[s-cars] blowoff valve questions

[Achint Sandhu]

I'll prefix this entire post with - I think it works this way. I hope the
others will jump in and correct me if I am off base...

As I understand it, the MAF measures the amount of air entering the system
through the airbox and reports this number of the ECU. The MAF by itself
does not do any compensation for re-circulated air etc.

In the case of a bypass scenario, air from the turbo compressor outlet is
re-routed back to the compressor inlet. Given the way that hoses are
attached to the compressor inlet and the fact that the re-circulated air is
under pressure, it is much easier for the turbo to suck in the re-circulated
air than it will be for the turbo to get "new" air from the airbox.

Hence in the case of re-circulation, the MAF will report less air entering
the system while the throttle is closed.

In the case of the blow off, the MAF will report the same amount of air
entering the system even when the throttle is closed.

That having been said, autospeed editor Julian Edgar has "created" a blowoff
setup for his audi. Details at: http://www.autospeed.com/A_0344/page1.html

I suspect that the ECU can compensate for small "leaks", but I'm not sure
about large ones...

I have no idea how the systems on other cars work, so I can't comment on
that. I think that the Subaru WRX (old style) used a MAP as opposed to a
MAF, but I don't know if that has anything to do with the blow-off valve....

That's my $0.01...... (I can't claim $0.02 since I don't know enough ;-))

Cheers,
Achint

-----Original Message-----
From: Chris Covington [mailto:malth at umich.edu]
Sent: Monday, June 18, 2001 11:38
To: Sandhu, Achint [SKY:QT77-M:EXCH]
Cc: s-car-list; 200q20v; quattro
Subject: RE: [s-cars] blowoff valve questions


That seems to be the consensus, but wouldn't the MAF just compensate for
the fact that the air is gone?  Doesn't it measure to make sure that the
bypassed air is back in the system before it enrichens the mixture, etc.?
Or does it always use an assumption that X amount of air will be rerouted,
post MAF?

Chris
'91 200q20v


On Mon, 18 Jun 2001, Achint Sandhu wrote:

> 	There is a major problem with using a Blow Off Valve instead of a
> bypass valve in our cars and this relates to the fact that any air
entering
> the turbo has already been metered by the MAF. The ECU is expecting this
air
> to be somewhere in the intake system.
>
> 	If the Bypass is replaced with a Blow off valve, metered air will
> get lost and the ECU will assume that there is a leak somewhere in the
> system.
>
> Cheers,
> Achint
>
> -----Original Message-----
> From: Chris Covington [mailto:malth at umich.edu]
> Sent: Monday, June 18, 2001 10:30
> To: s-car-list
> Cc: 200q20v
> Subject: [s-cars] blowoff valve questions
>
>
> 20VTers,
>
> Has anyone here ever run a blow-off valve instead of replacing their BPV
> when it blows?  I see that many manufacturers use those from the factory
> (Mitsubishi, Nissan, Mazda) and this could help eliminate our blown
> hoses, etc. issues with chipped cars and/or upgraded turbos, since the
> pressure is relieved instead of being re-routed.  It seems that
> Audi never has though, I assume even with the RS4, RS2, etc.  Thoughts?
>
> Chris
> '91 200q20v IA3
>
>
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