemblies, sort of like what you'd find inside of a skateboard wheel. Both kinds are located at each end of the shaft, supporting it all despite whatever radial, thrust, and axial loads it's subjected to, which can be a lot depending on the circumstances. To be sure, even larger, slower-spinning turbos typically suited for heavy-duty applications still spin as much as 10 times faster than an engine's crank ever would.
CHOOSING THE RIGHT TURBO
Before doing anything, you've got to have a target horsepower range in mind. But be honest with yourself; going with the biggest turbo you can find in hopes of impressing whomever might look underneath your hood will never be a good idea. Don't lose sight of what your car will primarily be used for, how much traction you'll be able to muster up, and whether or not your engine and driveline will be able to handle it. A little bit of honesty might reveal that now, all of a sudden, a smaller-framed, quicker-spooling turbo could actually make your car a whole lot quicker than something bigger that won't do a whole lot more than spool up at seven-grand and roast a perfectly good set of tires.
Right about now you might be wondering how much boost you ought to be pushing through whatever turbo you're considering. Stop wondering. Boost pressure matters a whole lot less than the Internet would lead you to believe. Instead, concern yourself with horsepower and airflow. It's true that lower boost pressures mean whatever turbo you've got will generate less heat and not work as hard, but all of this is of little consequence to your engine, which will decide whether or not it wants to blow itself to smithereens or make a whole lot of power based on cylinder pressure, not boost.
Speaking of heat, when selecting compressor and turbine housings, choose ones that can pump the most air into the cylinders but without raising temperatures beyond what complicated thermodynamics laws say they should. Fancy-looking compressor maps are needed to reveal a given turbo's efficiency, surge limit, boost potential, and shaft speed. The labyrinth of considerations doesn't end there, either. You've also got to look at sophisticated things like pressure ratios, compressor surge, trims, and A/R ratios, which we'll get to in a bit.
Now's also a good time to mention oversized compressors and turbo lag. In short, there's little correlation between the two. Lag is mostly associated with the speed at which the shaft spins, which is determined by the turbine wheel. But that doesn't mean there aren't consequences of an oversized compressor. As size goes up, efficiency drops and heat rises. Yes, 10 psi will always be 10 psi, no matter the size of the turbo, but air quality can differ between two turbos, as will power. As efficiency drops so does air density, in turn, yielding less air volume for the combustion chambers to do anything with.
GETTING THE COMPRESSOR RIGHT
Choosing the right compressor is the most important part of turbo sizing and the most often bungled. Here, efficiency rules, not size. Look for something that'll pump the most air into your engine's cylinders but do so as efficiently as possible; you'll need to review a proposed turbo's compressor map to verify any of this with certainty. Most turbo manufacturers provide these graphs and charts, which make choosing the right compressor a lot easier. Before you go reading a turbo's compressor map like you know what you're doing, you've got to know two things: your engine's proposed boost pressure ratio and its airflow rate.
Figuring out an engine's boost pressure ratio isn't hard, but you'll need to set your ego aside for a minute. The calculation is simple: Divide the absolute outlet pressure that you think you want (14.7 + boost pressure) by the absolute inlet pressure that the Earth says you can have (14.7) and you've got your pressure ratio. Limiting yourself to a reasonable number is the hardest part. Start with a realistic boost pressure, like, say, 10 psi for an engine that started out naturally aspirated and play around with larger numbers for bigger-power, track-only configurations. Also, if you aren't near sea level, then you'll need to determine the appropriate absolute inlet pressure because it won't be 14.7 psi.
Knowing your engine's airflow rate isn't as simple and isn't up for debate. It is whatever Volkswagen and physics say it is. Airflow tells you how much air is entering your engine for a given period of time and can be quantified at a given engine speed by factoring in displacement and volumetric efficiency. Come up with this number and, with an honest pressure ratio and the right compressor map, you'll have a hard time landing a turbo that doesn't stink.
As it turns out, though, there may be more than one turbo that won't stink. Narrow it down to one by matching a given compressor's maximum efficiency point to the most useful part of your engine's rpm range, which is typically where torque peaks. If you're like most people and are looking for proper midrange response and good top-end power, compare compressor efficiency at more than one point and see what you get.
GETTING THE TURBINE RIGHT
Without a turbine housing and wheel, the compressor wheel would never spin. Because of the shared relationship between the two wheels, smaller turbine wheels have the ability to allow compressor wheels to spin faster and, ultimately, generate more airflow. Go too small, though, and exhaust gases can back up in the combustion chambers, making things worse.
When considering a turbine, you've got to think about its overall size as well as its A/R, or its area-to-radius ratio, which describes the relationship between the size of the turbine housing and its opening. Most of the time, the size of the turbine depends on its wheel's exducer diameter, or the part of the wheel air passes over last. A larger bore in the housing will typically yield more power. Sort of. The trick is keeping the turbine wheel's diameter within 15 percent of the compressor wheel's, give or take.
A/R is just as critical and will determine how well and how quickly exhaust gases are able to escape the housing. Go too small and spool-up time will improve but exhaust gases will revert back into the combustion chambers. Go too big and you'll find a bit more power, only a whole lot later than you'd probably like. The housing's radius also matters and directly affects turbine speed. Increase it and everything has the ability to spin faster. Settling on the right A/R can be tricky, and involves all sorts of complexities, like exhaust gas pressure, turbine inlet pressure, and, of course, boost pressure. Most of the time, once the right compressor housing and wheels are selected, whoever manufactured the turbo should have a pretty good idea of what it is that you'll need, so don't be afraid to ask.
A compressor or turbine wheel's trim is the relationship between its minor and major diameters. Every wheel has an inducer—the section of the wheel that air passes by first—and an exducer—the section it passes by last. Because compressor and turbine wheels are oriented away from one another, their inducer and exducer sides are reversed. In other words, a compressor wheel's inducer represents its smaller-diameter side and its exducer its larger-diameter side. It's the opposite for the turbine. Generally, numerically larger trims mean more airflow, assuming not much else has changed. There are trade-offs with larger trims, like reduced efficiency on the compressor side and less backpressure on the turbine side. When increasing trim, it's usually a good idea to do so without increasing the overall diameter of the wheel if possible.
As discussed, A/R ratios separate compressor and turbine housings further by yielding various flow characteristics for otherwise similar housings. Calculate the A/R by dividing a compressor inlet or turbine outlet diameter's cross-sectional area by the distance between the center of the wheel's shaft and the center of the previously measured inlet or outlet area. Do it right and the A/R will remain constant throughout the housing. Playing around with A/R ratios won't affect the compressor side as much as it will turbine characteristics.
UNDERSTANDING COMPRESSOR MAPS
Compressor maps aren't a whole lot different looking than something you avoided in the physics classes you never took. Each chart displays compressor efficiency by expressing the boost pressure ratio along the map's Y axis and airflow ratings along the X axis—the two figures you came up with earlier. Oval-shaped islands within the graph represent various efficiency zones. Any given boost/flow point plotted on an island will yield an efficiency point, ideally as close to the often mythical center island as possible with efficiency decreasing as points move outward. Where the two points intersect on the map represents the maximum amount the compressor can flow in that particular situation. Compressor efficiency is a percentage, with most peaking in the 70 percent range. Stay above 60 percent and you're in good shape.
There are all sorts of places you don't want to end up on a compressor map, most of which will result in surge or some sort of choke point. Locate the chart's choke line, look to the right, and you've just found the least efficient realm where shaft speeds are excessive and a larger wheel should probably be considered. Points to the left are just as bad. Here, surge is bound to happen, which can lead to a loss in power as well as bucking and jerking when stabbing the throttle. All of this happens when an engine's unable to inhale what the compressor's trying to feed it, which leads to air backing up in the intake tract, inside the compressor itself, and against the compressor wheel. Let all of this go on long enough and you can say goodbye to your turbo's thrust washers.
Suppose you've shunned the compressor maps, though, and are wondering whether or not you're experiencing surge. Identify it easily by listening for chattering sounds that, in some cases, can be mistaken for a blow-off valve releasing pressure. Ward off all of this in the first place, though, by reviewing those maps and choosing the most efficient compressor, which, by default, will result in the lowest surge limit.
WHAT YOU'VE GOT ALL WRONG
A bigger turbo means more power: Not always. Most of the time, a turbo that's too big will lead to all sorts of trouble, including the inability to spool up and less power than what you started with.
Turbo lag vs. boost threshold: You hate turbo lag and all that it stands for, except what you really hate is boost threshold. Boost threshold is really just the lowest engine speed at which positive pressure can be generated. Lag only happens once you've passed that threshold, pounced on the gas pedal, and waited for that boost to come on.
Boost pressure matters less than you think: The amount of air pressure in your intake manifold isn't what'll potentially blow your engine to bits. Cylinder pressure's quite good at doing that on its own, which rises alongside boost pressure but is exponentially more powerful. For example, a larger turbo churning out a measly 12 psi can just as easily annihilate an engine as a smaller turbo pushing out twice as much boost.
When 15 psi doesn't equal 15 psi: Unless you're comparing identical turbos and engines, then stacking up your 15 psi against somebody else's doesn't mean much at all. That number really doesn't tell you how much power the two of you are making, either. For example, 20 pounds of boost out of a wee T25 will likely generate half as much power as the same amount from something like a GT35R on otherwise identical engines.
Calculating the airflow rate: You'll need to know how much air your engine can flow before plotting anything on that compressor map. You won't be figuring out your engine's brake specific fuel consumption, so estimate yours somewhere between 0.50 and 0.60 for best results.
airflow rate = horsepower x air/fuel ratio x (brake specific fuel consumption/60)
Calculating the pressure ratio: You'll need to know your desired pressure ratio—or how much boost you want to run—before that compressor map will mean anything to you.
pressure ratio = atmospheric pressure + boost pressure / atmospheric pressure
Calculating trim: Knowing a turbo's compressor and turbine wheel trims can tell you a whole lot. Here's how to figure yours out.
(inducer squared / exducer squared) x 100