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Will my car pass the smog check using higher octane gas?

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
Answer:

Depending on your particular vehicle's specific smog test results, using higher octane gas may help or hurt smog test results. It is our recommendation to always follow manafacturer's requirments. A properly maintained vehicle should pass the smog check with the correct fuel octane. Late model vehicles have emission computers specificly programmed for the octane rating required by the manufacturer for efficient combustion. posted by SmogTips Support

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