[Gmecm] Big cap HEI on 7747

[davesnothereman at netscape.net]

I think it makes more sense if you replace "cam" with "crank" in Ludis' post.  Seems like a reasonable substitution.  Otherwise, "2 firings per intake valve opening" and "one firing per cam revolution" indicates that the cam actuates the intake valve on every other rotation, which is extremely unlikely.  
 
Zaphod
 
-----Original Message-----
From: Robin Handley <Robin at FuryWorld.fsnet.co.uk>
To: gmecm at diy-efi.org
Sent: Thu, 22 Jun 2006 19:26:02 +0100
Subject: Re: [Gmecm] Big cap HEI on 7747


<SNIP>
I don't know, but I did some digging.  Here's a monthly archive that
contains a post from Bruce Plecan:

http://diy-efi.org/diy_efi/archive/archive_num_98

Search for "triggered", the second occurance should be his post, which
claims that advance is 1.5 degress per 1000 RPM.
</SNIP>

Trying to get my head around something that Ludis said in that archive:

"In batch PFI/TPI (at least for 1226870), hardware in the ECM divides the
distributor reference pulses by 2 (L4), 3 (V6), or 4 (V8).  The injector
firing is triggered by this divided signal.  The result is one firing
per cam revolution, aka two firings per intake valve opening."

This doesn't sound right to me. IME with L4 engines, the distributor is
driven directly off the cam shaft and so rotates once for every cam
rotations. The distributor has 4 trigger points (whether it's Hall or
reluctor). So, one rev of the cam shaft generates 4 pulses. The cam shaft
rotates once for a complete engine cycle - which includes each cylinder
firing once (hence the 4 trigger points per cam revolution). Therefore, if
the ECM (or, pedantically, the software :-) divides the distributor pulses
by 2, then the result will be 2 per cam revolution. If the injectors are all
triggered together by this divided signal then surely that means that they
will fire TWICE per cam revolution (not once).

Sanity check, anyone?

Robin

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