200T Head Gasflow?

[Phil@isham-research.demon.co.uk (Phil Payne)]

In message <199801141255.HAA24015@camel14.mindspring.com> audidudi@mindspring.com (Jeffrey J. Goggin) writes:

> >John Robinson has gasflowed his MB engine and finds that performance has
> >_suffered_ as a result.  If you put more air in, ya gotta put more fuel in to
> >keep the mixture right.  That's basically controlled by the air mass sensor
> (up
> >to a point) and then by the ECU.  How do you tell the ECU that more air is
> >going in for a given boost level?  The ECU is not connected to the air mass
> >sensor.
>
> On CIS-equipped cars, the fuel-to-air ratio is self-adjusting, up to a point
> ... in my experience, most Audis respond well to mild porting work (really
> nothing more than cleaning up casting flash, match-porting gaskets, etc.)
> without making ANY changes to the fuel system.  If the head is apart anyway,
> it's kinda silly NOT to do this as the only downside is paying someone to do
> it if you're not capable of doing it yourself...

I think it's the 'up to a point' that's the issue at present.  Scott
Mockry and I have been playing around with air mass sensor potentiometer
readings that tend to show air plate deflection ceases to be a useful
input at the normal design max bhp point of these engines.  From there
on, you're flying blind on ECU maps driving the fuel frequency valve.

One interesting 'blip' - it seems impossible to use max/min meters,
because to do the test you have to drive the system into the fuel
pump cut-off logic.  At the point where the fuel pump cuts, system
and control pressures cease to exist - the sudden lack of control
pressure counteracting the air mass sensor's displacement seems to
fling the paddle to the farthest point of its travel.

Another major surprise - though it may be coincidence - is that the
two MB engines so far measured have air mass sensor potentiometer
outputs that are _identical_ within the accuracy of the DVM I've been
using.  Did Audi calibrate this device - and if so, what the &$@# for?

--
 Phil Payne
 phil@isham-research.demon.co.uk
 Phone: +44 385302803  Fax: +44 1536723021