Originally Posted by TurboLX

While I certainly appreciate someone using my book as a reference, please keep in mind that it was written not as a college text, but rather as a medium to educate those with little or no engineering background about the underlying principles of engine calibration. I intentionally avoided going too deep on some subjects to keep from losing interest from the majority of readers. As it stands, I already get enough complaints along the lines of "it's too technical, I just want to know X..." When I have questions, I reach to my copy of "Internal Combustion Engine Fundamentals" by John B. Heywood. It's 930 pages of college textbook glory and hasn't failed me since my days in engine design class nor through my current role as an OEM calibration engineer.

Now back to the topic at hand. The quote from my book above is taken a little out of context here. I wasn't referring to the difference between 87 and 93 octane pump fuels, but rather pump premium versus dedicated race fuels such as VP C16, where there is a radical shift in the blend of chemicals. Many race fuels have higher percentages not only of 2-2-4 trimethyl pentane, but also things like aromatics (think Benzene rings) that are even harder to break than simple H-C chains. Changing fuels certain DOES have an effect on laminar flame speed, as seen in Heywood's figure 9-25 on page 403:



Even there, one can see a difference in flame speed between isooctane and "gasoline" with a mean difference of roughly 4cm/s. (table 9.2 shows isooctane at 26.3 cm/s and gasoline at 30.5 cm/s) So changing which chemical cocktail one burns in the engine can have significant impact on required timing if you wish to maintain a CA50 number of roughly 7-9dg ATDC under the same conditions. Am I being specific enough here? I think it's no secret that pump gas is a cocktail of chemicals, which includes some octane, but also plenty of other things like detergents, stabilizers, light ends that evaporate over time, oxygenates, etc. Each can have its own effect upon final performance, so you'll see that factory calibrators spend a lot of time making sure the ECU is flexible enough to handle these, especially as seasons and fuel specs change.

Back on point again. To the original question... To the best of my knowledge, while I was at GM "premium fuel" for us as 91 pump octane. This means that cars with either "premium required" or "premium recommended" had their high octane spark tables calibrated for 91 octane pump gas and the low octane table for something less (usually about 86 or 87 pump number). The ECU has a knock learn factor that is uses to determine how far in between these two tables it should be at any given time. In the presence of knock, one would see this knock learn factor swing toward the low octane setting. If after some time and higher load events without knock, it would creep back toward the "good fuel" table. The GM logic did not have the ability to add base spark beyond the high octane table values like some other OEMs do.

That's all great info, but what about my car? Can I add timing and make more power?

Probably not. It has been my experience that N/A engines like this don't gain much reliable, consistent power from adding the last one or two degrees of spark advance available at full load due to the difference between 91 PON and 93 PON. You'll see bigger differences run to run than the actual change due to improved cylinder pressure profile if you're honestly running the tests under identical conditions for IAT, ECT, EOT, cam position, lambda, and sweep rate.