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.
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.
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.
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.