quattro digest, Vol 1 #4806 - 5 msgs

John Forbes John at craincorporated.com
Sat Apr 5 16:46:42 EST 2003

Its the way CIS works. On a non-CIS car it doesn't effect the mixture at all. In your CIS system there is a plate which air flows past that measures the ammount of that air. More air = plate moves farther = more fuel. In the stock setup with no BOV when you close the throttle less air flows past the plate (obviously) because the throttle is closed. When you add in the BOV however the air still flows because even though the throttle is closed the turbo is still spinning and the BOV is venting the air allowing the air to flow, which the plate thinks is air flowing into the engine. What happens when the plate sees all this air moving and thinks its going to the engine? It assumes the throttle is open and it sends fuel to match, which is why it loads the motor full of fuel.

Incidentlly I have a BOV valve question too though. I installed a BOV last night on my 5ktq  (removing CIS this week and going to EFI) and I'm getting some odd results. Whats happening is the BOV is "lagging", as in it doesn't immediately open when I lift, it takes a few seconds before it opens. I'm thinking theres 2 possible reasons. The first is I installed it in the inlet side of the IC (if you take a look at one you will see a 25mm circle spot that looks like a plug in the IC, I drilled that out and put a fitting there), so maybe it takes some time for the pressure to build that far back in the system before it has enough pressure to open the BOV. Idea 2 is that the Michelin Man hose is doing its job before the BOV has a chance to. Which of them seems right? The Michelin Man is comming off this week too, so I guess I'll find out soon enough.

Pics of the BOV install are going to be put on my site later tonight if I have time.

-Cody Forbes
 '86 5ktq (waiting on EFI injector inserts, ETA Tuesday, then EFI is go!)

Original message:
Message: 5
From: "Brady Moffatt" <bradym at sympatico.ca>
To: "Steve Sears" <steve.sears at SHOESsoil-mat.on.ca>,
<quattro at audifans.com>
Subject: Blow-off Valves
Date: Sat, 5 Apr 2003 14:22:44 -0500

Hi Steve/All,

I'm starting to learn about all this turbo stuff...

>From what I understand, the BOV releases pressure in the intake between the
turbo and the throttle plate when the throttle is abruptly closed and the
pressurized air has nowhere to go. The air "bounces" off the closed throttle
and goes back to the turbo, slowing it significantly, and accelerating wear
on the turbo. With a BOV, the air escapes through it, and does not slow the
turbo as much.

But, BOV or no BOV, the bulk of the air isn't getting past the closed
throttle. So I don't see how the mixture would be affected by the BOV.

Am I missing something?

Brady Moffatt
Montreal, Quebec, Canada
83 UrQuattro with 50,000 miles
86 4ksq with 308,000km
86 4ksq parts car
72 Datsun 240Z with 180,000 miles
92 VW Golf with 200,000km FOR SALE
in middle stages of quattrosis accumulatus

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