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----- Original Message -----
From: "Jim Adney" <jadney@vwtype3.org>
> Sorry, I don't think that was a particularly good explanation. I do much
better
> with sketches and graphs.
I know what you are saying.I can visualize it well enough,I think.
How about this:
" Special Cases: I only have one specific case, the introduction of
the 2.0L motor for the 914 in 1973. It's clear that this motor had a
different speed correction curve than the 1.7L, but the motor was introduced
using the 1.7L's ECU. My conjecture is that Bosch and/or VW-Porsche had not
completed the development and design of the 2.0L ECU, so they had to go with
the 1.7L's. Higher flow rate injectors were used to account for the basic
mixture difference, but that still didn't handle the changes in the Ve
curve. To accommodate, Bosch made a slight change in the MPS - they tweaked
it to have a richer full-load response, and they changed the resistance
characteristics of the head temperature sensor. First, they changed the
set-point of the sensor at 20 C from 2.5 K ohms to 1.3 K ohms, and added a
ballast (static) resistor of 270 ohms in series. This brought the cold
engine (sitting overnight, ambient temp 20 C) resistance to 1.57 K ohms,
considerably leaner than before - this was usually dealt with by running the
idle mixture richer, but '73's were always a bit harder to get going when
cold. When hot, however, the ballast resistance kept the hot resistance
value considerably higher (richer) than would the standard sensor. The
richer mixture produced good part-load and full-load response, even with the
more choked-off Ve response of the 1.7L ECU. Note that Automobile Atlanta
has sold a "hot European setup" for many years - a '73 MPS, temp sensor,
and ballast resistor. Gives you a richer part-load mixture for more power -
oh, and more emissions and higher fuel consumption, too."