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      04-20-2010, 10:56 PM   #42
rick_vor's Avatar

Drives: 2011 1M, 2008 M5
Join Date: Feb 2010
Location: Lafayette, CA

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Originally Posted by e.n335 View Post
We are all getting older

The ECU calls for 5.5 psi. Thus spinning of the turbos is not directly related to the engine revs the turbos will provide absolute 5.5 psi at sea-level or absolute 5.5 psi at 5000 feet. In the latter the turbos will have to produce relative 8.0 psi due to less dense air. To be able to provide the same absolute boost pressure at different altitudes the wastegate is used. So 5.5 psi absolute boost pressure at sea-level and 5.5 psi absolute boost pressure at 5000 feet.


The Supercharger is driven by the crankshaft and has no wastegate. So it provides always the same relative boost at a specific rpm. Using a 8.0 psi pulley will provide 8.0 psi absolute boost at sea-level or 5.5 psi absolute boost at 5000 feet. That's the difference compared to turbocharging when we look at boost only.

BOV's / DV's:

Blow off valves or diverter valves are used both on turbo- and supercharged systems. They don't control boost pressure, they just blow off the boost spike in the air intake manifold when you close the throttle ( 8 throttle bodies in case of the S65 ). The difference between BOV's and DV's is where they vent to.

Anything missing ?

Longtime Troller, 1st time poster...

Spot on. To reflect on my own experiences, I have a 2005 GMC Sierra that has been both Supercharged and Turbocharged through its lifetime. Both systems were built for 10lbs of boost at sea level of a 6.0l V8. When driving to Tahoe, CA (6k' above Sea level) from the SF Bay Area, the Procharger would drop down to about 7 lbs max of boost. When I replaced the Procharger with twin Garrett GT3071r's with no other changes to the motor, I still pull the same 10lbs of boost at Tahoe that I would see at Sea level.

2011 E82 1M - AW - loaded
2008 E60 M5 - Interlagos Blue - SMG - loaded
2008 E90 M3 6sp - Jerez Blk/Blk - sold
2003 E46 330i ZHP Sedan - Stock - sold
2005 GMC Sierra Denali - 6.0l - Twin Garrett GT3071r's - smokes all the Beemers...