wrote:
> Paul DeRocco wrote:
>
>>the knock sensor just prevents damage,
>>it doesn't make the engine work as efficiently
>>as it would have on higher-octane fuel it was designed for.
>
>
> Correct me if I'm wrong, but the optimal efficiency for any engine
> (given the current AKI, temperature, throttle position, pressure, etc.)
> occurs when the engine is just about to knock.
>
> That is, to tune the engine for maximum efficiency under any given
> conditions, you advance timing until the engine knocks ... and then you
> back off a tad. That, I'm of the understanding, is the
> maximum-efficiency point for that engine.
>
Not true at all. If you plot torque vs. ignition timing you get roughly
a parabola. As you advance spark from the retard side torque rises but
at a diminishing rate until you reach a peak. The peak is referred to as
MBT, Minimum for Best Torque, spark timing. If you continue to advance
spark, torque will start to decrease with the slope getting steeper as
you continue to advance spark. The shape of the torque vs. spark curve
(and height) is completely independent on fuel octane. It is a function
of the basic engine design.

If the engine is knock-limited such that you cannot run at MBT timing
then using higher octane fuel will allow you to advance spark and get
higher torque. But, if you can run at MBT, there is absolutely no reason
to run higher octane fuel. You definitely do not want run near the knock
limit if it is beyond MBT. More spark advance is NOT always beneficial.