Warm spark bias

[TheApocalyptican]

I've had a problem with bogging in my 90 Sub with 350 tbi and 770r4 w/ 7747 ecm. Not sure if I posted about it here before. Basically have a intermittent low rpm bog. Have been chasing my tail trying to find the problem. Today I decided to play with the tune some and tried pulling out the warm spark bias which was at 9.XX. All the other bins I have downloaded suggest that this is the correct number to have. However, when I pulled it out, my bog disappeared almost completely. Had it do it a tiny bit, but nothing like before. What is the reason for the warm bias? I understand the cold bias, but I would think if there was a subtraction in spark wanted when warm it would be easier to just set the main table correctly. What's the reasoning behind having the warm bias this way?

[FSJ Guy]

It's explained in the XDF file, IIRC. The memory can't hold negative numbers, so there is a warm spark bias applied to the ENTIRE table.

Thus, if the table says 10 degrees and the warm spark bias is set at 15 degrees, the actual computed timing is now NEGATIVE 5 degrees.

Some XDF tables AUTOMATICALLY compensate for the warm spark bias. Some do not.

At least that is how I understand it...

[TheApocalyptican]

I guess that makes sense as a whole....but in my main spark table, the only cells that are low enough to have the warm bias make the spark negative, are cells that basically can't ever actually be achieved. Like the 400rpm 100map cell. That's the big reason I don't know the point of it(in my case).

[FSJ Guy]

That's GM for you. LOL!

I don't know why it's there, either. Maybe some engines need the negative spark and they just left it in the code to cover all situations. <shrug>

[pima.1971]

Is this the coolant temp bias were talking about. If it is, it's supposed to pull timing once it hits those temps. But to me it seems like it pulled timing no matter what. I had results by zero'ing out the coolant temp bias as well as the egr bias. I just about zero'd out most of the bias's. Now I'm not saying to do this as I have been told to be careful as the timing may not be retarded if it needs to. Just don't do too much at once and watch knock counts. HTH

[EagleMark]

FSJ Guy is exactly correct.

In the 1227747 $42 small block you just added 9.84 degrees to your entire spark table!!! Now if you look at a Big Block bin file warm spark bias is 0. There's so many other spark corrections and bias that doing this takes time and testing with timing light to ensure you know..... wait for it......

So where is your timing really at?

I'd put it back for now and look into other things first. Like where is your base timing set? This is timing light marks at balencer with wire disconnected stock should be 0. Lock distributor and recheck, reconnect wire, clear codes. Then you can use the hack in $42 XDF and see actual timing in ADX. if you need more you can add to timing table, with bias and vehicle fully warmed up you'll notice what your timing table says is 9.84 degrees off. Look up data tracing as well when data logging.

This could still be a mechanical issue, these small cap distributors cap and rotor can be completely destroyed and still run, there's also a possibility of a faulty EST or even a fuel pressure issue? Injectors clogged, injector O rings leaking, do you see a good spray pattern or lots of drips, etc... is it a high mile vehicle? Min air setting? What's your IAC count totally warmed up closed loop? What I'm getting at here is with age the wear closes throttle blades and when engine is running there's hardly any air mixing with fuel through throttle blades and all air is coming in from IAC, when you hit gas throttle blades are being sucked closed and it jerks open dumping fuel that has been collecting on top, when engine not running throttle blades open smoothly...