For 25 years, turbos were champ and superchargers played palooka. Now the roles are reversed,
and engineer-journalist Norman H. Garrett is ready to analyze why. Henry Rasmussen shoots the pre-fight pix.
I had this whole thing wrong for a good portion of my life. When I first contemplated the internal-combustion engine (while walking behind a smoky Briggs & Stratton), I could tell I was dealing with a "suck-boom" device—as opposed to blenders, which I knew were "zap-spin" devices, and model rockets, a.k.a. "zap-boom" devices.
But after years of fiddling with suck-boom thingamabobs and trying to make the inhale part go a little smoother, I’d pretty much reached the limits of these poor devices. I mean, how much can you expect of a piston that’s being asked to suck in air and fuel through an air filter, a carburetor, the manifold’s turns and twists, and then a half-open valve? I have fits when I try to go snorkeling, and I’m only sucking in a little bit of air at a time: That one-lunger B&S was trying to get a cup of air through a dime-sized hole fifteen times a second. When it comes to tuning, getting enough fuel is never a big deal: Getting enough air in to burn with the fuel is the bottleneck.
Then it hit me: I’d held my hand against carbs enough to realize that there just wasn’t all that much sucking going on. Put it under pressure—that’s the ticket. If you actually force the air into the engine it just has to take as much as you can cram in, right? So what if the intake valve is a little small or the intake ports are mismatched? We’ll be pushing and shoving like Fudgesicle day at the cafeteria. Now we’re talking about a push-boom device, that that’s really something to think about.
Round 1 - Rocky’s Road
Of course, the mass-market auto industry has already figured this out. Beginning with a supercharger here and there (at Ford and Studebaker in the ’50s) and an occasional turbo (in the early ’60s at GM), the idea of pressurizing the intake charge was already catching on decades ago. But it wasn’t really until cheap cubic inches collided with expensive gasoline in the ’70s that things got serious, and along with that seriousness came a revival of the long-fought battle between engine-driven superchargers and exhaust-driven turbochargers. (That’s really the only theoretical difference between them: A supercharger is powered by a direct connection to the crankshaft while a turbo gets spun around by the blast of outrushing exhaust gasses.)
From the 1970s until recently, turbos were very much the favorite—just about every carmaker had one, in fact. Recently the tables have turned, and now supercharged cars outnumber turbocharged ones by a wide margin. The point of this article is to figure out why.
Currently, the most widely used supercharger is manufactured by Eaton, the first company to make the famous Roots-type design commercially viable. Mercedes’ C-class Kompressor cars use it, Jaguar fits one to its 4-liter XJR Six, Aston Martin uses a single Eaton on the DB7 and two on the 550-bhp Vantage, and Pontiac, Buick, Ford and Oldsmobile have all joined the same party. Toyota’s current Previa has its own Roots blower, meanwhile, and Mazda now fits a screw-type supercharger to the Millenia’s Miller-cycle engine. There are more examples, but that’s enough for now.
Turbos, meanwhile, have virtually disappeared from the general-use market and are now found almost exclusively on very-high-end flagship sports models: The 911 Turbo, RX7 and 300ZX-T, for example. There are also a few exceptions from Sweden, which we’ll talk about momentarily.
But first, let’s take a look at the whole picture: Anytime you pressurize the intake tract of an engine you’re talking about forced induction. Instead of simply asking the air to flow into the combustion chamber as the piston moves down the cylinder, you’re pushing it in with anywhere from five to 20 psi of pressure.
Done correctly, forced induction can let a small, light, fuel-efficient engine make the power of something twice its size. Done correctly, that is—most turbo applications fall far short of their theoretical promise (as do most supercharged engines). As with everything else in the car business, some jobs just turn out better than others.
Turbochargers do their stuff with a small blower that works like a water wheel. Exhaust gasses leaving the engine spin a little fan (the turbine) in one half of the turbo; a shaft connects the turbine to a similar fan (the impeller) in the other half of the turbo, and this compresses the fresh-air charge going into the engine. For many years this was advertised as "free power," since the energy contained in the exhaust gasses was going to waste anyway.
Round 2 - The Ring of Truth
There were a few problems with the turbocharger unit itself, but these have been largely ironed out today. One growing pain centered around the bearings that support the high-speed shaft connecting the turbine and impeller. Turbos spin at up to 120,000 rpm and they’re constantly exposed to 2000-degree exhaust gasses, so their lubrication and construction requirements are understandably stringent.
Another problem comes from the inherent nature of all centrifugal fans: They have a "sweet spot" where the air gets flowing great guns, but at every other speed, things are down a bit. Turbochargers exhibit a slight delay between the time you punch the throttle and the time it reaches this sweet spot: Some of the famous turbo "lag"—the delay between stomping on the gas and the engine making real power—is caused by the inertia of the rotating masses themselves, but the rest comes from this favoring of one rev range over another.
Turbocharger manufacturers answered the heat issue by using engine-coolant lines to cool down the turbo’s shaft bearings. They also created lightweight ceramic components to lower the unit’s inertia and variable-vane impellers to broaden its sweet spot. And the latest strategy is to simply fit a smaller unit, which will spin up faster and reach peak efficiency sooner at the cost of some top-end power. Volvo and Saab have been particularly successful in using this strategy with their 4-cylinder engines. Another spin—forgive me—on this idea has been put into play by Mazda. They use two separate turbos on the RX-7: a smaller unit to provide some low-speed grunt and a second, larger one for the air-hungry top end.
By now, decades of development have left turbochargers with just one real flaw remaining: By nature they are non-linear feeders bolted to linear consumers.
What does that mean? Well, as an engine’s revs increase from idle to redline, the pistons suck in air in direct proportion to how fast things are turning. But a turbo delivers air at a square-function of its speed—in other words, its performance is exponentially non-linear. The practical result is that while the driver expects performance to be more or less a function of throttle position, it’s actually the turbo’s current rotating speed that determines whether stomping on the gas will give a lot of acceleration or just a little.
Round 3 - Tale of the Torque
It’s important to note that where driveability is not a major concern (racecars in particular), turbos are still the blower of choice. When you’re looking for maximum power at wide-open throttle, you can place the turbo’s sweet spot right right where you want it and get some great numbers. Two other factors can also make turbos a winner: There’s negligible engine power involved in driving them, so the absolute maximum output can be higher. (Hence their use in flagship sports cars, where advertised power is critical.) Second is the fact that in the rare sedan applications still out there, the turbo engine is usually bolted to another non-linear device—an automatic transmission. This, combined with careful tuning and the increased use of smaller, lower-volume turbos tends to mask what little lag remains.
Okay, back to the matter at hand—turbo versus supercharger. We’ve already seen that for some uses the turbo’s unpredictability is a problem and for others it isn’t. So that alone isn’t enough to explain the big recent boost—so to speak—in the supercharger ranks vis-a-vis turbos.
That’s because other factors are also involved. First, emissions regulations have numbered many turbocharged engines’ days. Catalytic convertors love heat, need heat, crave heat. But turbos absorb heat, especially during cold-weather starts where emissions are the worst. Delaying the cat’s wakeup call bounces many engines out of the running, emissions-wise. Turbos hurt emissions one other way, too—via increased combustion-chamber temperatures. With the turbo in the way, the exhaust heat can’t get out of the engine as easily. That can elevate the chamber temperatures and lead to premature detonation from hot spots. The easy fix is to retard the timing, throw in more fuel for cooling, or both, but you’d wreck the emissions either way.
Round 4 - Blown Chances
Then comes the manufacturing issue. In practice, the technology for mass-producing quiet, efficient superchargers has just recently been perfected, thanks to new advances in computer-controlled machining techniques and material developments. The modern Roots-type blower uses extruded-aluminum rotors that spin at 12,000 rpm with barely enough space to pass a human hair between them. Think of two gears meshing but never touching at that speed and you get an idea of the engineering challenges involved. This newfound affordability of a truly first-class blower has advanced the supercharger’s fortunes more than anything else.
So, re-enter the supercharger. Now, all superchargers fall into two distinct categorie, either being centrifugal or positive- displacement. Centrifugal superchargers are essentially belt-driven turbos. They use the same sort of impeller as a turbo, and share some of the same sweet-spot problems in driveability. But positive-displacement superchargers (like the Roots type) flow a fixed amount of air for each revolution and are therefore very linear in their delivery.
Superchargers are also generally easier to build than turbos from an engineering standpoint. Their shaft speeds are around 12,000 rpm, instead of ten times that; bearing and seal designs are therefore a bit easier. Not having hot exhaust gasses flowing through the unit also means less strain on the design.
Not surprisingly, properly designed superchargers are proving as reliable as starter motors, while many turbocharger applications still show a fair number of failures during the first 36,000 miles. Superchargers also leave the exhaust tract clear, and they’re easier on engine oil and cooling systems.
The most obvious downside of superchargers versus turbochargers is that when full boost isn’t required, the unit is still sucking power out of the engine via its drive mechanism. There are ways around that, of course: Toyota and others use an electro-magnetic clutch—much like the one on an a/c compressor—to disengage the blower when it’s not needed, while cars with the Eaton unit generally use an air-bypass system upstream of the unit to recirculate its output during part-throttle operation. Through careful balancing, the bypass effectively lets the unit freewheel at cruise but still offer seamless, virtually instantaneous boost on command. Parasitic losses under light throttle fall to an insignificant amount—perhaps a tenth of a horsepower—this way.
That doesn’t stop the supercharger from sucking down power under load, of course, but this argument is something of a red herring. At full song the modern supercharger will consume up to 6% of the engine’s power, but in return it may yield a 50% or more improvement in gross output. That’s an equitable trade to say the least.
Most important of all, engineers can use today’s fairly inexpensive superchargers to cash in on the excellent feel a positive-displacement unit lends to an engine. Now you have a linear air-consumption device—the engine—being force-fed by a linear output device—the supercharger. In the Roots-type application, boost begins at idle and rises in a linear progression straight up to redline. The supercharger always spins at the same relative speed to the engine, and this means a proportional amount of boost is always available. Squeeze on 25% more throttle and you get 25% more power. Or choose 26%—it doesn’t matter. The linearity is preserved, the throttle angle is directly related to the power output, and real-world performance and usability are generally improved.
Round 5 - The Decision
In brief, the equations are fairly simple. Ultimately, turbos can create up to 7% more power, but superchargers are usually easier to install, get the power flowing sooner (often for better overall acceleration), and feel better at the pedal.
Thus, most of the cars currently sold with turbochargers are vehicles where the highest quotable peak-power figure is critical. On the other hand, most of those being offered with superchargers are more concerned with delivering real-world, over-the-road drivability.
Add the fairly recent emissions and manufacturing issues differentiating the two and superchargers seem likely to extend their newfound sales lead even further in the future: A technology that seemed pretty easy to pick on ten years ago is now the likely future champ. That knowledge may come in handy the next time you’re out on the showroom floor trying to decide between a suck-boom device and one of them push-boom jobs.
