730 Spark Advance

[Shannen Durphey]

quickie note, with the 749, the timing advance reported is correct
when the initial timing value in the prom and the base timing at the
dizzy match.  The ECM calculates total advance, then subtracts the
initial advance value in the prom, to produce the correct ignition
timing.  If you've got 6, and ya want 30, you only need to add 24
more.  Quick glances at the 730 code lead me to think it's similar.
Shannen

gmecm at diy-efi.org wrote:
> 
> On the gmecm web site, under the '747, there is a write-up
> on how the spark works. It is similar to how it is done
> on the '730. Covers inital, bias's, and the math.
> 
> I think that the way Gm implemented hiway spark was to
> pass emissions testing. I've just added what worked to
> the main spark table, and let it at that.
> 
> BobR.
> 
> >I'm doing some last minute tuning on the spark tables, and need to get a
> >few things straight.   Base cal is AUJP.
> >
> >1.  The actual measured timing at the crank is the amount in the table (32
> 
> >degrees for example), subtracted from the inital advance (6 degrees in
> >AUJP).   Measured at the crank, this would be 26 degrees?    This is what
> >the scan tool reports as spark advance.
> >
> >2. Is the value for max advance at $026 is the max advance the ECM will
> >allow to be delivered, or the max value allowed in the table?  Given that
> >$026 is 39 degrees, the max advance possible measured at the crank would be
> 
> >39 + 6 = 45 degrees?      If this is correct, a value of 42 degrees in the
> 
> >table would result in 38 degrees crank advance, right?
> >
> >3.  What are average highway cruise SA numbers?  I realize this varies
> >greatly, but what's a ballpark?  Stock AUJP table looks to only put in
> >about 31 deg. at highway cruise, less than I'd expect.
> >
> >4.  Is there a good reason to use "Hiway SA mode" instead of just directly
> 
> >tweaking the spark table?
> >
> >-> Bob Valentine
> >-> bob at tecmark.com
> >
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