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Behind the Speed: Turbocharging

An in-depth look into the anatomy of the tuning world’s most versatile power upgrade: the turbocharger.

Turbocharging 101

Forced induction is the only way to get, say, a four-cylinder engine to huff in enough air to behave more like a V8 twice its size, or to get that V8 to belt out the sort of horsepower that will get you down the quarter-mile in six seconds. Out of the three distinct methods of forced induction, though—turbocharging, supercharging and nitrous oxide injection—none is as capable as the turbocharger. Their components are many, their tuning demands strict, and their ability to play well with the rest of your engine is vital to whether or not power is quadrupled, for example, or whether one of your connecting rods decides to all of a sudden take a hike through the front of your block.

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Turbo 101

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TURBOCHARGING VS. SUPERCHARGING

Like a supercharger, a turbo generates more power by compressing air, allowing more of it inside of an engine at any given time. They do all of this in entirely different ways, though. A turbo relies on engine exhaust flow to spin its turbine, which, through a common shaft, spins an opposing wheel that compresses incoming air and does everything you care about. Superchargers aren’t driven by exhaust gases but instead by a belt that’s linked up 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 supercharger requires pulley swaps and even then can be limiting. Turbos aren’t perfect, either. Strapping one onto your engine will generate a whole lot more heat, 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—and that includes those supercharged Top Fuel dragsters you’re thinking about that generate more than 8,000hp. For example, mid-1980s turbocharged Formula One engines that produced significantly less power but from smaller engines resulted in some of the most impressive specific outputs in automotive history. The advantages of a proper turbo system mean the same thing for you as they do the multimillion-dollar racing program: more power more efficiently than anything else.

 

THE AIRFLOW PATH: COMPRESSORS AND TURBINES

 

"As far as efficiency goes, no other form of forced induction compares to the turbo."

 

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, supported by a system of bearings that connects to both wheels. The two snail-shaped housings haven’t changed much over the years and are really just a place to store the pair of wheels, featuring inlets and outlets to draw in and direct air to all the right places.

 

Fire up your engine and air automatically 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 the shaft they share. It’ll take you dutifully 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 principles and airflow path won’t change.

 

"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."

 

After making its way out of the compressor, the air charge moves through a series of pipes, possibly through an intercooler, 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 up the turbine housing, creating a whole lot of back pressure, all kinds of heat and, because of the housing’s unique shape, the wherewithal to keep that turbine wheel spinning, repeating 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.

 

Airflow Path Airflow Path Airflow Path Airflow Path

Airflow Path

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 back pressure on the engine 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 lies the center cartridge, which carries the load of the common shaft and supports its bearing assembly. A pressurized oil stream redirected 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 in some cases. Some center cartridges also include inlets and outlets that may be integrated into the engine’s cooling system, which can help regulate internal temperatures to some degree, literally.

 

Like your engine’s crankshaft, a turbo’s common shaft has got to be supported by a series of bearings. Here, journal bearings like you’d find around your engine’s crank are most common; however, higher-end applications are often made up of more sophisticated and longer-lasting ball-bearing assemblies, not unlike what you’d find inside a skateboard wheel, for example. 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 significant depending on the circumstances. To be sure, even larger, slower-spinning turbos typically suited for heavy-duty workhorse applications still spin as much as 10 times faster than an engine’s crank ever would.

 

"Before doing anything, a target horsepower range is critical."

 

CHOOSING THE RIGHT TURBO

Before doing anything, a target horsepower range is critical. 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 the best choice. Never forget 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 might 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’re going to make. Stop wondering. Instead, concern yourself with horsepower and airflow. It’s true that lower boost pressure means your turbo will generate less heat and not work as hard, but all of this is of little consequence to your engine. Cylinder pressure, not boost, will decide whether or not the engine wants to blow up or make a whole lot of power.

The Right Stuff The Right Stuff The Right Stuff

The Right Stuff

Speaking of heat, when selecting compressor and turbine housings, choose ones that can pump the most air into the cylinders without raising temperatures more than complicated thermodynamics laws say they should. Complicated-looking compressor and turbine maps are needed to reveal a turbo’s efficiency, surge limit, boost potential and shaft speed. The labyrinth of considerations doesn’t end there, though. You’ve also got to look at fancy terms like pressure ratios, compressor surge, trims and A/R ratios, which we’ll get to in a bit.

 

"As size increases, efficiency drops and heat rises."

 

Now’s also a good time to mention oversize 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 with an oversize compressor. As size increases, efficiency drops and heat rises. Yes, 10 psi will always be 10 psi, no matter the size of the turbo, but while air quantity may be equal, 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 that will 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 sorts of graphs and charts, which make choosing the right compressor a whole lot easier. Before you go reading a turbo’s compressor map, though, 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 moment. The calculation is simple: Divide the proposed absolute outlet pressure (14.7 + boost pressure) by the absolute inlet pressure (14.7) and you’ve got your pressure ratio. Limiting yourself to a reasonable number is the hardest part. Start with something realistic like, say, 10 psi, and play around with larger numbers for bigger-power, track-only configurations. Also, if you’re not anyplace near sea level, then you’ll need to determine the appropriate absolute 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 what it is. Airflow tells you how much air is entering your engine for a given period of time, and it can be quantified at a given engine speed by factoring in displacement and volumetric efficiency. Come up with this number—with an honest pressure ratio and the right compressor map—and you’ll have a hard time landing a turbo that doesn’t work for you.

 

As it turns out, though, there may be more than one turbo that’ll seem to suit you. 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.

Turbo Facts Turbo Facts Turbo Facts

Turbo Facts

GETTING THE TURBINE RIGHT

 

"The trick is keeping the turbine wheel’s diameter within 15 percent of the compressor wheel’s, give or take."

 

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, allow for 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 overall size of the turbine housing and its opening. Most of the time, the size of the turbine depends on its wheel’s exducer diameter. 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 also 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 prefer. The housing’s radius also matters and directly affects turbine speed. Increase it and everything has the ability to spin faster. Settling upon 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 appropriate compressor housing and wheels are selected, whoever manufactured the turbo should have a pretty good idea of what it is that you need, so don’t be afraid to ask.

 

TRIM

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 been changed. There are trade-offs with larger trims, though, like reduced efficiency on the compressor side and less back pressure on the turbine side. When increasing trim, it’s often a good idea to do so without increasing the overall diameter of the wheel if possible.

 

A/R

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 than something you’d see in math class. 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 different efficiency zones. Any given boost/flow point plotted on an island will yield an efficiency point, ideally as close to the 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.

 

AVOIDING SURGE

There are an infinite number 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 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 accelerating. All of this happens when an engine’s unable to inhale what the compressor’s trying to supply, 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 all of this off in the first place by reviewing those maps and choosing the most efficient compressor to begin with, which, by default, will result in the lowest surge limit.

 

MYTHS AND MISCONCEPTIONS

 

A bigger turbo means more power:

 

Not always. In fact, 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 occurs once you’ve passed that threshold, stabbed the gas pedal and waited for that boost to come on.

 

How much you boost matters less than you think:

 

The amount of air pressure in your intake manifold isn’t what will potentially blow your engine to smithereens. Cylinder pressure’s quite good at doing that on its own, rising alongside boost pressure but 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. 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.

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