Blow-off Valves

[Brady Moffatt]

Hi Dan/John/Bob/Joe/Steve/All.....,

I'm still confused, for a few reasons!

I just bought an 83 UrQ (woohoo!) with a BOV, pssshht and all, and no black
puffs of smoke occur.

The conditions in the combustion chamber should be very similar behind a
closed throttle, BOV or not. The fuel metering takes place at the beginning
of the system, ahead of the turbo and throttle plate and it doesn't know
that the throttle has been slammed shut until after the fact, so it should
cause a rich condition regardless of BOV.

I understand that the same fuel with less air=richer mixture, but there is
definitely a time variable here. Immediately after the throttle is closed,
there is a momentary rich condition no matter what. It may be a bit richer
with the BOV, but when you get back on the throttle, due to the faster
spinning turbo (with BOV) shouldn't the extra air take care of the slightly
rich mixture from the BOV "event?" IOW, with a BOV, once back on the gas,
the mixture "recovers" more quickly. Without the BOV, you get the momentary
over-rich condition when the throttle closes, but then you lose lots of flow
due to the pressure back against the turbo. Open the throttle now, and you
have low flow, and probably a longer-lasting over-rich condition before
things spool up again.

With a bypass valve, you'll still get the momentary rich condition as the
throttle closes, the faster-spinning turbo as with the BOV, but instead of
venting to atmosphere, the trapped air does another "lap" around the turbo.
So, instead of losing that metered air, its journey to the combustion
chamber is just delayed, right?  Might that not still cause a momentary rich
condition?

So, worst to best on a CIS Turbo app: nothing, BOV, BPV?

Is all this bunk or does it make sense?

Cheers,
Brady

----- Original Message -----
From: "Dan Cordon" <cord4530 at uidaho.edu>
To: "Brady Moffatt" <bradym at sympatico.ca>; <quattro at audifans.com>
Sent: Saturday, April 05, 2003 3:19 PM
Subject: Re: Blow-off Valves


> > 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?
> >
>
> Kind of.

<snip>
> So, to your question.... since the audi system is a flow meter system,
> it is highly effected by leaks. However, a BOV is kind of like a leak in
> the opposite direction. So instead of running lean (because of the
> engine ingesting un-measured air) it will run rich because of releasing
> measured air. In any CIS system with a BOV, just after shifting (and
> hearing the "pppphhhffffff" sound) you will see a large puff of black
> smoke come out the tailpipe. This is because the fuel metering system
> thought the engine was still intaking lots of air, when it was actually
> not. Thus the engine was fed lots of fuel and out came a puff of black
> smoke.
>
> So, for airflow metering systems it's common to use a bypass valve. This
> is just like a blowoff valve except instead of sending the air between
> the turbo and throttle to the outside world, it diverts it back to the
> turbo inlet (after the airflow meter though!). IIRC the later audi 5 cyl
> engines had this, and it's a common add on for the earlier 10v engines.
>
> Hope that helps some. Thanks for the bandwidth!
>
> --
> Dan Cordon
> Mechanical Engineer - Engine Research Facility
> University of Idaho


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