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Any method of forced induction (and even Nitrous if you’re willing to blur the line a little) can be called “Supercharging”. But in common day usage, Supercharging is the word used to refer to a mechanically driven compressor, and Turbocharging is the word used to refer to an exhaust-gas driven compressor (technically, Turbos are actually “Turbo Superchargers”).

Both systems are designed to do the same thing, pump air in to the engine at a rate that’s faster than the engine would be able to consume the air if it was just breathing normally. This oversupply of air raises the pressure of the air inside the engine, packing more air in to the engine cylinders. The engine takes advantage of this added air by adding more fuel to each cylinder. The engine displacement is effectively increased, it behaves as if it's a larger motor. But Supercharger and Turbocharger systems achieve their common design objective in very different ways.

“Turbocharging” always uses the engine’s exhaust gas to spin the compressor, and the Turbo compressor is always a centrifugal type. Variants of Turbo systems differ solely in their arrangement on the engine. There are single Turbo systems and Multi-Turbo systems. Multi systems can be parallel or sequential. They also vary in location with respect to the engine. Many Turbo installations strive to get the Turbine as close to where the exhaust is hottest as is possible, and so they hang off the exhaust manifold (V configured engines usually need two Turbos, one for each side), but some systems like STS mount at the rear of the car.

“Supercharging” is always a mechanically driven system – a transmission of some type couples the compressor to the engine that the compressor is feeding. Compressor RPM always varies linearly with engine RPM. There are a couple of types of Supercharger compressor designs, “Centrifugal” and “Roots” being the primary classes (“Whipple” and variants, for the purpose of this discussion, can all be considered to be “Roots” type). The two classes of compressor behave very differently (more, below).

A Supercharger doesn’t need to be plumbed in to the exhaust system. Besides simplifying the installation process, there are advantages to be had in terms of reliability. Heat accelerates failure rate – everything wears out faster the hotter you run it. Superchargers also tend to deliver slightly cooler intake charges. The Turbo’s compressor is attached to the impeller (turbine) by a connecting shaft. This places the compressor housing in close proximity to the turbine housing. The turbine housing has hot exhaust running through it. So, Turbos run hotter by their very nature (and they transfer more heat to the air they're compressing. Engine oil, too).

Because superchargers are driven from the crank, the boost they produce is always proportional to engine RPM. But the boost curves are different for the two types of compressor. Roots compressors are constant displacement pumps. They move the same volume of air for each rotation of the compressor assembly and airflow through the compressor varies linearly with RPM. The engine is also a constant displacement pump, it consumes the same volume of air for each rotation of the assembly. Because the Supercharger and engine have a fixed “volumetric” relationship, Roots compressors present the same "additional" air volume to the engine across all RPMs, which makes them good at making boost at low RPMs. Centrifugal compressors accelerate air radially to achieve boost. Air enters at the center of a rotating vane and is accelerated outwards by centrifugal force. When the air exits at the outer edge of the rotating vane, it enters a “scroll housing” which slows the air down, which causes the pressure to increase. Unlike a constant-displacement pump (like a piston), the volume of air that's moved is proportional to the *square* of the compressor's RPM (as dictated by the V^2/r equation). This characteristic presents design challenges that don't exist for a Roots type. Because boost builds with the square of RPM, the design has to take in to account the maximum acceptable boost level (which will occur at the maximum expected operating RPM). Unlike a Roots type, this often means the compressor makes little or no boost at lower operating RPMs.

Centrifugal Superchargers are more compact than Roots. They can be installed in the space around or in front of the engine. Roots types are larger, and install on top of the motor in place of the intake manifold. While this is a simpler "modular" installation, Roots blowers will require a high-rise hood, which is an added expense (and PITA). The fact that Centrifugals don’t make boost at lower RPMs also means they don’t place as significant a parasitic load on the engine (though many Roots types include low-RPM bypass valves to help address this discrepancy). As a result, Centrifugals can be more “drivable” than Roots, and might also have less of a negative effect on highway-cruise MPG.

The "best" choice for a Corvette Supercharger is ultimately a function of your priorities. Roots types like Magnusson are straighforward, simple installations but you need a high-rise hood. Vortech and ATI are more complex (Vortech needs a hole in the oil pan and ATI needs plumbing for the air/air intercooler) but they fit under the factory hood.

Turbochargers are also centrifugal compressors. They differ from Centrifugal Superchargers in that they're not driven off the Crank, they use energy from the exhaust gas to spin. This makes them *much* more efficient than crank driven compressors. You can think of it in terms of simplistic "thermodynamics". The energy contained in the hot exhaust is "captured" by the Turbo’s impeller and converted in to mechanical force to spin the compressor. If the turbo wasn't there the exhaust “energy” would just flow out the tailpipe anyway, so the boost you get from a Turbo is essentially "free".

Besides the advantages Turbos have over Superchargers with respect to operating efficiency, they also have another huge advantage. Turbos are "load sensitive". The boost they generate is proportional to the engine’s load and NOT the engine RPM. This means that a Turbo will generally be able to produce good boost at low RPMs like a Roots, even though it's a centrifugal type compressor. You need exhaust heat, and not so much exhaust volume.

But as mentioned earlier, Turbos live in the exhaust, and the exhaust get's *HOT* (it’s not at all unusual to see a Turbo’s exhaust housing glow red). This heat can create problems for reliability. Failures most often manifest as bearing or seal failure due to buildup of abrasive "coke" on the bearings ("coke" is what's left of your oil after you cook off all the volatile components). Good Turbo systems will include a water-jacketed center bearing housing to help keep this critical component cooler during operation. Turbo systems are also often equipped with a “Turbo Timer”, that either keeps the engine running for a short while after you remove the key, or uses an electric pump to circulate oil through the Turbo’s bearings for a short while after you remove the key. The objective being to allow the Turbo assembly to cool down some before the oil flow is shut off.

Note though, there isn’t a huge mechanical force to be had from the engine’s exhaust gas. Turbo’s have to be small to minimize inertia or they take forever to spin up (AKA “Turbo Lag”). Smaller compressors have lower operating limits, they have to spin faster to move a given volume of air and they can’t move as much air (max out sooner). Larger displacement Turbo applications almost always need to use two or more Turbos. In contrast, a centrifugal Supercharger compressor can be quite a bit larger than its Turbo brethren because it’s got the power of the entire engine to get it spinning. So while V8s like ours can get by just fine with a single centrifugal Supercharger, they always require two Turbos to achieve acceptable operating performance.

Intercooling is a feature found in many (but not all) FI systems. Intercoolers reduce the temperature of the air exiting the Supercharger or Turbo compressor. You want to do this because compression increases the air’s temperature (according to Boyle's ideal gas law: pv=nRT) (and heat soak from the exhaust system is also a factor in Turbos). Lower temperature air is denser, less likely to contribute to preignition, and allows a higher maximum boost level (= more power). Since both Turbos and Superchagers benefit equally from Intercooling, you should always select a system that is Intercooled. But note that having an intercooler is a function of the "completeness" of the design and is not a requirement.

Intercoolers come in two basic types, air-to-air and air-to-water. The debate about which type is more efficient has raged and will continue to rage between aficionados of the types for years. What it really comes down to is this... Air/Air require a large radiator up front, with correspondingly long ducts/plenums to route the charge to/from the intercooler. Fitment can be a challenge. Air/water intercoolers don't need long ductwork, and they get by with smaller radiators, but they do need plumbing for the water and pump and electrical hookup to drive the pump. In Roots type blowers, you don’t have a choice, it’s no Intercooler, or air/water, but Turbo and Centrifugal Superchargers can use either type. Consider the types to be equivalent. What matters is you have one, not which type it is.

When the throttle slams closed at high RPM/Boost levels (like when you lift during a shift), the engine continues to turn at high RPM, so the Supercharger continues to turn at high RPM, and all the air being pushed through the Supercharger compressor suddenly has no place to go. This can cause a "compressor stall" (in really high boost applications it can also damage throttle plates). “Stall” doesn't mean that the compressor stops spinning, it means the compressor stops compressing (think of a cavitating motor boat propeller and you'll get the idea). This is a bad thing, since the system has "inertia" and when you crack the throttle open again it takes a little while for the boost to "bounce back". Superchargers use a Blow Off Valve to address this issue. A vacuum operated valve provides a path away from the high-pressure side of the compressor for the compressed charge air to "dump" into when the throttle is closed. In race applications, BOVs usually just dump to “outside” the intake manifold. In an emissions-compliant application like our cars, the air has already been metered by the MAF, so the BOV allows the compressed air to circulate back to the low-pressure input side of the compressor.

Turbochargers have something called a Waste Gate (though they can also make use of BOVs). The WG is a valve that sits in the exhaust flow between the engine’s exhaust manifold and the Turbo's turbine. The valve is pressure-actuated off of the high pressure side of the Turbo. When the boost coming out of the turbo exceeds a given level (set by a spring inside the waste gate diaphragm/actuator), the pressure operates the actuator to open the waste gate valve allowing engine exhaust to bypass the Turbo’s turbine. The turbine slows and since it’s coupled to the compressor, the compressor slows and boost levels quickly drop. Waste Gates make a Turbo self regulating. Even if it's capable of spinning fast enough to deliver 100Lbs of boost pressure, the Turbo will never boost more than the waste gate allows. And since the compressor speed can be regulated independent of the engine RPM, a Turbo system can be designed to produce better boost at lower loads (which are also generally lower RPMs) and not blow the top off the motor at high loads/RPMs.

An Electronic Boost Controller allows boost level to be set arbitrarily (rather than by adjusting a spring). EBCs insert an electronically operated control valve between the Waste Gate actuator and the high-pressure side of the Turbo. The valve is normally kept closed, so the WG never sees any "boost" pressure at all. This prevents the WG from opening unless the EBC wants it to be open. The boost pressure is monitored by a pressure sensor connected to the EBC. When the EBC measures a boost level greater than the currently selected limit, it opens that valve allowing pressure to reach the WG actuator, opening the WG. An EBC is able to regulate boost pressure to any level it wants across the Turbo’s entire operating range. This allows a Turbo design to produce higher boosts at lower RPMs (load, actually) without having to worry that the Turbo will produce too much boost at higher RPMs (load, actually).

A mechanically driven supercharger can also use an EBC. But with a Supercharged application, the EBC regulates the opening/closing of the Blow Off Valve not the waste gate. This allows control of boost in a fashion that's similar to the Turbo EBC (except the turbo EBC regulates the source of the energy that spins the compressor, whereas a Supercharger EBC would regulate the compressed charge that goes to the motor). This allows the selection of a centrifugal compressor that could provide better boost at low RPM, without over boosting at high RPM. Unfortunately, while there are plenty of Turbo EBCs available, Supercharger variants are much harder to come by.

Bottom line: Turbos are often the more efficient and capable systems, and deliver higher levels of performance, but they are mechanically more complex, harder to install, more expensive, and can be less reliable/durable than Superchargers.
 
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