#24: Dr. Octane

Today’s rather than answering a specific reader question, let’s rebuke a common misperception that comes up every few days on every scooter forum:

My (dealer and/or manual) told me to use premium gas in my Asian scooter.

Dr. Buzz: First, you have to understand that “Premium” is a bad name for gas, because it’s not BETTER, it just has different properties that work better in certain engines. For that matter “High-Octane” doesn’t even mean “more octane,” the Octane rating simply quantifies the anti-knocking characteristics of the fuel. If those Wikipedia links are too intense for you, here’s The Straight Dope.

So if your scooter manual specifies 92 octane, you should use it right? Probably not. Your manual is likely full of mistranslations, typos, “Engrish” and sloppy conversions. For instance, I’ve seen Chinese and Taiwanese manuals simply translate the word “km” to “miles” without actually changing the numbers!

Octane ratings aren’t metric, but Taiwan and most other Asian counties use RON octane ratings and the US uses AKI octane ratings. At any gas station in Taiwan, you’ll see 92, 95, and 98 RON octanes available. 92 is the lowest rating available there. In the US, you’ll find 87, 89, and 91 AKI octane, where 87 is the lowest rating.

If that’s not enough proof, here’s some actual math:
RON Octane Rating x 0.95 = AKI Octane Rating,
So 92 x .95 = …you guessed it, 87!

In some cases, using higher-octane-rating gas than specified is actually bad for your engine. And it’s certainly costing you money.

But what about the people who insist they’re getting better mileage or higher top speed with high-octane-rating gas? Ask them to see their dynamometer readings from tests performed under scientific conditions They’re either lying, deluding themselves, or crediting one of the dozens of factors that influence mileage and top speed to their gas.

Do you have a question for our so-called experts? Email Dr. Buzz! Your confidentiality is guaranteed, to the degree that we’ll spell your last name with one letter.

Note: Dr. Buzz is an unlicensed, mostly-fictional doctor. Take his advice with a grain of salt.

10 thoughts on “#24: Dr. Octane”

  1. I think the one caveat to your comments is that especially in midwestern states, Premium is the only gas (beside de-ox) you can get that doesn’t have significant amounts alcohol in it. The modern Piaggio/Vespa bikes can be particularly sensitive to the ethanol blends. Then again, that’s an italian bike rather than the Taiwanese machines you’re referring to here. It wouldn’t surprise me at all if a Buddy ran perfectly fine on 87.

  2. According to Modern Vespa’s FAQ, Piaggio does recommend 95 RON which would be 91 in the US.

    The alcohol argument as I understood it was that it was bad for seals in vintage scooters, but you may have a point, I’ll look into it more.

  3. Some quick research shows that, predictably, manufacturers and mechanics blame ethanol for all kinds of problems, and gasoline manufacturers, the government, and environmentalists insist that it’s not a problem. Anecdotal evidence on the web will support any argument you want to make (I found everything between a post insisting Ethanol made a 50cc scooter faster to vast, pages-long conspiracy theories that the U.S. sugar industry is using mind control drugs on aliens at the EPA.)

    It is a fact that alcohol/ethanol can damage plastic and rubber components on older vehicles, it’s unclear whether EPA specs and current scooter manufacturing practices mitigate or eliminate that issue.

    It does seem likely that ethanol contributes to the faster decay of gasoline in recent years, so it may increase the amount of maintenance needed and the likelyhood of a gummed up carb. And some manufacturers suggest changing jetting for fuel containing ethanol.

    I’ve been running whatever gas and oil is at hand on all my scooters for years, and I don’t feel I’ve had any problems not common to scooters in general, especially rarely-ridden scooters, but my anecdotal evidence isn’t worth any more than those say that 97 octane gives them “more go.”

  4. One set of problems with ethanol that I can imagine are that when in contact with non-synthetic rubber it can accelerate the loss of elasticity making it brittle, swell and more prone to cracks. I’m not sure how something like 10% ethanol blends would so drastically damage a rubber part or seal, but surely the higher percentage blends would do quite a number on them. The question is how many ethanol-sensitive parts are in a modern scooter. Viton is an example of a synthetic rubber used in some seals. These should be ethanol resistant. I have no idea what is put into modern bikes in terms of tubing, seals, o-rings or other plastic rubber bits. I think octane is octane (in any given rating system). It’s an objective measure. It’s not like USDA rating of Choice, Prime or Select cuts of beef. If the Octane rating is not reflective of combustion properties of some engines, then that’s a problem. I doubt this.

  5. Well if 92 octane has less/no ethanol when compared with 87 octane containing 10%; and ethanol contributes to a faster degradation of fuel, then that would explain why I rarely/never experience carburator troubles from improper storage when I use 92 octane, but experience quite a few problems when I am running 87.

    No performance gains, but for infrequently used vehicles like lawn mowers and snowblowers that I always store wet (lazy); it saves a headache when I need to use it.

  6. Maybe just spend the extra 19 cents/foot on the better fuel line!

    I kid. But I think that any sort of gas shouldn’t go ‘bad’ over the winter. Maybe if one’s winter storage season is longer than 8 months. Though I think someone deserves troubles on principle. I almost always use 87 and the same gas starts up the scooter every month or two all winter. I think if it clogs carbs it’s that it’s degrading fuel lines or other non-resistant rubber somewhere between the tank and the float bowl. I don’t know if the varnish deposits in ethanol containing fuel are any different than in non-ethanol containing fuel.

    But 92 doesn’t have any less ethanol than 87. In MN we all have 10 percent Ethanal and in 2013 the mandate will jump up to 20. So 10% in regular unleaded and 10% in ‘premium’.

    What is the Octane of 2% mix?!

  7. Ethanol blends shoudn’t “gum up” your carb – in fact just the opposite should be true since the ethanol will dissolve the gummy/shellacy deposits that precipitate out of gasoline. On the other hand some of that stuff that accumulated in your fuel system that won’t dissolve in gasoline can be loosened and dissolved in ethanol – which could then stick to your fuel filter.

    Also, ethanol is hydroscopic – it combines well with water, as well as with gasoline. Which is why it is used in Dri-Gas to deal with water in your fuel system – it allows the water to be mixed with the fuel and blow through the combustion cycle without stalling the engine. But when ethanol reaches its water carrying limit, it (and the water) separate from the gasoline, causing the same fuel interruptions that water in regular gas causes. You can’t really blame the ethanol for that, since it is not the source of the water contamination. Ethanol blended fuels can deal with small slow sources of water contamination that straight gasoline can not, but it isn’t a cure for a serious water problem.

    Modern engines and fuel systems should be able to handle regular gasoline ethanol blends, but I wouldn’t count on that with vintage bikes or cheap Chinese knockoffs.

  8. The issue with ethanol is water and tubing. The Master 500’s from 2004-2006 are notorious for needing new fuel pickup hoses and clamps. The problem doesn’t really show up until the bikes use ethanol fuels, then the hoses soften and voila, fuel pressure falls off. Replacing the tubing and using a clamp instead of the compression fitings. So there are issues with Ethanol, but I don’t think they are things that cannot be dealt with.

  9. I use whatever cost less and add 1oz. of Sea Foam to each gallon of gas. Good ?, Bad ?, I’m not sure but I’ve had little to no carb problems over the years.

  10. I’m under the impression that Higher compression pistons that are used in many of these small engines in order to get any HP out of them.
    This is the reason for the Higher octane. If your spec’s read a lower compression ratio of 9.5 or 9.7:1 a mid grade 89-90 octane is fine. 87 might not be ok?? I’m Not positive.
    Most automobile manufacturers use 8 – 9.5:1 compression pistons in their engines and precise fuel injection to keep the need for higher octane eliminated.
    However if a manufacturer is squishing the compression ratio to 10 or 11:1 to get that extra hp you better run the Premium needed for higher compression or you will be taxing your engines drive train through increased heat from inaudible detonation..
    The bottom line is it’s true there is no there is no benefit to high octane if you don’t need it but if you have a high compression engine (and you probably do) you do need it and will in all likelihood be doing damage if your not.
    If you don’t belive me Google Compression & octane.
    Forget about conversions that can be mis-translated and check the ones that can’t.
    If you have a compression ratio of 9.5:1 or higher be safe for a few pennis per tank.

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