One of the funniest, clean, movies ever . . . . .

 

That’s a good question, but one that can’t be answered with precision. There are two issues. First is octane itself. The higher octane should allow more spark advance without the knock sensors kicking in to shove the spark advance back down. That will help mileage by a bit. But the other factor is gasoline blending, and it’s probably just about as big a factor. Premium is at least somewhat tough for refiners to blend. The difficulty is that most high octane components are also high boiling, but gasoline needs a mix of low and high boiling components. Aromatics are dense and high octane, but are also high boiling. So you tend to get a lot of components called alkylate and light cat in premium. Both are low to medium boiling and decent octane, but they are also the lowest density among the low to medium boiling components. The higher density low to medium boiling components are lousy octane and thus tend to go into regular blends. So on average, premium blends are a smidgen less dense than regular blends. That means fewer pounds of gas per gallon of gas, which also means lower mileage. Now mind you, any individual blend can disprove that average. But I’d say that overall, if you measured average mileage on 1000 tanks of regular versus 1000 tanks of premium, you wouldn’t find much difference. The smidgen more mileage you’d get from higher octane is balanced by the smidgen lower average density of premium.

 

Thanks. I actually understood the last sentence.

 

Top tier 93 for me.

The owners manual for my car specifically says if 87 octane is used do not perform any aggressive driving maneuvers such as WOT and to fill with 91 as soon as possible.

Cracks me up the people that use 87 to save a few bucks. Really? The gas tank is 18 gallons with a .35 cent difference between 87 and 93 you'd save a whole 6.30 per fill up at max! A big ol $300 a year filling up once a week.

 

I used 93 octane in my old P-928S, in my LS3, and currently in the LS9.

Performance cars deserve performance fuel...and the modern top tier fuel
additives are better for your engine. If 93 isn't available I use 91.


(did someone say Prius ?)

 

You can't go wrong by following the OWNER'S MANUAL.

 

And your answer is why I love this forum.

One question though.. Your answer seems to imply that fuel is measured gravimetrically rather than volumetricly. I would have thought that the pump meter would simply be measuring volume and not weight. A liquid gravimetric meter is typically more expensive than a volumetric meter.

The C6 system is inferring volume from injector timing. A lighter fuel should go through easier (faster) at the same pressure so the DIC should read higher mileage with a lighter fuel. How's that for throwing sand in a precise answer.

 

You are correct in that “none of the above” measure things in mass. But the various systems have daisy chain linkages that if you think it through to the end, are driven by mass. For example, if you think about the car’s engine computer, the thing that drives the rich/lean logic is the O2 sensor reading. It commands the fuel injectors to deliver a certain number of pulses of fuel per millisecond, which in essence, is a volumetric flow rate command. While not absolutely true due to your observation that lower density material flows easier, it is more nearly true to say that each pulse delivers a certain volume of fuel rather than a certain mass of fuel, so the computer’s mileage calculation should still be reasonably accurate even as fuel density varies. But getting back to the question of whether the system as a whole is driven my volume or mass, if the fuel has less mass than the computer is expecting, because it is lower density than it is expecting, the O2 sensor will sense too much excess air in the exhaust because there hasn’t been enough fuel injected to use up all the air. Thus it will command a bit more fuel be injected to bring the O2 sensor back to target. So even though the system doesn’t measure or inject anything directly in mass, it ends up behaving as though it does, by virtue of the feedback loop from the O2 sensor. Hope that makes sense. It isn’t a trivially simple answer, but it’s as simple as I know how to make it.

 

I don't know that I agree - yet. You are assuming that the fuel has the same composition in terms of energy per unit mass (or volume). Imagine an engine running on hydrogen gas. Now change to atomized carbon vapor. The engine produces almost 3 times the energy for the same amount of oxygen running on carbon. But the carbon is 6 times more dense. Gasoline is a combination of both and the the ratio can be slightly affected by how the octane level is obtained. Add ethanol to the mix and the ratios change significantly as well as adding oxygen which is going to mess with the oxygen sensor calculation. At this point my head is starting to hurt. Oh, wait, that's probably just a result of the ethanol.

As the resident expert in this area you had the correct answer the first time - there is no precise answer to this question. My OPINION based on real world experience is that I get a couple of percent better mileage on 93 octane than 89. Enough to pay for the increased cost. By my OPINION is based on less than 50 gallons of 87 so the difference could well be inside the margin of error.

 

You're not the only crazy one here. When I bought my '13 GS, I insisted that my salesman fill the tank himself, and warned I'd better not catch anyone else in the car.

My sales guy is in his early 50s, and very understanding of the 'vette owner mentality, and he can be trusted. Some of the others I've seen, would probably do a 5k burnout on their way to fill the tank.

 

You are correct, mass isn’t the precise answer either, but it is more nearly correct than volume. The fully correct answer is stoichiometry, but that gets messy to explain, so I had been ignoring it. I got into that mess a little bit in a recent thread, where a guy claimed ethanol not only reduced mileage, it reduced power. I answered mileage yes, power no, and made the mistake of getting into stoichiometry. The two posts are near the end of this referenced thread. Read posts #21 and #23 in that thread first, and then continue with this one.

http://forums.corvetteforum.com/c6-c...uestion-2.html

The actual thing that the engine computer controls is air to fuel stoichiometry, at a ratio of just a teeny bit over 1 to 1. As all of the things you mention vary (fuel density, energy content of the fuel, ethanol in the fuel, carbon to hydrogen ratio of the fuel, and so forth), the computer adjusts back to target stoichiometry by looking at the O2 sensor reading, and injecting whatever amount of fuel is required to keep the exhaust gas oxygen content just a teeny smidgen above zero. The O2 sensor only responds to molecular oxygen in the exhaust gas, it does not see any oxygen that might still be tied up in any small amounts of unburned ethanol that may remain. So even though oxygen atoms that came in with any ethanol in the fuel do reduce the engine’s air requirement, they do not affect the engine computer’s ability to do the stoichiometry calculation based on the exhaust gas O2 sensor reading.

Your last post also has one other implied question, the statement about you seeming to get better mileage on premium. The most likely answer is your suggestion that since your observation was only based on 50 gallons or so, your result was simply within the noise level of random variations. The 2% that you reported is well within the random variation of mileage result from a given refinery, and if you look at comparing refinery A with refinery B, it can get even bigger. That’s because one refinery might be trying to blend more or less premium than another, and the processing configuration of one refinery may be somewhat different than another. There’s even the possibility that the regular that you tried had more ethanol in it. While you’d normally think ethanol would be at least as, if not more likely to be blended into premium due to its high octane, there is a counter-argument that applies in some situations. There are certain low octane streams in a refinery that simply won’t fit into premium no matter what, but if you put in enough ethanol to get the octane back, you can sneak them into regular. So if a refinery is long on those low octane streams for one reason or another, they may actually be doing the counter intuitive move of putting more ethanol in regular than premium. And to head off a question in advance, yes, I’m well aware that the ethanol is added at the terminal, not the refinery. But when the gas leaves the refinery, it must go with a “tag” so that they know how much ethanol to put in at the terminal.

 

So, for those of us that only speak English, 93 Octane is better right?

 

Steve, I was thinking the same thing.....and then when something happens they're the first to blame GM !

 

The one word answer is yes. The lengthy stuff was in response to specific questions about mileage and then about whether the car’s control system works in volume or mass units. The short answer to the mileage question is, it doesn’t make much difference, because certain factors in gasoline blending tend to cancel out the advantage you’d get from higher octane if everything else except octane was exactly equal. As illustrated by several followups, there isn’t a very good short answer to the volume versus mass question. That’s because the answer is neither, it is stoichiometry, and there for sure is not a good short answer to stoichiometry.

 

Precisely why I've been chuckling throughout this, and other threads.

So many of these "concerns", on so many threads on these forums, are clearly answered in the owner's manual, often with illustrations as well.

 

Thanks, you're a gentleman, and I was just being a smart ***.

 

Better than being a dumb one.

 

True, but like up here (Reno, Nevada, and across the state, many station do not offer 91 Octane...As stated 85, 87, and 89 are the choices...some only 87 and 89 [CostCo, Sam's Club, 7-11 etc.] )

The Chevy dealer here tells its new Corvette owners to only run 87 Octane. (Even though the manual states 91)

Another disturbing fact is, recently, all the gas coming out of California is now 10% alcohol. That ought play havoc on fuel systems....wonder how auto manufacturers are gonna deal with that on cars that are specifically warned to not use alcohol based fuels or their emission warranties will be voided. Hmmmmmmm.

MC

 

Octane of the gas sold in places like Reno can be lower due to altitude. Air pressure decreases with altitude. At an elevation of 5000 feet, the air pressure is only about 85% of what it is at sea level. With your car’s engine, that means that if you floor it at an altitude of 5000 feet, you get about the same power and acceleration as you would at 85% throttle at sea level. The fact that the engine is developing less power means that it needs less octane. The only thing you have to watch out for is that if you fill up with lower octane gas in Reno, then drive down to the coast, once you are on the coast, it would behoove you to fill up with higher octane gas before you floor it. The difference in octane requirement at altitude 5000 feet versus sea level is about 2. So if you were trying to use your owner’s manual to decide what octane is the minimum to use at 5000 feet for your engine, you would just subtract 2 from whatever it says.

On the ethanol content, all cars sold since the early 90’s have fuel lines, pumps, and hoses that are perfectly ok for ethanol up to 10%, both for the car and the warranty. Cars older than that have some risk associated with fuel system corrosion and fuel hose deterioration. There is debate about how much, if any risk is added by proposals to increase from 10% to 15% ethanol.

 

The LAST brand name gas is now gone from our town (Sunoco) so all I have left to choose from are no name/generic gasolines like Sheetz, Get Go, and whatever is open. None of them sell 93 grade. Guess it will be a bottle of octane boost each time I fill up from now on.