Turbocharing vs Supercharging
This posting of mine was taken from my response on a www.bimmerforums.com 7-series thread regarding the differences between a turbocharger and supercharger in relation to a V12 engine. I am posting it here because the general concept applies to all engines regardless of size and cylinder count. Hopefully this information will shed some light on the true differences between the two forced air induction methods used to create more power in combustion engines.
"The main difference between a turbocharger and supercharger is the form in which the turbine is spun. A turbocharger uses redirected exhaust gases from the engine to spin its fan while a supercharger uses a pulley and belt attached to the existing pulley/belt system on the car.
For this reason it (usually) takes a turbocharger longer to produce "boost" because it needs to wait for the flow of exhaust gases to spin the turbine fan. By basic reasoning this takes longer because exhaust is the byproduct at the end of the entire combustion process. The supercharger has almost no "lag" because its turbine spins the moment the engine RPMs rise because it is, in a sense, directly attached to the crank.
The general rule of thumb, and I stress general, is this: if you have a small displacement motor that produces little torque it is generally better to turbocharge because you don't directly drain the engines low-end power having an extra pulley and accessory attached that is always running. There is more wear and tear on the engine when running at lower speeds. If you have a high displacement engine with more available low-end torque you can supercharge because the amount of energy the supercharger demands at low engine speeds doesn't hinder the available power of the engine. At high RPMs the effects are the same for both systems.
Once both systems 'spool' up to their configured boost pressure you essentailly get the same result. This is all assuming you're running a turbine style supercharger (resembles a standard turbo). If you decide to run a top-mount twin (or single) screw supercharger, like those found in the 2003+ Mustang Cobra (among other cars), the use of low RPM engine power is the same but the boost delivery is more direct. It is a more direct boost injection because the supercharger is bolted directly to the intake manifold, rather than running boost through an intake tube to a remote location. If you want that "supercharger whine" you would opt for the twin-screw and if you want the "turbo hiss" you would go with the turbo-turbine supercharger, or just a standard turbo.
The reason many manufacturers have been twin-turbocharging their large displacement V12s is a result of space restrictions on the top of the engine and wear and tear during normal driving. With the development of new bi-turbo technology all but eliminating turbo "lag," the space saving and power saving benefits of running a twin-turbo setup are enticing.
Proper bi-turbo setups run two turbochargers. If the engine only has one intake for both banks of cylinders the turbo's are configured to provide the same boost pressure at different RPMs so they compliment eachother and nearly eliminate the dreaded boost "lag." If an engine has two separate intakes, such as the BMW V12, one turbo is used for each bank and configured to boost at at the same time with the same pressure.
When separating the turbos it makes it impossible to have them compliment eachother thus giving in to boost "lag." The partial solution to this problem comes from the displacement of the engine. Because the V12 engines push such a massive volume of exhaust per bank (6-cylinders worth) the turbos can be configured to maximize boost at low RPMs and maintain that throughout the rev range; again, nearly eliminating turbo "lag."
The latest amazing turbo development was made by Porsche and is implemented in their 2007 997 Turbo. It is called VTG (Variable Turbine Geometry) and basically adjusts the pitch of the turbine blades inside the turbocharger in real-time to account for changes in engine speed and exhaust volume. This allows the turbos to produce their maximum boost pressure at almost all engine speeds without compromise. You can be assured this technology will soon be applied to nearly all new turbo systems, eliminating turbo "lag" forever, both in single and twin-turbo configurations.
Wow, that was long. Hopefully you guys read it and become experts on turbos and superchargers."
- Mitch G
posted by Mitch @ 9:40 AM
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