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Turbochargers Vs. Superchargers - The Basics

Turbochargers and Superchargers both force more air into the combustion chamber, which, along with added fuel, makes more power. By "more air", I mean more than can be acheived by just the "sucking" of the piston on the intake stroke. The extra pressure they make is commonly measured in psi in North America.

Turbochargers and superchargers both use a compressor or turbine as a sort of "pump" to do this.

A supercharger's pump is driven mechanically by a belt or gears off of the engine's crankshaft. As the motor runs, the crankshaft spins a belt or gears which spin the compressor or turbine, which pumps more air into the combustion chamber, and along with added fuel make more power. Because of this direct link to the engine, superchargers typically produce boost pressure immediately, without "boost lag". Typically, but not always - a supercharger with an overly big turbine or one that is undergeared can suffer from boost lag. Becuase a supercharger is hooked up mechanically to the engine, it uses a small amount of the engine's power to run. So, even though a supercharger might add 100 extra horsepower, it might be using up 10 to 20 horsepower or more just to mechanically spin the turbine. However, no matter how you slice it, you are still getting the 80 to 90 extra horsepower.

A turbo's pump is driven by exhaust gas energy. As exhaust gas leaves the combustion chamber and goes out the exhaust manifold, it is fed into a turbo. The turbo has two separate chambers. The exhaust gas flows into one chamber, causing a turbine to spin, then exits the chamber and out the exhaust system. The spinning turbine is connected by a shaft to the second chamber, where is spins a compressor wheel that pumps intake air through the intake tract and into the combustion chamber. Because turbos rely on exhaust gas energy to make boost, they can sometimes suffer from "boost lag" as the turbo waits for exhaust pressure to build before pressure and power is produced. However, with proper sizing of the turbo and exhaust plumbing, "boost lag" can be avoided. Because a turbo uses exhaust gas energy to spin the turbine (energy that would otherwise be lost out of the tailpipe, unused), it does not use up any of the engine's power the way a supercharger does.

Because the turbo uses exhaust gas energy to make more power, and more power produces more exaust gas energy, in theory a turbo will produce more and more psi in a cycle until the engine can't take any more and blows to smithereens (a bit of an over-simplification). This is why turbos need a wastegate to control boost pressure. When the wastegate senses that the boost pressure has gone over a predetermined psi, it opens up and "leaks" the extra pressure out, keeping the psi at a safe level.

For turbos (and superchargers that are mounted upstream of the throttle body), whenever the throttle is shut, the intake air under pressure from the turbo slams against the closed throttle butterfly. This slows the intake air down and also can back the pressure up back into the turbo. This is not great for the turbo, and also means the boost pressure must be built up again once the throttle is opened again. This is why blow-off valves (BOVs) exist. Blow-off valves "leak" extra intake pressure when the throttle is shut, keeping the turbo spinning for good boost response when the throttle is opened again, as well as preventing the pressurized air from backing up into the turbo. The blow-off valve is what causes the "psssst pssst" or "squeak squeak" sound when you listen to turbocharged rally cars or other cars with turbos a BOVs shifting.

Which is the best system? There are way too many variables to consider, including how the system is to be used (street, drag racing, road racing?), how much money one wishes to spend, emissions concerns, the quality of manufacture, and the details of the engine to be boosted.

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