Gasoline Octane for your VFR

by Mike Guillory


This article was written by Mike Guillory, posted on the VFR List and he has graciously allowed me to post it here. I think it is offers a clear explanation about premium gasolines.

Today, during a nice ride in the Texas country, two (2) of us on VFRs stopped for gasoline. The other rider was surprised that I use "Regular." He had been using "Premium" because he recalled the manual specifying 91 octane or higher. So here's the real story if anyone is using Premium and are not sure if they need to. Keep in mind that there is about a $0.20 to $0.30 difference in price per gallon, depending on where you are.

The manual says "use gasoline which has a pump octane number (R+M)/2 of 86 or higher, or a gasoline with a research octane number of 91 or higher," or something very similar to that. Translated, that means "use Regular gasoline."

Can you get additional benefit by using Premium? Probably not. If you keep your motor in good shape, you will not have "pre-ignition" with regular, just as you won't have it with Premium. (Pre-ignition, or dieseling," or "knock" is what "octane" prevents). In fact, you normally get better fuel economy with Regular because it has a higher "heating value" in BTUs per gallon than does Premium.

What causes the confusion is the way octane ratings of gasoline are displayed. (R+M)/2 simply means "the average of the Motor and Research octane numbers." Without getting into the differences, for conventional refined and blended gasolines the Research number (R) is about 9 or 10 octane numbers higher than the Motor number (M). They are both determined using special "knock" engines, but the "Motor" determination is done under more severe conditions thus you get lower numbers.

So, getting back to that "Regular Unleaded" at the pump with an "average" octane number of 87 (in Texas, and most other states). To get an average of 87, the gasoline blend will have a Research number of about 92 and a motor number of about 82. So, if you go back to the statement of octane requirement for your VFR, it meets both a Research number of 91 or higher AND an average octane (R+M)/2 of 86 or higher.

What about "additives" and fuel system cleanliness? Most major gasoline marketers put in a cleanliness additive mixture which will allow their gasoline to pass standard requirements and they usually use the same additives at the same dose rates in Regular as they do in Premium.

Will using Premium harm the motor? Nope! Motors like Premium just fine.

What about locations where Regular is only 85 or 86 average octane? If you are at elevation, say 5,000 ft or higher, there is no problem because motors have a lower octane requirement at altitude. If you are nearer sea level, I'd probably alternate Regular and Premium, filling up about half-tank each time.

I hope some of you find this interesting, if not helpful. If you suspect that I used to have a job in the gasoline manufacture, quality and blending business, including surveys of all major competitors' gasolines, then you are a sharp devil!
Then another Lister asked this question.

Now what about the concerns some people have regarding gasoline quality at the various "generic" filling stations? I've heard it said that the gasoline one gets at these places is of inferior quality to that found at filling stations bearing names such as (major Oil Companies). Any truth to this?

And Mike Guillory answered with:

First, understand that the "base gasoline" (correct octane, volatility, etc) without additive is widely traded or "exchanged" across the USA. Depending on where the refinery is located relative to the marketing area, companies will receive gasoline in one area and give back gasoline, diesel, or other fuel in another area. This saves transportation costs. Also, much gasoline is moved via "common carrier pipelines" so that what you put in at one end may be different from what you get out at the other end. That's all not really as bad as some might suspect, because the government and the pipeline monitors fuel to assure it still meets all basic specifications. In addition, quite a bit is purchased foreign-made gasoline. That is done by majors as well as smaller independents.

So, about your question. In many cases, perhaps the majority today, the only thing that differentiates one marketer's gasoline from another is the "additive package," which is usually a mixture of 2 to 4 components put in the help keep deposits from forming in your fuel system and in your combustion chamber. The additive is normally added to the gasoline truck as it loads up at the fuel terminal. This is important, because any given fuel terminal may supply gasoline that ends up in any number of different "brands," but each may have a different additive package. But not necessarily, depends...

Some companies, particularly the "majors," develop their own gasoline additives and dose them to try and be "better" that the competition. Some companies, probably most of the independents, buy a standard additive package from a vendor and can put it into the gasoline at any number of different concentrations to give from "just passing" industry cleanliness tests, all the way to "superior" cleanliness. Of course, the majors can do the same thing with their "unique" additives.

So there is no simple answer to the question of whether "generic" stations have inferior quality gasoline. I think it is more likely, so I try to always buy from a major. In my area that is either Shell, Exxon, Chevron, or Mobil. As some of you may already know by experience, one shot of "bad" gasoline can totally screw up your fuel intake system. You can get a shot of bad gasoline from a major but I think it is less likely.

The other factor I haven't mentioned yet is "base fuel quality." Although all gasoline presumably meets all the octane, volatility, stability, etc specifications, there still are a number of characteristics that affect cleanliness and performance within those specifications. I wonder if some of the small independents have the monitoring to assure all the gasoline purchased on the open market, often produced half-way around the world, is as good as fuel made right here in the good old US of A.

Hope that helps.

Mike in Houston
'94 VFR750 "XENA"
'85 V65 Magna "YELLOW SONIA"

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