Whether it’s the whine of a supercharger or the “shhhhsshh” of a turbo system’s blow-off valve, the lure of forced induction has as much to do with emotional appeal as the effect on performance. But when it comes down to spending your hard-earned money on a serious power adder, you’ve got to use your head as much as your heart.
In simple terms, a turbo system and a supercharger system increase horsepower in a similar way. Each force a greater amount of air into the engine than it would normally draw without mechanical assistance. Forcing more air into an engine than it would draw naturally can substantially increase the output of a smaller engine and give it the power of a larger engine. The forced air – delivered to the intake manifold at a pressure greater than the outside, or ambient, air – is denser, delivering more oxygen to the combustion chamber. When mixed with the appropriate ratio of additional fuel, the result is a more powerful combustion. That’s the essence of forced induction – whether through an engine-driven supercharger or exhaust-driven turbocharger.
At their most basic, turbochargers and superchargers are air pumps, but with different pumping characteristics. The turbocharger is an exhaust-driven pump that saps no engine power when not making boost. A supercharger is an engine-driven pump that is essentially another component on the accessory driver system that requires a modicum of power to drive, even when it’s not producing much boost.
The thermal efficiency – the amount of combustion energy that is converted to power – is greater with a turbo system than a supercharger, because it recycles a significant amount of exhaust energy to spin the compressor. That exhaust energy is lost to the exhaust system in normally aspirated and supercharged engines. That said, centrifugal and Lysholm (screw-type) superchargers are up to 85-percent efficient.
In general terms, superchargers deliver greater power and torque at lower and midrange rpm levels, with nearly full boost available immediately at wide-open throttle. A supercharger’s effectiveness tends to trail off at higher rpm, while turbochargers typically deliver their greatest power contribution at mid- and higher-rpm levels, with boost building progressively with the increase in engine speed. Turbochargers are also very good at building mid-range torque and, when properly sized, can deliver excellent low-end power, too.
Centrifugal vs. positive-displacement blowers
When it comes to supercharged horsepower, positive displacement superchargers (Eaton-style and screw-type) and centrifugal blowers (Vortech and ProCharger compressors) produce it differently. In simple terms, a centrifugal supercharger’s boost increases exponentially with engine speed, while a positive displacement supercharger’s airflow is linear – with maximum boost occurring very low in the rpm band. That means a Roots or twin-screw blower that delivers, for example, 500 cubic feet per minute (cfm) of air at 2,500 rpm will push 1,000 cfm at 5,000 rpm.
With a centrifugal supercharger, boost builds in a non-linear way, like a turbocharger. As rpm increases, the airflow from the compressor will increase at a faster rate. Because of that, maximum boost is not achieved until the engine’s red line, or maximum rpm level.
The differences in airflow delivery create very different performance curves and driving experiences. In general terms, a positive displacement supercharger will have a flatter power curve, with more lower-rpm power. The centrifugal will deliver a greater feeling of increasing power as the revs climb. On the street, and all other things being as equal as possible, a positive displacement blower well feel stronger on the low-end, especially directly off idle. A Roots or twin-screw blower makes a small amount of boost whenever the engine is running. The centrifugal, on the other hand, “rolls” into its boost and is generally easier to launch, with a stronger feel through the mid-range and upper rpm levels.
Basically, a street vehicle with a positive displacement blower will feel the effects of the blower immediately and at all low-rpm levels, while a centrifugally blown car will feel more like stock until around the 3,000-rpm level. There is also a more pronounced application of the power with a centrifugal blower, but not the “on/off” feeling of a turbocharger.
There are a number of factors to consider before purchasing a bolt-on system for a street vehicle:
Horsepower and power adjustability:Although supercharger and turbocharger kits will deliver approximately the performance their manufacturers advertise, turbocharging generally delivers more power for the equivalent dollar spent on a supercharger kit. Turbo systems also offer almost unlimited upgrade potential.
Apart from the capacity to change the drive pulley on some superchargers, the output of a blower is pretty much determined by the size of the compressor. With a turbo system, there are a number of elements that are easily manipulated to increase power. In fact, the almost infinite adjustability of turbo systems is one of their primary appeals.
Performance range:As noted at the beginning of this section, superchargers – particularly Roots/screw-type – will generally deliver gobs of low-end power and become less efficient at higher rpm. The opposite is generally true of turbochargers, which tend to deliver their greatest performance as maximum boost is delivered with higher engine speed. When it comes to the basic power curves of most four-cylinder and many six-cylinder engines, a supercharger gives a car an advantage in low-speed and stoplight-to-stoplight driving.
Drivability:Because an engine-driven supercharger is “on,” it tends to give a street-driven vehicle an abundance of off-the-line/low-speed pull – to the point where it can be difficult to manage part-throttle driving in some instances, as tire spin becomes an issue. The higher-rpm power application of turbo systems typically makes them more tractable at low speeds. The driver who wants to guarantee a win off the line at stoplights, as well as the instant application of full boost, will probably enjoy a supercharger more, while the enthusiast seeking a wider performance range will likely find a turbo system more rewarding.
Noise:Generally speaking, the compressors of most supercharger and turbo systems are very quiet these days. Turbos are essentially silent until they start spinning at a high rpm, and the same is true for most Roots/screw-type blowers. Centrifugal superchargers are much quieter than they used to be, but at idle, they’re not as quiet as turbos or a Roots/screw supercharger.
Tuning:There’s no real advantage between tuning a supercharged or turbocharged engine, since the need to maintain an adequate air/fuel ratio to avoid detonation is most important with both methods. Both types of systems have unique needs for delivering safe, optimal performance, but the basic approach to tuning is similar.
Maintenance and reliability:When installed and used properly, supercharger and turbo kits are very reliable, with the compressors for both lubricated with engine oil – although some Roots/screw-type blowers feature self-contained lubrication systems. Over time, the drive belt for a supercharger must be inspected, just like the engine’s standard accessory belt, and after a few years, the compressor may require an inspection to make sure the tolerances and clearances are within specification limits for the rotors. Turbochargers are very susceptible to heat, and even with adequate lubrication, the internal seals and turbine can wear and allow oil blow-by. This requires the turbo to be rebuilt.
System cost:Because of the myriad of extra equipment – from the wastegate to the exhaust manifolds – turbocharger kits generally cost more than supercharger kits. In fact, the cost of a typical bolt-on kit can be two or three times that of a supercharger system. Along with that, turbo systems generally take longer to install than supercharger kits, which adds up when outsourcing the project to a professional shop.
Installation impact on the vehicle:Assuming all turbo and supercharger systems will employ an intercooler, the Roots/screw supercharger systems generally require the fewest compromises and/or fabrication modifications during installation. Because they install in place of the manifold, few changes are required at the front of the engine or engine compartment. Consequently, they offer the most integrated, “factory”-looking appearance under the hood. Centrifugal superchargers require a mounting bracket on the front of the engine that can require moderate modification of factory components, including the removal or relocation of some components.
When it comes to bolt-on turbocharger systems, the requirements for the exhaust manifolds, turbochargers and associated plumbing typically require considerably more fabrication, modification of stock parts and relocation of stock parts than supercharger systems.
Installation cost:Again, because of the extra equipment associated with them, turbo kits are generally more time-consuming to install and, therefore, cost more for the labor. So, while a turbo kit offers greater performance potential, the costs involved with such an investment may steer some toward a supercharger. In fact, there are other factors to consider before giving ordering a system for your car. For one thing, the tight confines of the engine compartment may make packaging and installing a turbo kit very difficult. This not only makes the installation a painstaking, difficult procedure, but can make future servicing all but impossible without an extensive teardown of the vehicle’s front end.
Bottom line:For most front-drive, four- or six-cylinder-powered tuner cars, turbocharging ultimately delivers greater performance potential when judging against important features such as engine displacement and application of power on the street or racetrack. Supercharging has its merits as a basic, economical power adder, but is better-suited to larger-displacement engines.
Used on four- and six-cylinder-powered cars, superchargers and turbochargers delivered an era of unprecedented performance for smaller-displacement engines. Credit:(Photo courtesy of GM)
Detroit automakers have experimented with production forced induction setups since the 1960s. This 1984 Mustang GT Turbo delivered about 145 horsepower from its 2.3-liter four-cylinder engine, using a single turbo and no intercooler. The 145 hp doesn’t seem like much today, but in 1984, the V-8-powered Mustang GT was rated at only 175 hp from its 5.0-liter engine. That’s only 30 more horses from more than twice the displacement.
The engine of the 1984 Mustang GT Turbo was a technology leader. It used electronically-controlled, sequential fuel injection – the same basic fuel delivery system used on just about every new vehicle today. Ford introduced this engine in 1983, and later editions used on Mustang SVO models employed an intercooler to make even more horsepower.
Subaru helped drive the current popularity in turbocharged tuner vehicles with its compact and very powerful Impreza-based WRX and WRX STi models. Those vehicles make a very strong argument for turbocharging as the preferred method of forced induction.
The brief production run of the Saturn Ion Redline (shown here) and Chevy Cobalt SS Supercharged demonstrated supercharging was a viable and affordable method pumping up small-displacement engines. The 2.0-liter engine of these cars was rated at 205 horsepower. Credit:(Photo courtesy of GM)
The ultimate example of supercharged performance in a production vehicle is the Corvette ZR1. With a 2.3-liter supercharger – larger than the entire engine displacement of many four- and some six-cylinders – forcing air into its 6.2L V-8, the ZR1’s engine churns out 638 horsepower.
One of the advantages of a supercharger when building a modified engine is space. Typical Roots- and screw-type superchargers that mount in place of the stock intake manifold are relatively easy to fit – if a manufacturer offers a kit for your vehicle. The plumbing required for a custom turbo kit can make for an installation nightmare on vehicles without much room in their engine compartment.
Although relatively easy to mount and install (a GM supercharged 3800 V-6 is seen here), superchargers have a relatively limited performance range. A smaller drive pulley is about the only thing that can be done to increase the boost from a blower, apart from installing an entirely new, larger-displacement compressor.
This is an example of a factory-blown car taken to drag racing extremes. It started life as a Grand Prix GTP with a supercharged V-6, but the performance limit of the blower caused its owner, Scott Cook, to turn to turbocharging.
Here’s the engine compartment of Scott Cook’s Grand Prix. The supercharger has given way to a custom sheet metal intake plenum that’s fed by a larger turbocharger. The combination boosted the engine’s output to about 620 hp and enabled quarter-mile passes in the 9-second range.
A turbocharger is driven by the force of exhaust gases that spin a turbine. The larger the turbo and the greater the engine speed, the more power a turbo system will generate. In contrast, a supercharger is driven by the engine and is limited in maximum rpm by the engine’s speed.
One of the greatest advantages of a turbo is the capability of adjusting its output to tune power in an almost infinite manner. In this photo, a new wastegate actuator is added to an existing turbo system, enabling the system to generate more boost.
Subaru’s WRX series is an excellent example of the tuning capability of turbo systems. The basic WRX offers 275 horsepower from its 2.5-liter inline four, while the STi model cranks out 305 horses from the same engine. The primary difference is boost pressure: the WRX’s engine generates 13.3 lbs of maximum boost, while the STi’s pushes up to 14.7 psi. Credit:(Photo courtesy of Subaru)
On vehicles with sufficient engine compartment space, custom turbo systems can be plumbed so that the turbo(s) are located more conveniently. Turbochargers generate tremendous heat and can cause damage in tight confines. An adequate intercooling system is also a must to keep intake air charger temperatures lower.
Another advantage of turbocharging is its on-demand performance. When not generating boost, turbo engines perform like a naturally aspirated engine – including fuel economy. It’s when the throttle is cracked wide-open, such as with this MazdaSpeed 3 at the drag strip, that the extra horsepower is tapped.
At the drag strip, turbocharging is used almost exclusively in classes that permit power adders. You can simply get more power out of a small-displacement four-cylinder than you can with a blower. Credit:(Photo courtesy of GM)
Turbo vs. supercharger: The Chevy Cobalt example
When it was introduced in 2005, the Chevy Cobalt SS Supercharged (and its GM cousin, the Saturn Ion Redline) delivered performance thrills on the cheap. Its supercharged 2.0-liter engine delivered 205 horsepower and lots of low-end, neck-pulling torque (peaking at 200 lb-ft). The blower was an Eaton-manufactured Roots-type and helped the Cobalt SS hit 60 mph in about 6 seconds flat and run the quarter-mile in the mid-14-second range.
A few short model years later, the Cobalt SS was reborn with an intercooled turbocharger system generating 260 horsepower and 260 lb-ft of torque on the same basic 2.0-liter Ecotec engine platform. Not surprisingly, that extra 55 horses and 60 lb-ft of torque greatly enhanced performance, shaving about half a second off both the 0-60 and quarter-mile times.
More than that, the turbocharger system of the Cobalt SS enables greater performance through modifications of the wastegate and turbocharger itself. The previous supercharged version could be modified only through moderate means, such as a smaller-diameter drive pulley and careful tuning. Cobalt turbo tuners are easily pushing output to more than 25 pounds of boost, taking the engine past the 300-horsepower mark. Simply put, that output isn’t possible with the standard supercharger on the SS Supercharged model.
Chevy’s Cobalt SS models have been offered with both a supercharger and turbocharger, with each power adder integrated on the same, basic Ecotec 2.0-liter engine. With the blower, the engine made 205 hp, but with a turbo, output jumped to 260 horsepower. Aftermarket kits allow the supercharged engine to make up to about 250 hp, while more than 300 are possible with relatively minor mods to the turbo engine. (Photo courtesy of GM)
The Cobalt SS Supercharged engine... ...and Cobalt SS Turbocharged engine.
Credit:(Photos courtesy of GM)
Understanding boost – including psi vs. Bar
Whether a supercharged or turbocharged system, the measure of pressurized air fed into the engine is referred to as boost. It is the difference between the ambient air pressure and the increased pressure the boost-producing device generates at the intake manifold. Boost is the opposite of vacuum, which a non-boosted engine makes during typical operation.
When an engine isn’t running, it generates no vacuum or boost (negative pressure), meaning the pressure in the intake manifold is the same as the ambient air pressure – about 14.7 pounds per square inch. At idle and low-throttle conditions, an engine will generate vacuum, indicating the pressure in the intake manifold is lower than the ambient pressure.
In a supercharged or turbocharged engine, boost is created as more throttle is applied and the boost-generating device forces air into the intake manifold at a higher pressure than ambient (positive pressure). The air pressure at the intake manifold swings from negative to positive – that’s why high-performance boost gauges indicate both vacuum and boost measurements.
In North America, boost is generally measured in pounds per square inch (psi), while it is also measured in Bar. When measuring in psi, the ambient air pressure is regarded as the base, or “0” pounds of boost. The positive pressure builds on that base, with 1 pound of boost indicating 1 psi greater than ambient pressure.
With Bar measurements, Bar is roughly the equivalent of ambient air pressure. Technically, 1 Bar is equivalent to 14.5 psi, not 14.7 psi, but many enthusiasts equate it to the equivalent of normal atmospheric pressure, so a 0.5-Bar pressure reading is the same as roughly 7.25 pounds of boost. A full, 1-Bar reading would indicate 14.5 pounds of boost.
A typical boost gauge, with measurements in pounds per square inch. Vacuum is what the engine generates when it is not producing boost.