[Gmecm] Big cap HEI on 7747

[davesnothereman at netscape.net]

AFAIK, the 7749 fires injectors with each distributor pulse when synchronous mode is enabled.
 
Zaphod 
 
-----Original Message-----
From: Robin Handley <Robin at FuryWorld.fsnet.co.uk>
To: gmecm at diy-efi.org
Sent: Thu, 22 Jun 2006 19:59:51 +0100
Subject: Re: [Gmecm] Big cap HEI on 7747


To add to this, the Turbo P4 doc, which claims to describe a PFI system,
discusses 3 fuel modes:

Synchronous - presumably fires every ref pulse? (the 'normal' mode)

Quasi-asynchronous - 'used when the synchronous fuel base pulse width
becomes so small...l' and 'fires the injectors every other ref pulse, for
twice what it would have for synchronous mode'

Asynchronous - 'used to deliver fuel pulses for acceleration enrichment'

So, I understand from this (if it's right), that, for an L4 engine, normally
the injector(s) fire 4 times per cam rev - i.e. the distributor pulse is not
divided at all.

Have I got this right?

Robin

----- Original Message -----
From: "Robin Handley" <Robin at FuryWorld.fsnet.co.uk>
To: <gmecm at diy-efi.org>
Sent: 22 June 2006 19:26
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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