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In Reply to: Emissions are identical, see more... posted by RussC on April 25, 2002 at 15:59:43:
The turbo is going to sap a lot of heat from the exhaust, and a single cat in the usual place is going to take a long time to get hot - causing A LOT of cold start emissions problems.
This is the reason for the cat between the head and the turbo - cold start emissions. This is also the biggest detriment to the WRX power output.
With a supercharger, you're not sucking heat from the exhaust system, so you can get away with a more traditional cat setup.
Also, roots and screw superchargers are FAR less thermally efficient then the compressors on turbochargers, and create much more heat in the intake charge then a turbocharger at the same amount of boost.
Centrifigal superchargers (basically a belt-driven turbocharger) are much closer to turbos, but generally lag behind the compressor design in current turbo technology, and are generally not quite as thermally efficient.
Both systems SHOULD use intercoolers, but Turbos have no more need for an intercooler then a supercharger at low (5-8 PSI) boost.
Turbo kits generally USE intercoolers because it is alot easier to do. You can use an air-to-air intercooler and not have to worry too much about how restrictive the core is, like you would have to do with a roots or screw supercharger. (Again, a centrifigal SC is set up much like a turbo).
Also, using an IC with a roots or screw (I'll start saying 'positive displacement') SC increases throttled volume, as you have to have the throttle body before the SC, not after, as you would on a turbo or centrifigal SC.
As far as cost and simplicity, on an OEM basis, they're quite similar.
Much of this is moot, since nobody would (or probably even could) put a positive displacement SC on an M5.
But, I don't know of an OEM that is using a centrifigal SC at all. All that I know of are using positive displacement pumps (most are using Eaton/Magnusson roots pumps and a couple are using Lysholm screws (like the Mazda Millennia)).
My arguments are from an OEM basis, which is where this thread started.
From an aftermarket basis, YES, a SC is going to be easier and cheaper. But it is still by no means more efficient, it will produce a higher intake temp then a turbo.
I don't have a lot of experience with Centrifigal SC's (as most of my forced induction experience is on small displacement 4's, and a centrifigal is a joke for that application) but have a fair amount of experience with roots/screws and turbos.
Once the forced air gets to the cylinder, the systems(exhaust wise) are completely the same. How the forced air gets there is different. Whether the WRX had an SC or turbo has nothing to do with exhaust set up. The suby would need that exhaust regardless of forced induction type.
This two biggest factor for in order SC is 1:cost, 2:simplicity, plain and simple. SC systems cost less and are simpler to install. You can get away with good power gains and not have to use an IC. Turbos almost always need a IC as the heat added to the intake is higer.
RussC
turbos are more effiecient by far than SCs. And can always make more boost. SC are primarily used now because of the exact reason you mention, cost.
I was under the impression that the reason SC's are beginning to be used more on OEM installations now is because of Emissions.
It is VERY difficult to get turbo cars to pass emissions requirements - view the WRX as an example:
Cat between manifold and turbo.
Cat between turbo and downpipe.
Cat in exhaust.
But, yes, Turbos are far more efficient then superchargers, especially the roots-type SC (eatons) that are mostly being used in OEM applications.
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