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Friday, July 25, 2008

Turbocharging vs. Supercharging

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Turbochargers and superchargers are similar in that they both compress air to higher than atmospheric pressures. Normal or standard atmospheric pressure is about 14.7 psi (pounds per square inch or "psi"). The job of the compressor common to both turbochargers and superchargers is to increase air pressure so that more air is forced into the cylinders ("forced induction"). This increased air volume ("boost") is mixed with a proportionately increased fuel volume which, when burned in the combustion cycle, results in increased horsepower and torque production. However, this is where the similarities between the two types of systems ends.


Power Curves

Because they are belt driven from the engine crankshaft, centrifugal superchargers build boost as rpm increases in a linear fashion. As engine rpm increases, the supercharger compressor speed (and boost level) increases to the point of peak boost occurring at peak engine rpm. For example, a supercharger designed to produce 8 psi at 6,000 rpm may produce as little as 2.5 lbs. of boost at 3,000 rpm. Turbochargers, on the other hand, because they are exhaust driven, come up to speed very quickly (almost instantly if properly sized), and will reach the same 8 lb. peak boost level as low as 2,500 rpm. The result is much more horsepower and torque being produced earlier at lower rpm levels with a turbocharger vs a supercharger.


Just like the air conditioner compressor on a car, all superchargers require horsepower to turn them. This "parasitic" drag is always present, even when the car is being driven normally, and can rob 20%-30% of the power being produced by the engine. The result is a significant decrease in fuel economy and less net power produced. Turbochargers, however, are exhaust gas driven and don't require any horsepower to spin the compressor. When driven normally, a turbocharged car will not consume more fuel and, in fact, gas mileage can actually increase. Even when under full throttle, a turbocharger system will produce as much horsepower at 9 psi as a supercharger at 12 psi.


Both superchargers and turbochargers require high compressor rpm to compress the air. This ranges from 30,000-65,000 rpm in superchargers and can be even higher with turbos (over 100,000 rpm). In order to achieve the high rpm levels required to compress the air to the psi required, superchargers must have a step-up mechanism (gears, belts, pulleys or a combination thereof) consisting of numerous moving parts, to convert 6,000 engine rpm to the 40,000+ rpm necessary to build boost. Turbochargers need no step-up mechanism and have only one moving part, the compressor/turbine wheel assembly. The simplicity of the turbocharger is therefore less prone to mechanical problems. Superchargers must have a belt to drive them, and belt slippage or breakage is a common problem. More serious problems include crankshaft, bearing and engine damage caused by belt tension forces on the crankshaft. Turbochargers have no belt and no direct mechanical connection to the crankshaft, thereby eliminating these problems. It is interesting to note that many automobiles and nearly all large over-the-road trucks use turbochargers that regularly log in excess of a million miles of reliable performance.


Some superchargers have a separate lubricating system that must be maintained, but turbochargers are lubricated by the engine oil and require no additional maintenance beyond what is normally required for a naturally aspirated car.


Superchargers are always connected to the engine, they are always producing some level of boost and cannot be "turned off". Because turbochargers only produce boost when under load (as in full throttle acceleration), performance under normal driving conditions is no different than if the engine were naturally aspirated. Turbocharged cars exhibit excellent driveability characteristics.

Upgradability and Adjustability

Superchargers are generally not upgradeable. When higher performance is required beyond the capabilities of a specific supercharger system, the entire system must be replaced. Turbocharger systems, however, are usually upgradeable by simply upgrading or installing a larger turbocharger without requiring replacement of the entire system. Further, adjusting the boost levels on a supercharger requires removing and replacing pulleys, idlers and belts. Adjusting the boost levels on a turbocharger may be accomplished with a simple turn of a boost controller knob from the comfort of the inside of the car.


What does this all mean? Basically, an 8 psi turbo kit will produce more peak power due to the fact that a supercharger is using a fairly large amount of power just to get it spinning. What is more important for a street car is "power under the curve" meaning the average horsepower produced. This is where the turbo really shines since you can have full boost at as little as 2500 rpm! This will make the turbo car feel like it has 50% more cubic inches (or more). The difference in torque at low rpm's can be as much as 100 lb ft in favor of the turbo due to the additional available that's performance!