[Gmecm] Big cap HEI on 7747

[Steve Ravet]

> -----Original Message-----
> From: gmecm-bounces at diy-efi.org 
> [mailto:gmecm-bounces at diy-efi.org] On Behalf Of bcroe at juno.com
> Sent: Tuesday, June 13, 2006 1:01 AM
> To: gmecm at diy-efi.org
> Subject: [Gmecm] Big cap HEI on 7747
> 
> I agree it fires at the threshold.  But the point is, the 
> threshold is very near the polarity crossover voltage
> (zero) relative to the signal amplitude.  So amplitude 
> variations (due to temp, velocity, etc) have negligible 
> effect on timing.  There are no perfect circuits out there, 
> and I won't have time to measure how close that threshold is 
> to zero for a while.  Maybe you know?  
> 
> Bruce Roe

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.

The IC is a motorola MC3334, datasheets are probably floating around
even though the part is obsolete.  The SAE paper is SAE 750346 -- HEI-A
New Ignition System Through New Technology.

Here's a WWW page with some information, including a block
diagram/schematic of the trigger circuit:

http://www.cyberdave.org/HEICoilInfo.html



--steve




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