[Diy_efi] VAF MAF

[Alexei Pavlov]

brian at dessent.net wrote:
> 
> Alexei Pavlov wrote:
> 
> > There is no 'accelerator pump' effects with the Bosch MAF (VAF).
> > The flap follows the flow rate, that's all. If you see spikes
> > on the MAF output, that means your intake manifold needs more air.
> >
> > If you replace a flap MAF with a hot-wire/film MAF you need
> > introducing a temperature/ambient pressure compensations.
> 
> I have a vane-style airflow meter, and I've driven around with a scope
> attached to its output.  It indeed does have some overshoot, especially
> on transitions such as rapidly opening the throttle from closed at
> higher RPMs.  It's an underdamped system with fair amount of overshoot.
> I don't know how exactly the ECU uses this fact, but it's there and I've
> seen it mentioned in documentation about this particular engine
> management system.

That kind of responses would be observed with any MAF or MAP sensor.
This is due to the fact that the intake manifold is a physical unit
that has a finite volume, and the air takes some time to change
the filling level of that volume when throttle position is changed.
To be exact, there is a delay between the air flow and the pressure
level in the intake systeme.
This is something described in any motorbook source (like
Heywood's book). This is also the reason why we distinguish between
steady and transient engine modes. Without this phenomena the static
3D ECU maps would work 10 times better, especially in pressure-n
systems.
The Bosch flap MAF has in fact 2 flaps: the main one and the
'antidump' one. I've compared a flap and a hot-film MAFs and
believe me, they both react in the same way.

Alexei

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