Blow off valve control

[Jörgen Karlsson]

> So, I want to learn more:
>
> 1:  How does the sudden loss of boost in the system affect the AFM, and
> give a rich mixture? I have to find out *which side* of the AFM on a
> car with a swinging gate type AFM the boost is vented by a standard
> "closed circuit" BOV. I have to guess it is to the air filter side, as
> otherwise the sudden rush of boosted air into the inlet could / would
> slam the gate against its stop, probably damaging something.

The air _must_ be released after the air meter, other wise you can just as
well release it in the engine bay. Once the air has passed the air meter it
must enter the engine, otherwise the mixture will be off.

But as I said earlier hot wire or film type air meters will not react fast
enough to notice a gear shift, the fuel is mostly controlled by the manifold
pressure sensor and the throttle position sensor during shifts. That means
that air can be released to the atmosphere without a resulting overly rich
mixture. This does not seem to be a problem with some swing type air meters
either, my car has a swing type sensor and I have no black smoke during
shifts.

> *BUT* , on the Skyline, the boosted air is vented by the BOV's to the
> *turbo* side of the AFM's. In this case does the hot wire type AFM read
> the air going through it the opposite way to normal air travel? Not
> having a swinging gate to slam shut maybe it doesn't matter if boost is
> vented to either side? It just flows either way?

The meter is still no fast enough to respond and the engine is still
consuming huge amounts of air with closed throttle at high rpm. The turbo
takes the recirculated air instead of the fresh from the air meter.

> I had one e-mail from someone suggesting I disconnect and plug the vac
> pipe to the BOV altogether. This wouldn't work. OK, at idle the valves
> would be shut, but as soon as enough boost was created to push against
> the BOV's valve head harder than the internal soft spring could
> control, boost would bled either to atmosphere, with the valve
> disconnected from its trunking to the turbo inlet, or even if it was
> ducted to the turbo inlet, as standard, boost would be lost to the
> intake box. The BOV needs to see a pressure signal under boost to stay
> closed...

I have a soloution for this, if you put a restriction in the hoses from the
maifold to the BOVs and on the BOV side of the restriction you install a T
with a one way valve that lets air from the outside enter the line freely.
This way the valve will never see any vacum but it will still see boost
pressure. This will give the same results as the solenoid soloution you
mentioned but it is a lot cheaper, I would guess $10 instead of $100.

Do you have any performance figures for the car with the tweaks you have
done?

Jörgen Karlsson
Gothenburg, Sweden.

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