What Octane Means
Drifting
Joined: Jan 2005
Posts: 1,937
Likes: 1
From: Costa Mesa CA
CI 6-7-8-9-10 Veteran
St. Jude Donor '06-'07-'08-'09
That's the answer I seek before I buy a 2006 Z06.
C6 recommends 93, runs fine on 91, Z06 requires 93 per Dave Hill, and more importantly, per Chevrolet.
Still haven't found out if the Z06 just drops a few HP on 91 , or whether there is an engine life and/or warranty issue.
C6 recommends 93, runs fine on 91, Z06 requires 93 per Dave Hill, and more importantly, per Chevrolet.
Still haven't found out if the Z06 just drops a few HP on 91 , or whether there is an engine life and/or warranty issue.
Drifting
Joined: Nov 2004
Posts: 1,923
Likes: 62
From: California
Z06 needs 93 octane??
Originally Posted by icedancr
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?
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?
Drifting
Joined: Nov 2004
Posts: 1,923
Likes: 62
From: California
Questions..
Originally Posted by LDB
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.
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.
Great quote LDB, I have a few questions. If I happen to own a car that requires a 93 octane and out here in CA highest octane is 91. Would and Octane booster at Pep Boys do the trick. How good are those over the counter Octane Boosters. Will it do its job and will it be as good as a gasoline after the booster is added versus the 93 octane in other states?? Thanks
Drifting
Joined: Jan 2005
Posts: 1,937
Likes: 1
From: Costa Mesa CA
CI 6-7-8-9-10 Veteran
St. Jude Donor '06-'07-'08-'09
There is no "if" as to GMs statement.
"Recommended unleaded fuel: Z06 - 93 octane (required)"
See the engine specifications for the Z06 at http://www.chevrolet.com/corvette/specifications/
I just don't know what that means to the warranty and engine life.
"Recommended unleaded fuel: Z06 - 93 octane (required)"
See the engine specifications for the Z06 at http://www.chevrolet.com/corvette/specifications/
I just don't know what that means to the warranty and engine life.
Thread Starter
Drifting
Joined: Jan 2005
Posts: 1,949
Likes: 1,336
From: Houston Tx
Answers to various questions. Gasoline is blended at the refineries. The only exception is that sometimes, oxygenated RFG (with MTBE or ethanol) is shipped without the oxygenate, which is then added at the terminal rather than refinery. But terminals never blend components other than the oxygenate. And when you ship by pipeline, you ship in batches, or plugs. The pipeline knows where all the plugs are, so you do have the opportunity to pull a certain refinery’s plug out the other end, and maintain its identity. Some readers point out that at some terminals, they do not maintain branded identity, and that is true. But it is possible to maintain branded identity, and that is done at many pipeline terminals.
As to 91 versus 93 octane premium, the basic problem in the west is the special California specs. They severely limit olefins content and slightly limit aromatics. From the main discussion, recall that while olefins and aromatics aren’t as good as isoparaffins on M number, they are still fairly high. So reducing them makes it more difficult to generate octane in California. And that is aggravated by a second issue in the CARB specs, lower end point, which also robs potential octane. That leaves choice “A”, make premium octane a bit lower in California, or choice “B”, make the extra cost of premium much higher. For better or worse, most companies took choice “A”.
As far as using additives, the problem is that it’s extremely difficult to predict octane blending response. You would think that adding 10% of an additive that is 10 octane higher than the fuel would add 1 octane. But you’d almost certainly be wrong, and what’s worse, sometimes you’ll be wrong high, and sometimes low. Blending response is a very complex function of compositional parameters which you have no way of knowing with an analytical lab at your disposal. So the danger of adding “mouse milk” is, you’ll think you’re adding enough, but you won’t necessarily be doing so. I don’t have any easy answers for you.
As far as why the Z06 “requires” 93 octane, I can only take an educated guess. Perhaps spark retard is not as effective in reducing knock in that engine, or perhaps it’s fundamental knock response transitions quicker, such that the knock sensor can’t react fast enough. Orrrr, maybe they’re just being conservative. I don’t know enough about the Z06 parameters to know for sure. Maybe some GM reader does, and will comment.
As to 91 versus 93 octane premium, the basic problem in the west is the special California specs. They severely limit olefins content and slightly limit aromatics. From the main discussion, recall that while olefins and aromatics aren’t as good as isoparaffins on M number, they are still fairly high. So reducing them makes it more difficult to generate octane in California. And that is aggravated by a second issue in the CARB specs, lower end point, which also robs potential octane. That leaves choice “A”, make premium octane a bit lower in California, or choice “B”, make the extra cost of premium much higher. For better or worse, most companies took choice “A”.
As far as using additives, the problem is that it’s extremely difficult to predict octane blending response. You would think that adding 10% of an additive that is 10 octane higher than the fuel would add 1 octane. But you’d almost certainly be wrong, and what’s worse, sometimes you’ll be wrong high, and sometimes low. Blending response is a very complex function of compositional parameters which you have no way of knowing with an analytical lab at your disposal. So the danger of adding “mouse milk” is, you’ll think you’re adding enough, but you won’t necessarily be doing so. I don’t have any easy answers for you.
As far as why the Z06 “requires” 93 octane, I can only take an educated guess. Perhaps spark retard is not as effective in reducing knock in that engine, or perhaps it’s fundamental knock response transitions quicker, such that the knock sensor can’t react fast enough. Orrrr, maybe they’re just being conservative. I don’t know enough about the Z06 parameters to know for sure. Maybe some GM reader does, and will comment.
Pro
Joined: Jul 2005
Posts: 658
Likes: 0
From: Nicey Pleasant
Originally Posted by LDB
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.
And, yes, I can fully understand you innoculating yourself against what can be relentless criticism, by properly giving disclaimers...
Drifting
Joined: Feb 2005
Posts: 1,311
Likes: 1
From: Los Angeles CA
lol SO THE z06 requires a fuel that as far as I know ONLY ONE gas station in all of southern ca provides... guess if I ever get a Z06 Ill have to stop by at the airport and fuel up next to the Bonanzas and Mooneys!
Le Mans Master
Joined: Sep 2000
Posts: 5,635
Likes: 4
From: Orlando Floriduh
Using fuel with ANY amount of lead will ruin the catalytic converters. Aviation fuel is a leaded fuel... It probably will scew up some of the sensors too. I wouldn't try it.. I'd consider unleaded racing fuel though. P.S. I grew up in the San fernando valley. Where are you?
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Melting Slicks
Joined: Jul 2004
Posts: 3,316
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From: San Antonio Texas - Any sufficiently advanced technology is indistinguishable from magic
Wow! We go from Gen Chem 101 to Organic Chem 202, but with emphasis on the alkane molecules. Interesting stuff, but complicated. Thanks for shedding some light on a subject we all should know more about... 
