Blow-off Valves

[Dan Cordon]

> 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. It depends entirely on the fuel injection system. With
electronic fuel injection there's two primary ways used to decide on the
amount of fuel to inject. One system uses manifold pressure/vacuum to
approximate the amount of air entering the system, then injects fuel
accordingly. In this system the airflow isn't measured at all, but a
system of tables are used to correlate manifold pressure and airflow
rate. This is done by the MAP (manifold air pressure) sensor. This is
*not* the audi system (at least not the older ones -- I know nothing
about Audis newer EFI systems).

The other common way is to use some sensor to measure the airflow
through the intake. On most domestic vehicles this is done by a MAF
(mass air flow) meter. On the early audis (CIS) it's done by the flapper
valve under the distribution block. In either case the sensor monitors
the amount of incoming air and adds fuel to keep the air/fuel mixture as
desired. The advantage to these systems is that small modifications are
adjusted for by the ECU because it's still measuring incoming air. Mild
cam changes, or boost changes work great, where they usually won't in a
MAP system. The drawbacks to this system is that the flow meter is
usually the main restriction in the system, so changes to the engine
airflow usually don't help much. The second problem is that because the
measurement is usually done far upstream from the cylinders and intake,
any "extra" air sneaking by will not be measured and result in a lean
condition in the cylinder. Any leak in the system between the flow meter
and the cylinder (joints, gaskets, etc.) will be problematic.

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