[A4] [A4 venting some hot air, is this a better topic? :o)

Tyson Varosyan tyson at up-times.com
Tue Nov 6 15:12:54 PST 2007

Guys I have been watching this discussion the past few days and its
funny how many rocket scientists are really here.

In pure form, yes running open air BOV is not ideal as it does vent
metered air. Some cars (rather their ECU programs) are more sensitive to
this than others. I have never tried it on an A4 - my 1.8T was a lease
and my 6-cylinder is NA. However, I have ran open air BOV's (many types)
on the Twin Turbo 3000GTs that I've own(ed), which too uses a
draw-through MAS (as stupid as that system is).

To touch a few misnomers:
1. The stock narrow-band sensors are a joke. I would imagine that the
reason A4's get a CEL is because the slight rich condition caused by the
BOV is being mis-interpreted by the ECU as a failed Catalytic Converter.
Watching the output of those sensors on a datalogger is like watching
the scrambled Playboy Channel hoping to see an outline of a boob in all
the static - sometimes you do see something, but you are rarely 100%
sure what it was.
2. A little spike of rich fuel on a shift will not cause your plugs to
fail or your engine to build up any more carbon or explode. Sucking up
an extra rain drop into the intake will clean more carbon than running a
BOV will ever produce.
3. It will burn more fuel, but again the amount is equivalent to maybe
half a cup every fillup.
4. There are valid reasons to run an open air BOV (although 99% of
people running them don't know how to attain them). 

On most cars (again don't know much about the A4 in particular) the blow
off and bypass valves are set very conservatively to account for
variance differences in manufacturing process. Identical cars coming off
the assembly line one after another can have as much as a 5-8%
difference in power when tested, due to variances and tolerances. An
open air system is more efficient at venting air quickly because of less
restrictions in the plumbing and lack of backpressure. The trick is to
find a happy medium where you are venting enough air to prevent backspin
of your turbo impeller while keeping the most amount of compressed air
in your system ready to give power once you are in gear. It is a fine

That being said, on a stock car, the factory valves are usually set well
enough to where spending 1-2 hours on a dyno is not worth the potential
gain. However, once upgrades have been made to the intake plumbing
(anything from the filter to the head) - especially the turbos
themselves, tuning of the valve is critical. Stock valve may be too
restrictive and damage your new turbos or your newly upgraded plumbing
may be able to hold more volume, hence dumping all the pressure like
before could make your car slower.

5. The "cool sound" is a total rice mod. All noisemaker BOVs actually
have a whistle built into them! On many of them (like the HKS SSQV) the
whistle is removable and replaceable to allow the end-ricer to change
the tune of their kazoo. Without the whistle the things don't make much
of a sound and have less restriction while dumping air.

Just thought I'd weigh in my 2 cents to the conversation that has gone
on for about 3 too many days... 

Tyson Varosyan
Technical Director, Uptime Technical Solutions LLC.
tyson at up-times.com
206-715-TECH (8324)

-----Original Message-----
From: a4-bounces at audifans.com [mailto:a4-bounces at audifans.com] On Behalf
Of Brizax at aol.com
Sent: Tuesday, November 06, 2007 2:08 PM
To: thejimrose at gmail.com; caliban at sharon.net
Cc: a4 at audifans.com
Subject: Re: [A4] [A4 venting some hot air, is this a better topic? :o)

because he wants the cool whooooosh sound! :o) you guys ever see the
fake bov 
sound machine on ebay? some eletronic device for nt cars! now thats

> [simplifying] The ecu calculates how much fuel to inject based on this
> signal from the air meter. if your diverter is expelling metered air
out to
> atmosphere, then the ecu thinks there is more air entering the engine
> actual, so it overfuels. aka you're running rich. no bueno. will the
> blow up? probably not. is it ideal? nope. so why run it that way when
a 710n
> is 30$ from the dealer?
> oh and sorry, i should have qualified.. "any good manufacturer, that
> try not to sell you the wrong part". because "works" and "right" are
not the
> same.

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