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      04-12-2010, 10:03 PM   #21
e.n335's Avatar

Drives: e93 ///M3 DCT, 07/2009
Join Date: Feb 2007
Location: Switzerland, ZH

iTrader: (0)

We are all getting older but I can recall we discuss about supercharging at altitude .

Back to Bob's scuba-diving: If you fill a bottle @ sea level up to 220 bar you will get 220 bar of pressured air with 20% oxygen and 80% nitrogen. Filling the same bottle at 5000 feet you will get also 220 bar of pressured air with 20% oxygen and 80% nitrogen, you just have to apply a higher compressor pressure ( + 2.5 psi ) to achieve the 220 bar in the bottle.


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 ?