Dyno run... huge spark advance spike after runs

[USAsOnlyWay]

Is this normal? Spark will advance to 36 degrees then when the throttle is closed, spark advance jumps and plateaus at 47 degrees until about 2500 RPM. (The car is in gear, throttle closed, no brakes at this point.) Is this normal? If not, is this bad? Since max advance when on the throttle is 36 degrees, I do not know why it would jump to 47 like that. Thanks in advance.

[bjankuski]

Originally Posted by USAsOnlyWay

Is this normal?

Spark will advance to 36 degrees then when the throttle is closed, spark advance jumps and plateaus at 47 degrees until about 2500 RPM. (The car is in gear, throttle closed, no brakes at this point.)

Is this normal? If not, is this bad?

Since max advance when on the throttle is 36 degrees, I do not know why it would jump to 47 like that.

Thanks in advance.

This is normal, at light load high engine RPM and high engine vacuum the spark advance will be high because you need more advance to allow enough time for the lean and less dense mixture to burn. Flame speed is reduced when the mixture is lean and less dense so that is why the ignition advance goes up. I have seen more the 50 degrees of advance in low load high RPM conditions.

[USAsOnlyWay]

Awesome. Thanks for the explanation. I'm very relieved it is normal.

Thanks!