My AutoXray computers can read the spark advance. My car base timing setting now is at 12deg. (2deg more than the 10deg factory recommendation). At idle (1K RPM) the AutoXray reads 18deg. Is that suppose to be the advance in addition to the base (12+18) or the total from 0 (12+6)? In other word, is it advancing 18 deg or 6 deg at idle? How much it suppose to advance at idle? Thanks.

If the ECM isn't in by-pass mode the computer controls the timing. It will advance and retard as needed. The sensors tell the computer what's going on and it adjusts accordingly. Also the higher the RPM the more advance it will go. This is normal. On older cars they used centrifical weights to advance the distributor over the RPM range.
So it sounds like at idle the computer advanced it 6*

[This message has been edited by Oreif (edited 04-18-2001).]

Thanks Oreif. Yes, when throtle is applied ti goes up to 36-38*. Just was wondering if there was a set advance value for idle.

It's the sum of all of the advance calculations with the reference being considered negative.

Since you are now physically more advanced then the reference value set in the ECM, you have to add that difference to the numbers you read.

When you ground the ALDL connect, you force the spark advance to the ALDL mode spark reference set in the ECM code. Then you set the distributor to match that. If it's wrong, the ECM still assumes you set it right.

[This message has been edited by terryk (edited 04-18-2001).]

Alex,

I would say it is 18 degrees total. Otherwise you would be at 30 degrees at idle and the computer would advance it more as you move up in rpm from idle. I don't think that much advance could be used even on 92 octane without noticeable detonation.