I'm thinking about  replacing the ancient fuel  injection system on my 86
with one from a 2003 Subaru, and replacing the ancient 2-valve pushrod heads
with more modern 4-valve SOHC heads from the same car, not to mention a much
more efficient compression ratio, etcetera!

On 2/27/06, Geza Polony <gezapolony@sbcglobal.net> wrote:
>
> I've been trying to tune my '84 Westy, and reading about others' attempts
> to
> do the same, for a few months now, and I've come up with a question I
> can't
> answer. It regards the input of the various sensors to the ECU and the way
> the ECU determines fuel/air mixture.
>
> Let's say you have two identical, 22 year old Vanagons next to each other,
> and you're trying to tune both. The voltage from the sensors is going to
> be
> different for the same conditions, just because of age and manufacturing
> variability. So at 950 rpm at 185 degrees F (or whatever) the voltage from
> the AFM, the O2 sensor, the Temp II, the intake air temp sensor, and so
> forth, are going to be different from van one to van two.
>
> There's no way around this, if only because you can't get "correct"
> resistance specs for the potentiometer in the AFM.
>
> In practise, to set the FI mixture, mechanics put a sniffer up the exhaust
> pipe and turn the AFM screw, etc., until the readings are within specs.
>
> But this doesn't really tune the engine at all temps, RPMs, loads, etc. It
> just sets it for one condition.
>
> The sensors in the two vans are going to be putting out different voltages
> for 4150 RPM, 190 degrees F water temp, 85 degrees F air temp, open
> throttle, etc.  So the ECU will be sending a different mixture to the
> FI's,
> FOR THE SAME CONDITIONS.
>
> This leads me to believe that the ECU is in reality a relatively crude
> device, with huge margins for error from the sensors. Otherwise, the vans
> just wouldn't run. Or they would run at one set of condition, but not at
> another.
>
> This in turn leads to relatively inefficient operation, in terms of engine
> output and gas mileage.
>
> The variability of the sensors's output, take together, also makes these
> cars exceedingly difficult to tune accurately. And this isn't even taking
> into account the ignition!
>
> Am I on track with my thinking here? Is there something I'm missing about
> the fuel management system?
>
> Thanks for reading
>
>
> Geza
>

--
Jake
1984 Vanagon GL
1986 Westy Weekender "Dixie"
www.crescentbeachguitar.com