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I recently had a customer come into my store and asked that since he has to run premium gas (91 octane) could he run an 89 octane (which is cheaper by almost 15 cents a litre here) and add an octane boost to bring it up to a 91? I honestly couldn't answer this one....what do you all think

Unless it's that special canadian octane booster, most octane boosters don't do anything measureable. An octane booster that claims 6 points raises octane from 89 to 89.6. Not worth it, IMO.

Actually it was a NOS product that "claims" a 5 point increase so his thinking was 89+5=94 octane

No, 5 points is .5 octane. Each 'point' in an octane booster is 1/10 of an octane.

Gets pretty expensive that way, doesn't it?

Figured it was best to ask since I honestly didn't know. Thanks everyone.

If you simply used 89, you would have reduced ignition timing and reduced power and mileage. The reduced mileage would offset the extra cost of premium. It's a wash financially, so it's best to run the premium and prevent long-term engine damage.

I lost a motor due to an error by a pump jockey (we don't pump our own fuel in Oregon).

londonontman wrote:(91 octane) could he run an 89 octane (which is cheaper by almost 15 cents a litre here)

The difference between regular 87 and premium 91 is 13 cents here, not sure where you're getting 15 cents between 89 and 91.