Re: ignition timing question

[Mark Battley <M.Battley@xxxxxxxxxx>]

Allen Mitchell wrote:
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I have a some interesting info about 86 GTV6 ignitions:
As you know, the later GTV6's had a Bosch "digital electronic ignition."  
This was supposed to control ignition advance, inresponse to various inputs.

  These inputs are, specifically, RPM, temperature, and the throttle switch.

  However, if you've ever looked at your distributor, you will notice that 
there is ALSO a vacuum line going to the throttle body.  If you apply vacuum

to this line, the timing advances.  If you disconnect this line, and run up 
the RPMs while checking the timing, you will find that the spark advances 
independently of the vacuum capsule.  So the ignition box DOES advance the 
spark.  The distributor seems to be good for about half the advance, and the

spark box does the rest.  Together, they advance the spark to about 30 or so

degrees above 3000 RPM.  All the advance is in by then.  The temperature 
input to the spark box was subject to an Alfa tech bulletin advising 
disconnection.  The throttle switch retards (as I recall) the spark about 2 
degrees at idle.  Does anybody know why they used both vacuum and electronic

advance systems???
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I'm not familiar with the GTV6 system, but the vacuum advance sounds like
the common system that has been used on mechanical distributors for many,
many years. As I understand it its purpose is to advance the ign timing
beyond the dynamic curve (done by weights and springs in mech dists, or with
electronics more recently) when on light thottle openings which give high
vacuum. Once you put your foot down the vacuuum drops and the timing returns
to the basic curve.

On light throttle the engine can cope with more advance than at larger
throttle (when it would pink/knock), the extra advance gives a little better
efficiency/economy. It doesn't make any difference to maximum power, which
is probably why it was not as common on  (1960/1970s)Italian engines than
say British  :-))

It sounds like the GTV6 system was a bit of a hybrid mech/elect system -
more recent systems (mid/late 80s onwards?) use a vacuum sensor as part of
the computer to adjust the timing map.

Mark Battley
Auckland, New Zealand

1973 Alfa Romeo 2000 GT Veloce (no vacuum advance)
1989 Fiat Uno Turbo (vacuum/boost pressure sensor in ign ECU)
1999 Fiat Marea Wagon, 2.0 20V (who knows... a version of Motronic that does
all sorts of clever things..)
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