Turbo sytem flow question.
 

I am 90% sure I know this answer, but just want to clarify. On a turbo motor the intake air whether on boost or not passes through the turbocharger itself correct? I mean the bypass valve simply opens to let excess pressure out, not to bypass the turbo when not on boost right? If I am correct, does this "restriction" have any real affect on low rpm response? Just wondering, thanks.

I take it the Bypass valve is the same as a Wastegate. True?

Yes, it is used to avoid over pressure damage when the throttle plate is suddenly shut as in shifting and letting gas off. The air that is bled through the bypass has already been metered by the Air Flow Meter which goes immediately before the turbo intake on the turbine side.

What restriction are you refering to? I don't see any restrictions.

Yes - intake air does go through the turbo first. Bypass valve opens when throttle plate is closed to prevent the turbo from slowing down.
Effects on low rpm... umm - probably some i guess but then the engine is not sucking a lot of air at that time.

True, the compressor wheel is always spinning I would assume so I doubt it adds any restriction.

lendaddy, you left me thinking, I guess the restriction you are talking about is that at low rpm the bypass valve would open until boost comes up. Well this is not really a restriction because the bypass will open under negative pressure or vacumm. Turbo engines go to positive pressure almost immediately when you get on the gas then takes a little longer to build up high pressure.

The bypass valve bypasses the comp. any time there is high vacume. So any time the throttle is closed the bypass valve is open, bypassing the turbo comp.

No, they are different

The Throttle bypass valve does just that - it bypasses the throttle butterfly during off throttle events eliminating surging that would occur against the closed throttle plate from the still spinning turbo.

The wastegate opens at a specific pressure, such as 0.7bar, to control the engine compression under boost.

Nope, but the bypass description is right on except for it being on the compressor side of the turbo (turbine is the HOT side).

The wastegate diverts exhaust around the turbine side when intake pressure gets high. WG springs and controllers can modify when/how the exhaust bleed happens.

The bypass valve is triggered by vacuum that comes from behind the throttle body and fed to the small nipple. When there is enough vacuum on this line it lets the pressurized air from the compressor recirculate to the cold-side inlet. Some systems use a blow-OFF valve that isn't a good idea on pre-compressor MAF systems because it fools the ECU into believing more air is getting into the engine than there is.

I don't get positive pressure imediately . I'm still under ambient (.8 absolute here) until I get up to around 2000 rpm.

Wow, had to come back and fix this after 4 or 5 posts showed up before I was done typing the original version of this reply :confused:

I think he was talking about the comp. bypass valve which does what you said but it does not bypass the throttle. It basicly opens a valve in the compressor output to relieve pressure

BTW: Here's some BPV info put up by a gung-ho S4 quattro HP junky.

Look for the two links about Diverter Valves
http://www.s4biturbo.com/index.html

thebaer, Thanks for the correction.

Thanks for the clarification.