[Diy_efi] Re: Bosch L-Jet AFM question...

[Alexei Pavlov]

cice at trub.com wrote:
> 
> Alexei Pavlov wrote:
> > I'm really suprised by that product. They state they do elevation
> > compensation.
> 
> Yes indeedy.  There is a pressure transducer visible inside the signal
> conditioner.

OK

> 
>  > The problem is that the original ECU ALSO does this
> > compensation (by using some not stupid tricks). To connect a MAF
> > to an AFM based ECU one must de-compensate the MAF output, and for
> > that he/she must know not only the air temperature, but also atmo
> > pressure. And they don't state any usage of pressure gauges.
> > Moreover, some cars (like Porsche) use pressure switches mounted
> > apart the AFM, so they KNOW the atmo pressure.
> 
> Not that I'm an expert, but I doubt that this older system (Fiat/Lancia
> circa 1981) did this kind of elevation compensation inside the ecu or
> anywhere else.  I have personally built this installation (currently
> vane airflow), and there are no pressure sensors anywhere and I doubt
> there's one inside the ecu.  I could open one up and look visually as I
> have a spare ecu out of the car.
>

Bosch have been using reference tables for 2 known positions: idle
and WOT. When one of these positions was detected the real measure was
compared to the reference one to obtain the pressure correction that was
then applied to subsequent measures.

Porsche have been using external pressure switches (1000m reference).

 
> Air temp obviously has the air temp sender inside the afm, but the
> signal conditioner sends a constant 70deg F.  I *could* hook the
> original air temp sender up if I wanted to, but I doubt it's necessary
> as the MAF is already temperature compensated just due to how it works
> (ie. wheatstone based on temps).

Just keep in mind that volume measures of the AFM does depends on
pressure and temperature, not only volume. The ECU does compensation
job and need a not compensated measure. And the MAF measure is
compensated.


> 
> Again, the goal to measure actual air mass not just volume (and in my
> case, remove the intake restriction [rumor has it ~12% power hit] and
> prepare for a turbo).

My measures show 5% torque gain at all RPM on an atmo engine (2L AFM
on a 1.8L engine). If the AFM is undersized on a turbo engine that
could make a higher gain.

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