what does it do. why is my stock l03 set at 20. what does that mean/ have relation to? thanks.

Just think of it as your initial timing. It's so an editor can display the tables in an easy to read form for the humans.

Im my opiniom it's best to have it set at the same as you inital timing.
Is that *20* a hex or binary number?.
If its hex out of the bin file then your really looking at about 6d.
Value= Entry * 90/256

the 20 is from my constants menu in tunercat.

my initial spark advance is set at 0 as stock. so should i have the bais also set at 0, because its not as stock.

let me see if i understand right, which i dont think i do. that number is for the editor display only and wont effect how the engine runs?

Timing is calulated using a bunch of parameters.
From the load table, coolant temp., if ESC is active, etc. One of those items being bias. In order for the commanded timing to be shown correctly (like on a scan tool), the ecm needs to know what the intial timing is, or in this case the spark bias.

Take and change the value in your editor.
Compare that file to one you changed, you'll find the checksum, has been changed, and one other value. Say you changed the timing 10 degrees, that means your Hex value should have changed about 30. Then you can figure out if the number displayed is decimal or a hex value.

If $61 is like $42 there are 2 bias values.

Warm Spark Bias (what tunercat calls Main spark Bias) - a bias for the spark table

Cool Spark Bias (not listed in tunercat) - a bias for the coolant temp vs spark

Both Biases are subtracted from the total advance calc.

The purpose of the bias, as I understand it, is to allow for negative spark numbers.

If you zero the Main spark bias you will add 20 degrees to entire timing curve. Not nessesarily what you want to do.

The initial distributor setting is not listed in tunercat for $61 AFAIK.

Check out the 747 spark doc over at GM_ECM when it comes back.

I'm still a little cloudy on this myself.
So why 20 degree's bias if the max negative value on stock eprom is around -5?
I have ALDL SA value, EGR SA values (36.9 and 35.9 deg), and I added EGR on/off temp. I need to find the Cool Spark Bias value in the 8746 hack so I can add it to my definition file.

------------------
, Jon (350 TBI!)
91 Red My website

so like i said, it just helps readability in editors and other systems like scanners. as well as allow for - numbers. ok.

why doesnt tunercat have a cold spark bias?

shouldnt they?

so if i drop that 10 degrees, then do a full table subtraction of 10, the engine should run exactly the same? or will retards not be able to counter as much as they would want to? like say knock being able to retard 14 degrees?

I don't know. I don't see any reason to change the spark bias numbers.
The intial timing spec is useful if you need to exceed the dizzy's physical 42° limit. IE you can set initial to 8° and have 50° total.

Yes. The timing will remain unchanged. Has nothing to do with the retard function.

Check out:

http://www.diy-efi.org/gmecm/ecm_inf...ion/spk747.txt

for the rest of the story. I'd bet the 8746 is similar.

Brent, thanks a LOT for that 42 degree limit. I was getting no knock with my low compression and high octane so I increased the timing table but when I opened it up I got SES. I pulled code w/diacom and it said it was my distrib advance or something. It makes sence because I have a base timing at 6 and I tried advancing too much.

------------------
, Jon (350 TBI!)
91 Red My website

What code? Code 43 gets set when the ECM cannot induce knock during WOT. The ECM briefly spikes the timing high and listens for knock. If it doesn't hear any knock it flags code 43.