ckwhite

So many threads have been posted here on the Forum about octane ratings, what fuel to use in your Corvette and where to find it that I thought I’d put together a quick “primer” of what I know about this topic. Please feel free to comment – add any information or experiences of your own.

First and foremost, premium gas really is a better fuel in terms of the power it provides in the right engine. All gasoline is a heady brew of many different hydrocarbon molecules, ranging from heptane to decane and beyond. The hydrocarbon clearly identified on the pump is octane. This number, however, is not a measure of the percentage of octane actually in the gas itself. Rather, it is a measure of how that gasoline compares with a pure mixture of octane and heptane. The higher the number the harder it is to have engine “knock”.

Engine knock occurs when the compression of the fuel and air mixture alone, and not the spark plug, sets off an explosion. This results in a very loud noise and a lot of vibrations in the engine itself; it's very bad for engines mechanically, driving the piston down before it has reached the top of its cycle. Each hydrocarbon molecule behaves differently under pressure, but octane resists the temptation to explode better than the other molecules in the mix.

Most modern cars have a specific compression ratio, a measure of how much room is available to the fuel when the piston is at the bottom and the top of the cylinder. A compression ratio somewhere in the neighborhood of eight to one or lower can tolerate lower octane fuels (such as regular gasoline, good old 87 octane) without knocking. For these engines, there is no reason you should pay the extra money for higher-octane gas. High performance engines, however, often boast much higher compression ratios (The C7 Corvette’s LT1 engine has an 11.5:1 compression ratio). These cars require premium gasoline - 93-octane and above - and will definitely knock or perform poorly without it. Tailoring gasolines of specific octanes for different kinds of high-performance engines is a complex science in itself, and way beyond the scope of this quick explanation.

For many years, “racing gasoline” with octane ratings higher than 100 was produced by Unocal 76 (Union Oil Company of California). It was adopted as the official fuel of NASCAR and was available at many 76 stations across the country. Several years ago, Unocal merged with Chevron and the 76 brand was acquired by Conoco-Phillips, but none of these companies produce racing fuels anymore.

Aside from a number of small, “boutique” companies (Trick, Rockett), the only major oil company that produces racing gasoline (as much as 116 octane) today is Sunoco which has replaced Unocal as NASCAR’s official fuel and is available at many locations – usually racetracks – around the country. To find a location in your area, go online and use Sunoco’s “Fuel Finder”

And finally, in the event you find it inconvenient to travel to one of these locations, you can do what I did: simply buy and install your own gas pump. Then you can determine the grade of fuel you want, set the price, and fill up right in your own garage!

W88fixer

Not qualified to comment on most of your remarks but I will say that there are a lot of modern cars out there which are specified by the manufacturer to use 87 octane fuel that have compression ratios far higher than 8 to 1. Don

Steve_R

Good info.

At what point is higher octane "wasted" because it doesn't do any good?

Does octane add any additional available energy from the same amount of gasoline?

Might also want to discuss relationship between necessary octane and elevation - a topic I'm familiar with where I live.

ckwhite

You're right. There are. As I expressed, engine requirements for various octane levels depend on many things: ignition timing & fuel delivery, different ECMs, is the engine turbocharged, etc. Based on the manufacturers specs, even the new C7 Corvette can run on 87-octane gasoline. But that doesn't mean you'll get the best performance using a lower-octane fuel. Experience will also be a good guide; my little 213-CID aluminum-block Olds Cutlass had a 13.5:1 compression ratio. For general around-the-town driving, it ran fine on 88-octane fuel. But when I raced it at the strip, I HAD to use "high-test" (high-octane) gasoline.

ckwhite

Using a motor fuel with an octane rating higher than what your engine needs to have optimal performance really IS wasted money. That said, I think most would agree that an engine's requirements for racing are far more demanding than for everyday driving and in that case might benefit from using a higher octane fuel.

Octane and elevation? Good observation! I'd like to hear your take on the subject.

LDB

Your information isn’t seriously wrong, but it has some minor inaccuracies and isn’t overly complete. I’m a retired refinery engineer, and started the following two threads in the C6 section of the forum back in 2005 shortly after joining. Though they are 9 years old, the info is still up to date. The first one explains what octane means, while the second one talks about factors that influence how much octane your engine needs. Some people don’t want this much detail, and if you don’t fine, don’t read them. But I think the two posts below are about as simple and short as possible to give a reasonably firm understanding of octane issues.

https://www.corvetteforum.com/forums...ghlight=octane

https://www.corvetteforum.com/forums...ghlight=octane

ckwhite

[QUOTE=LDB;1588657055]Your information isn’t seriously wrong, but it has some minor inaccuracies and isn’t overly complete. I’m a retired refinery engineer.[QUOTE]

I can live with that. And BTW: Why aren't you writing this? I'm just a simple, retired petroleum geologist.

rcooper

Very good information and a nice addition to the OP's post, thanks.

LDB

I did write it, back in 2005, so smiles back at you. As far as the altitude question mentioned in posts #3 and #5 in this thread, as you go higher, air pressure drops, so the engine doesn’t pull in as much air because the air is thinner. So when you go full throttle at high altitude, it’s like going part throttle at sea level. That’s why you need less octane at high altitude. It would be like if you were in Houston at sea level, and put a block under the gas pedal so you could only open the throttle 3/4 of the way. Less throttle, less need for octane.

ckwhite

Good explanation. Thanks.

Highway 61

Is there an advantage to using non-oxygenated fuel over gasoline with ethanol in an C7?

0v3rc10ck3d

Soooo do I put 110 octane in my corvette and make +1000 horsepower or no?

LDB

Not hardly.

I’ve commented on ethanol in many threads, but don’t have a single post that covers it all very neatly. The short answer is that non-ethanol gas has a mileage advantage of about 5%, or 1 mpg. Claims of huge mileage differences are not credible. Ethanol carries slightly greater risk of trouble for long term, cold weather storage, but the emphasis should be on slight, very slight. Even non ethanol gas has some risk, and risk even with ethanol can be eliminated with a stabilizer in storage. And finally, there is some risk of fuel system corrosion and hose attack in very old cars. But realistically, the only difference that matters to the vast majority of people is the modest mileage difference. The bigger differences are political.

Democrats incorrectly view ethanol as benefitting the environment, while the Republicans love it because it benefits their farm state voters who grow the corn and fat cat investors who build the subsidized ethanol plants. Wouldn’t you know that one of the few things Republicans and Democrats both support is a boondoggle.

Some on the forum go to great lengths to find ethanol free gas. For my own use, if I can find top tier, ethanol free gas without paying much of a premium, I will use it. But that’s not available in many places. Risk related to engine cleanliness from using non-top-tier gas may not be gigantic, but it is clearly greater than risk related to using ethanol. So if your choice is top tier with ethanol versus non-top-tier ethanol free, it’s an absolute no-brainer. Top tier ethanol.

Fredvettec6

Thanks for the info and the references. Been awhile since my college chemistry days. What I do is alternate between highest octane available and next to highest between fill ups and I usually top of the tank when I get about 50% down. So I have a blend. No knocking in my C6. Now that gas is lower $ I will use highest octane in my C7 (when I get it, on order since 1 Nov. ). But obviously I won't use next lower octane if it is going to cause a problem going to

Highway 61

Thanks for the info, but a couple more questions.

Is it true that the non-ethanol gas has a higher burn temperature, which gives you a little more power with the high compression LT1 motor than ethanol fuel would?

My gas station is on the list as a Top Tier station, but does not sell 91 premium ethanol, only the 91 premium non-ethanol. Is this gas not Top Tier fuel?

LDB

Any flame temperature effect would be tiny. For context, drag racers go after this effect with nitro, but nitro’s flame temp is over 1000 degrees hotter than gasoline. Ethanol is only about 200 degrees lower than gasoline, so with only 10% ethanol in the blend, the gas plus ethanol blend would only be about 20 degrees lower. And that’s theoretical, with the actual difference always being less than theoretical. So while I admit that I don’t have much “tuner” experience, I’d be astonished if that translated into measurable power difference. I certainly never observed any in fuels testing of street engines. If someone claims that a measurable power difference exists, before I believed it, I’d look very carefully at his data to see if there are errors and/or possible explanations other than the fuel.

On your other question, you’re apparently in a place where non ethanol top tier is available. If so, it’s just as good on cleanliness as top tier with ethanol, so if the price is right, use it. Due to the mileage penalty of ethanol, you would break even economically paying 5% more for it than for gas containing ethanol.

Shaka

Pay attention.

Shaka

An unanswered question on one of those links.

No tuner that I know of has a fuel tester (chromatograph $8000 to $14000) includes octane measurement. Everytime you put in gas, the fuel properties will be different even from the same pump the next time you fill. Octane will vary 4 points, more or less. You can get 20hp more or less from one day to the next just because of the fuel on a standard day. Dyno runs are worthless without the knowledge of your fuel properties.
All the materials that go into the blends can cause the specific gravity to vary and that can affect how the fuel behaves in the injectors and the combustion chamber.
The use of ethanol as an octane booster is not a good idea. Octane has many structural isomers that differ by the amount and location of branching in the carbon chain which determine the octane rating. Vaporization destroys most of them which lowers octane rating. Only use top brand name gasoline. SUNOCO is the best. Big brand Gas companies add their additives and Isomers at their local distribution stations often with the same base blend from the refinery that everyone else uses.

OEM ECUs are designed for crap fuel. Be very careful when you mess with them. You are often running on fuel of a much lower octane rating than you think despite what it says at the pump. If you tune for race fuel, don't use pump fuel if you are going to get on it.

Maybe you have wondered sometimes why the engine is running hotter than normal. Well, it may not be your cooling system or the engine itself, it may be that you’re running a different fuel blend with a lower effective octane and didn’t know it. So you were getting by fine before but now because of the differences in the pump gas you’re using, this tank isn’t working so well.

Even racing fuel has ethanol. If you plan on running your car hard, better invest in some decent fuel. No pump gas will do on track days especially in high humidity regions. Just letting your 10% ethanol gas stand for a short while, reduces octane rating which can harm to your engine. Stock tune with pump fuel in your Z06.
Always gas up before rain storms and definately not during. When your intake is hot, vaporization occurs which further reduces octane rating. It stays cooler on track days.
Always purchase high quality fuel, test for water and alcohol before purchase, follow necessary precautions, and refill tank often to maintain octane rating. Store car with little fuel. Before you start it, ad fresh fuel enough to get to the SUNOCO gas station.
Reputable gas stations will give you a read out of their tanks water content. Come back when it is zero. 1%max. Change filter at least once a year.
Replacing fuel in your tank every 1-2 weeks will usually maintain the octane rating by limiting the occurrence of water absorption into the gas tank. Ethanol is very hygroscopic and a big con job by the crony capitalists working with the communists.

First year chemistry course required in mech. engineering 4 year degree with post grad work in thermodynamics and aerodynamics. Thesis:V4 OHC engine.

Kyflyer

Interesting posts and useful information. puregass.org has a listing of ethanol free fuel availability for those that are interested.

ckwhite

My off the cuff answer would be a simple “Yes”, but the full answer to this is a little more complicated than your question suggests, so here's a little background on the issue. LDB might tear it apart (he's better-qualified on the subject than I am) but read some of what follows and maybe you can make your own decision.

Oxygenated chemical compounds contain oxygen as a part of their chemical structure. The term usually refers to oxygenated fuels. Oxygenates are usually employed as gasoline additives to reduce carbon monoxide and soot that is created during the burning of the fuel. In the United States, the EPA had authority to mandate that minimum proportions of oxygenates be added to automotive gasoline on regional and seasonal basis from 1992 until 2006 in an attempt to reduce air pollution, in particular ground-level ozone and smog.

One of the first oxygenates was MTBE (methyl tertiary-butyl ether). MTBE is a chemical compound that is manufactured by the chemical reaction of methanol and isobutylene. MTBE was produced in very large quantities (over 200,000 barrels per day in the U.S. in 1999) and was almost exclusively used as a fuel additive in motor gasoline.

While the RFG (Reformulated Gasoline) program was advertised as being “great for the environment,” the benefits for the consumer were hard to find. Since the oxygenate additives cost up to twice as much as gasoline, reformulated gasoline cost up to 10 cents more per gallon than the non-oxygenated gasoline. Unfortunately, it gets worse. Both major oxygenated additives had a lower energy content than regular gasoline, MTBE roughly 20 percent less, ethanol 30 percent less. This results in a 2–3 percent loss in fuel efficiency. Translation: Consumers pay more to get fewer miles per gallon than before.

In 2005, Congress passed the Energy Policy Act that removed the oxygenate requirement for reformulated gasoline. At the same time, Congress also instituted a renewable fuel standard. In response, refiners made a wholesale switch removing MTBE and blending fuel with ethanol. According to EPA’s RFG Survey Data, MTBE has not been used in significant quantities in RFG areas since 2005.

In 2006 and 2007, North American automakers began to promote a blend of 85% ethanol and 15% gasoline, marketed as E85, for their flex-fuel vehicles, but these fuels are NOT acceptable for use in the C7 Corvette. While ethanol (alcohol) may provide for intrinsically cleaner combustion, due to its lower energy density it is not capable of producing as much energy per gallon as gasoline. Additionally, alcohol’s solvent properties may cause swelling damage to fuel system materials not designed for alcohols, corrosion of metal, increase water contamination, loosening of deposits causing clogging and destruction of fuel system components. This property was demonstrated with methanol in the 1970s and has reappeared to a slightly lesser degree with ethanol.

One last thing to mention: Most forms of automobile racing that require the use of gasoline as fuel (as opposed to higher-energy blended fuels or straight alcohols) prohibit the use of certain oxygenate compounds, since – in some specially-prepared racing engines - they can create a hotter fuel burn than the engine intake restrictions permit. Prior to the 2007 Daytona 500, for example, NASCAR driver Michael Waltrip and his team were heavily penalized when evidence of an unspecified oxygenate compound was found in the car's intake manifold during inspections.