There have been several octane posts lately. Maybe this will help clarify what octane really means. Like other things, octane is a complicated topic if you get into the fine details. Most reports are either grossly simplistic or mind-numbingly detailed. I’ll try to hit middle ground. This note does not try to tell you what octane you need. That is another complicated topic involving knock sensors, compression ratio, and spark advance. All I’m trying to tell you here is what octane means. Before you ask how much you need, you ought to know what it means.

First, be sure to recognize that octane refers only to how well a fuel resists knocking. It has absolutely and utterly nothing to do with the power of the fuel, the mileage of the fuel, or any other characteristic of the fuel other than how well it resists knocking. True, it can indirectly influence power, because higher compression ratio and greater spark advance give more power, so the engine can be set up to deliver more power. But once you have enough octane to prevent knocking at your engine’s compression ratio and spark advance, further increases in octane do not give further increases in performance.

Second, some very basic definitions. The specific compound 2,2,4 trimethyl pentane (224-TMP) is arbitrarily defined as 100 octane, while the compound normal heptane (n-heptane) is arbitrarily defined as zero octane. The reason the test is called “octane” is that the first compound (2,2,4 TMP) has 8 carbon atoms, and “octane” is the generic designation for any of thousands of compounds with 8 carbons, of which 224-TMP is only one example.

With those points out of the way, octane is determined in a test engine with variable compression ratio and variable spark advance. It is run on the two test compounds in the above paragraph, and then on the fuel being tested. The compression ratio and spark advance in the test engine are gradually increased to where it starts knocking. The test result is simply interpolated. So for example, if the test fuel starts knocking at the same point (in spark advance and compression ratio) as a blend of 90% 224-TMP and 10% n-heptane, it is rated 90 octane. 85%/15% would be 85 octane. And so forth. It is possible to be greater than 100 or less than zero if the test fuel is even better than 224-TMP or worse than n-heptane.

The difference between Research and Motor octane is simply the way that the spark advance and compression ratio are varied in the test engine. The research method is set at low compression, high spark advance, to simulate cruising condition. The motor method is set the reverse to simulate high power, accelerating conditions. Most engines are more sensitive to motor number than research number, but that is not universally true.

Now things are about to get really messy. This is where a detailed text gets into pages and pages, but I’ll try to hold it to a paragraph or two. For the class of compounds called paraffins, R and M number are close to the same. Both of the standard compounds (224-TMP and n-heptane) are paraffins. 224-TMP is a highly branched iso-paraffin (iso meaning branched chain, normal meaning straight chain), and its octane rating is vastly higher than the straight-chain paraffin, n-heptane. Trouble is, iso-paraffins are rare and expensive. There are two other, cheaper classes of compounds that also have reasonably good octane ratings: aromatics and olefins. Trouble is, while their octanes are very good when tested by the research method (cruising conditions), they are not as good when tested by the motor method (accelerating conditions). If you think about it, this means that for almost all blended fuels, motor number will be lower than research number. Remember, the test fuels are paraffins, whose R and M numbers are about the same. But blended fuels have some aromatics and olefins, whose R numbers are good, but whose M numbers aren’t as good.

Those last few sentences of the previous paragraph are why in the USA, we post (R+M)/2 octane number. Europe still allows posting of research number, which has the advertising benefit of being numerically higher, and thus sounds better. But I’d argue that’s potentially misleading because it’s possible to have a high R number with lots of aromatics and olefins, that would give a mediocre M number. And in most engines, M is really more important. With the USA posting system, if you try to play that game (lots of lower quality olefins and aromatics), while you can keep the R number high, the M number will suffer, so the (R+M)/2 will drop. Another piece of jargon in this topic is “sensitivity”. That is simply the difference between R and M numbers. So for example, if two premium fuels have the same (R+M)/2 of 93, and one of them is (98+88)/2 and the other is (100+86)/2, the former has sensitivity 10, and the latter 14. Even though the R number is higher in the 14 sensitivity fuel (100 versus 98), the 10 sensitivity fuel will resist knock better in most engines. And another side-issue is that the higher the fuel sensitivity, the more olefins and aromatics it contains. Olefins are bad from the standpoint of evaporative emissions, and aromatics are bad from the standpoint of toxicity.

I’ve dumped a lot of technical “stuff” on you, so what does it all mean? Unfortunately, unless you have access to the full specs of your fuel, it’s sometimes hard to tell. But you can watch out for a few things. First, check to see whether the octane is R or (R+M)/2. A fuel that is 100 octane by (R+M)/2 is an extremely high octane fuel, but 100 by the R method isn’t much, if any higher than typical premium. Next, if you car knocks on one company’s fuel of a given (R+M)/2 octane, try another company’s of that same (R+M)/2 octane. It might have a different sensitivity, and that could be important to your engine. And finally, though this is a general statement that has exceptions, and while I know full-well that in some areas, various brands share the same fuel source, it is directionally correct to say that cheaper fuels have higher sensitivities. That’s because olefins and aromatics tend to be excess, and isoparaffins tend to be rare, so the majors tend to sell excess cat gasoline (olefins) and tol-concentrate (aromatics) to the minors, who then blend a bit more cheaply. Is it a huge effect? No. Is it universally true? No. But is it a directionally correct statement? Sorry all you guys who swear all gasoline is exactly the same. It does happen to be a directionally correct statement.

 

Thank you for the lucid explanation. Very good information.

 

Thanks for the info .... now for the question that I cannot find any answer to.

Why does General Motors RECOMMEND 93 for the 400 HP LS2 C6 engine, but REQUIRE 93 octane for the 505 HP the LS7 Z06 engine?

 

Probably because the LS7 is more suseptable to knock. As aluded to above, higer compression and greater spark advance make the engine more suseptable.

 

Because the LS2 is 10.9 to 1 and the LS7 is 11.1 to 1

 

Thanks! for the write-up LDB, it's very informative.
I have one question: aside from seasonal reformulation, do the major suppliers generally keep thier manufacturing lot-to-lot R & M component (olefins and aromatics?) proportions consistant? Maybe another way to ask this question is whether a major will produce a (98+88)/2 batch of fuel this week and, due to manufacturing or procurement variations, produce a (100+86)/2 batch a month later?

 

check out this paper for all you ever need to know about gas.http://www.faqs.org/faqs/autos/gasol.../preamble.html

 

There certainly will be some variation from batch to batch, but one that large would typically not happen unless the refinery in question had a major incident. Even with incidents, if you happen to be in California or in one of the big cities with 49-state reformulated gas, variations that large would be rare, because emissions-related formulas and specs in those areas restrict blending flexibility, especially for high octane premium grade. But if you're on a cross-country trip, going in and out of various reformulated gasoline areas, with and without oxygenates, it would be easy to believe variations that large even if you stay with the same brand.

 

Agree that's a good reference, with the only drawback being possible risk of getting bogged down in detail. But some like details, and others would doubtless argue that what I wrote was already more than they wanted to know. So by all means, go into as much or little detail as desired. I'm easy.

 

The History Channel's Modern Marvels series has an edition on gasoline. It's pretty interesting and surely they will show it again. Imagine in the late 1800's crude was refined for lamp oil, with the gasoline considered waste and dumped into the river. Also in the 30's & 40's gasoline was only about 40 octane. Not to good for running in a Z.

 

<chuckle>

I enjoyed your treatise, and it was useful for the intended audience, but I could not help but note a predilection towards use of a particular phrase.

For those uninitiated with engineer-speak, the phrase simply means "At least the data wasn't pointing COMPLETELY the wrong way"!

 

Thanks for the great writeup , my head hurts.

 

PhD in writing!

 

Nice write up. I just like reading the things from those that really know they're saying and putting in terms the rest of us can understand. I feel like I have learned something tonight.

Thanks

 

Actually, repeated use of that term is more a defense against the various bulldogs on this forum. A few of my earlier posts have drawn some "humbug, it's all the same" responses. So I was simply acknowledging that sometimes, in some areas, that's true. But taken as a whole, what I'm saying is true.

 

Come on guys.... Give the Guy a break...Ya'll... He's trying to give us some good usefull information so the next time someone says "all gas is the same" you can "splain it to 'em.

 

With a petroleum engineering degree, I'll have to say........"Yeah, what LDB said." Good writing, but for most people each bump in the fuel octane only means about a dime per gallon for each grade.

 

The cost of making premium is about 6-8 cents per gallon over regular in the 49 states, and almost double that in California because certain CARB specs reduce octane, which then must be made back up with ever more expensive steps. As premium buyers, that may tend to make Vette owners yell that it’s another place where the dreaded oil companies rip us off. But it’s not quite that simple. If we only charged 6-8 cents more, then more people who don’t need premium would buy it, and the cost of making it would really skyrocket. The higher the base pool octane (as would be the case if more people bought premium), the higher the cost of making an incremental octane.

And as far as Larry B’s note to give me a break (which is at least somewhat ironic since I’m also a Larry B), you don’t have to worry too much on that score. I have a very thick skin, so am not sitting here feeling hurt by comments. It’s more a nuisance issue when somebody replies with a very strong statement containing a mix of truth and misunderstanding, which then must be countered. Seems to me that in a forum like this, it’s better to ask questions and clarify, or at least keep the tone a bit light, than to come in with guns blazing. And incidentally, I’m not calling any of the replies at least so far in this thread “guns blazing” variety. I’m referring to some past threads on that score.

 

Nice writeup LDB, but Ih have one more question which may, and probably is, the source of all those snide "Gas is all the same" rants. Of course, I'm not ranting...

Where is the octane formulation done, at the refinery, the local distribution hub, or somewhere else? I'm asking due to having watched that same Modern Marvels show where they talked about gasoline pipelines. So that, the barrel the oil company puts in at one end is not the same barrel they take out at the other end. If the formulation was only done at the refinery, then regardless of where you buy the gas, I don't see how you really would know what you got.

Or did I miss something really important here?

 

When I lived on L.I.NY the high test octane rateing was 92 to 93. Here in Las Vegas its 91. Cant find 92 or 93.
Why is it lower in the west?