DFI Revisited

[Howard Chu]

I hate to be a pain in the neck, but I've just spent 3 hours searching the
archives on the web site and came up emptyhanded. Do you remember a
year/month when Orin's message was posted? I've seen some references by
Garfield, but no hard numbers/specs. Also, is the message on this list, or
the gmecm list, or somewhere else? I've searched this list and the gmecm
archives already... My search may not have been exhaustive, but I am
exhausted...

To Bruce Roe: I don't actually know when to fire the plug. My intention was
simply to demultiplex the Spark signal from the stock ECM out to six
individual coils. The ECM decides when to fire, I just want to relay the
command. To recap, my original idea was to mount six Hall sensors inside my
distributor cap, with a magnet on the rotor to trigger them. A given sensor
would turn on a transistor, which would conduct the ECM's spark signal out
to the selected coil. There was to be no intelligence built into this setup
at all. (The sensor output is momentary, so I intended to route to a
flipflop to latch the signal. Each flipflop would be reset by the following
Hall sensor output.) In other words, the stock ECM still runs the show, all
timing etc. controlled there, I just route the signal. This will only work
as-is if the coils can fully charge in the available amount of time. I'm
dealing with a V6 engine, so there are 60 degrees available, but that
amounts to only 2.xx msec at the top end for this engine (stock fuel cut is
at 7500rpm, I have a tendency to run up to 7000rpm when I'm excited...).

I don't have exact specs for these coils, I don't know how long they really
need to charge. I've seen some posts talking about GM DIS coils charging in
only 1.5ms, which would leave me plenty of breathing room. I've also read
some specs on the Delphi web site showing some of their coil-near-plug coils
taking as long as 4.4ms, but I can't identify what vehicles use those coils:
http://www.delphiauto.com/pdf/eandepdfs/Ignition/CPC.pdf

It occurs to me that with a real direct ignition setup, you don't need
ultra-fast coils because they have so much time available between firings;
twice as much time as DIS and 4,6, or 8 times as much as a single-coil
setup. Of course, that requires an ECM with 4,6,8 outputs that knows to
start charging each coil in advance. My ECM doesn't know this, and I can
certainly set up a microcontroller to take care of it, but it would be
simpler if I didn't have to.

  -- Howard Chu
  Chief Architect, Symas Corp.       Director, Highland Sun
  http://www.symas.com               http://highlandsun.com/hyc

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