There may indeed be value to selecting higher octane. In Southern Ontario both Shell and Esso sell fuel with varying degree of ethanol content - 87 octane with 13%, 89 octane with 5% and 91 octane with no ethanol at all.
In some instances, selecting a slightly higher octane can increase performance by way of the greater BTUs and lesser moisture content in the fuel (since ethanol is hygroscopic). And we certainly see a decrease in NOx without a significant increase in HC or CO.
So, in some instances, buying a slightly higher octane fuel can both increase performance and help pass a failed emissions test.
Generally, yes, spending money on higher octane than your owner's manual recommends is a waste of money. But sometimes ethanol tilts the scales. Of the 6 vehicles I've owned, 2 benefited from slightly higher octane - the other 4 suffered. I've found that it's very much a vehicle dependent choice.
- SmogTips Contributor
