Originally Posted By: Colt45ws
Its a vehicle by vehicle thing. The engines Im familiar with that have a knock sensor, can only retard the timing by up to 3-5 degrees when it sees knock. It won't add timing as it can, just tries to run the standard table and if it knocks, pulls timing. There are situations where a little more octane might keep it from pulling timing. High load or high temperature. Or both.
I agree with you that it will vary by manufacturer.
In some cases, I think the standard timing tables use enough advance such that the engine is always adjusting timing dynamically according to input from the knock sensor. If you watch the load parameters on a ScanGauge or Torque, you'll find that the engine is at or above 80% load very often. Load (in this context) can be summarized as the actual airflow through the engine divided by the maximum possible at that engine speed. For instance, at just 2,000 RPM (beginning to accelerate from a stop sign for instance), 40% throttle may deliver all the air the engine can possibly consume at that engine speed. So engine load, at just 40% throttle, may be at or near 100%. A more simplistic measurement of this is manifold vacuum. If there is vacuum in the manifold, then the engine is trying to pull more air than the throttle will allow, meaning it CAN consume more. At no vacuum (or very close to no vacuum), the throttle butterfly is not a restriction, and the engine is consuming all the air it possibly can. This isn't necessarily at WOT.
I can see a difference in the timing advance values (with different octane) just pulling out of a parking lot. On 87 (in both of our current vehicles), the timing is retarded MORE than it is when running 93. There is also a difference at higher speeds in the form of fewer torque converter unlocks or transmission kick-downs. This is pretty repeatable if you can drive the same roads all the time, and find some with hills that are JUST steep enough to force a kick-down with one fuel vs. another.
I'm very sure there are vehicles with standard timing tables that "max out" with 87 octane. In this situation, higher octane won't matter because the timing is already advanced as far as it's allowed. I think this is largely driven by corporate powertrain programming standards, and how different manufacturers like to do things. Still, I wouldn't be surprised if these ECUs pulled timing more often than one might think.
Unfortuantely, most manufacturers don't make the "timing retard" events readable across the ODB-II protocol without proprietary scanners, so it's hard to be absolutely sure.
