[Gmecm] a question of timing

davesnothereman at netscape.net davesnothereman
Sat Mar 18 16:51:58 UTC 2006


Although it's a little esoteric, the signal from the distributor does 
provide crude crankshaft position information.  Every 90 deg of crank 
rotation (on an 8 cyl) there's a "piston pair at TDC" signal.  It's 
accurate enough for the ecm to calculate the correct time to deliver a 
pulse back to the ignition module for correct spark timing.

You can adapt the distributor to include a separate signal generated 
when the cyl 1/6 pair (on a GM v8) are at TDC to act as a synch signal 
for a distributor module.  Or you can use a sensor on the crankshaft to 
generate this information.

I think everything else was covered... there's only 4 pulses per 
crankshaft revolution for an 8 cyl engine.  Most tachometers expect to 
see this signal, and I'd guess your ignition system will produce a 
similar signal.

Although the aftermarket ignition system has a spark curve, you might 
want to consider using the gm ecm to generate your spark pulse.  The 
ecm may be more flexible in terms of programming.  I'm not sure why you 
want to eliminate the knock sensor... it can be a useful safety 
feature.  But it can be easily disabled in the ecm by adjusting a 
constant or two.  The factory ecm also has multiple spark tables for 
various grades of fuel, which it will automagically select.  These 
tables can be handy in a vehicle doing street/strip duty as one table 
can be programmed for street fuel, another will have additional spark 
advance useful with race fuel.  The downside is that you need knock 
sensing active to enable the "high octane" spark table.

In terms of signal sources, splitting the signal from the distributor 
between the ecm and ignition system may not be desirable.  You might 
consider feeding the signal from the distributor to the ecm, and 
feeding the spark output from the ecm back to the aftermarket ignition. 
  You'll want to lower the rpm at which the ecm "takes over" ignition 
pulses, and you'll need to use the maximum advance and retard limits to 
"lock" the ignition timing, sort of like a vacuum advance lockout on an 
old HEI distributor.

I would love to see some pictures of your Buick EFI system.

Zaphod


-----Original Message-----
From: Don Brown <gs455tpi at comcast.net>
To: gmecm at diy-efi.org
Sent: Sun, 12 Mar 2006 08:15:33 -0700
Subject: [Gmecm] a question of timing

  My question below is about the reference signal.  The application is

buick 455
90 corvette TPI (vin 8)
electromotive hpv1 dis


The tpi to buick is bridged by an adapter that connects the two.

My corvette manual comments that the reference signal provides both rpm 
and
crankshaft position.  It doesn't say what the amplitude of the 
reference signal
is.

Since the hpv1 has it's own advance curve I'm going to use it for the 
timing.
But can I use the tach output from the hpv1 for the reference signal 
into the
ecm?

My original plan was to just plug the distributor hole with a fixture 
that would
still turn the oil pump.  But as I was thinking (a painful process for 
me), the
tach output is not going to be 8 pulses for each revolution of the 
distributor
that the distributor would provide.  I have since modified a 
distributor to
provide the reference output needed by the ecm.

I think that I've answered my own question here but some comments would 
be great
since I don't think of everything.  I believe that I'll need the 
reference
signal from the modified distributor to give both tach and crank 
position to the
ecm (16197128) since the tach signal from the dis won't be giving crank
position.  I'll still be able to use temperature and map to adjust fuel 
delivery
and will program the ecm to work with the dis

A little additional thougt tells me that If I use the curves in the 
dis, I don't
need a knock sensor.  And hopefully I can adjust the hpv1 to eliminate 
the need
for it.

Does anyone know what is the amplitude of the reference signal is?

Thanks in advance for all comments.
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