Gasoline - Octane.. what's the deal

**ironforge** · July 19th, 2002, 10:46 AM

Just was at the pump this morning... put premium in my car.. took $40 to fill it up..

just wondering how much difference there is between regular, plus, premium.. etc..

what exactly is the octane rating? I have a little bit of knowledge on the subject.. but clarification is appreciative

(P+M)/2 method is typically what is used I think.. anyone can explain the formula?

**Lemon[H]ead** · July 19th, 2002, 10:50 AM

its probably to do with how environment friendly it is...

oh hang on you're in the states..."environment friendly" isn't in your vocab...

j/k

**Cruez** · July 19th, 2002, 11:14 AM

There is a little difference. Depends on what the car was designed for.

If it says "unleaded fuel only" you will be better off putting 87 octane in it...

If it says "premium fuel only" you can probably bet that it will be pinging away due to detonation....or not run very well on 87 octane..

Some cars may say "premium recommended" that means there is a noticable difference in performance, but it will run fine on lower grade fuel and have better performance with premium...(not many cars say this)

These are more or less generalizations.....if you are putting premium in a car not designed for it.....you are probably wasting your money....unless you have modded your car to take advantage of it...

**ironforge** · July 19th, 2002, 11:17 AM

Even if the performance gains might not be there...
would higher octane gas be cleaner for your engine?

I have been putting premium in my truck just because it is new.. heh.. got to feed the v8 something eh?

**vass0922** · July 19th, 2002, 11:18 AM

Agreed with cruez, everything I've heard says that if you're car is not designed for premium (and it will tell you!) dont' bother with premium gases..

Also from what I've heard no it doesnt help clean fuel injectors either

**Cruez** · July 19th, 2002, 11:29 AM

Originally posted by ironforge
Even if the performance gains might not be there...
would higher octane gas be cleaner for your engine?

I have been putting premium in my truck just because it is new.. heh.. got to feed the v8 something eh?

I doubt it... I had a Datsun Z car that the engine had over 365,000 miles on it...... very little buildup on the pistons....I ran plain jane 87 in it.....with an occasional FI cleaner.

**desmocat** · July 19th, 2002, 11:36 AM

Almost all grades of gasoline have a detergent additive in them.I think the EPA/Gov't more or less requires or mandates that.(in the US)
Also, a lot of engines have "knock" sensors in them, and will retard the spark timing to try and compensate for preignition "knock", and I guess you could try running regular, mid grade, and super to see what the lowest octane you can get away with running before you notice it pinging or the performance degrading. I have a 97 dodge ram 1500 with a 5.9 L andfor about the last 3 years, I have always ran 87 octane in it, even when pulling my boat and never had any problems. Mid grade and super were ran in it to the tune of about 3 months straight each, and I couldn't tell any difference in the mileage or the performance, so I went back to regular unleaded and been ther ever since. 147,000 miles on it too.

**Brainchild** · July 19th, 2002, 12:12 PM

regular is 83 plus is 86 and super is 90 but some companies carry 93. At least that's what it is around here. Mostly the higher the octane, the less the ping the engine will produce under load. In high performance engines, this means you can run more advance in the timing, and gain significant performance boost. I'm not so sure that with newer engines, designed for the lower octane fuels it will make that much difference, performance wise,I could be wrong.

**ironforge** · July 19th, 2002, 12:39 PM

here it is 87, 89, and 91

can it damage your engine if you run higher octane than designed for? or only if you run LESS than it was designed for?

**HfdWolfPack** · July 19th, 2002, 12:41 PM

Everyone has there own theory on this, but from what I gather, if your car says 87........run 87.

and if it isn't running well on said 87, something is wrong other than the octane level.

I was a big "higher must be better" but when the difference in price was .20 a gallon , I looked into it.
My car runs just as well on 87 as it does on 89 and I never bothered trying 90+ with my new car.

As far as being "cleaner".....I heard if the car is supposed to be run on 87, it is actually the opposite.

**Gomer** · July 19th, 2002, 01:28 PM

Agreed with the sentiments of WolfPack.

Ignition temperature is the only difference in octane levels. Octane plays a part in the temperature at which the gas ignites. Engines that run high compression ratios require high octane gas because of the higher temps (air gets quite hot when compressed). It will say in the manual what octane is required.

If you have pinging or other problems with low octane gas... and your car manual says that is what is required... you have other issues. For instance carbon buildup. The carbon stays red hot and will light off the gas too early. Also... various emission control systems, such as the EGR valve, may be failing.

Save yourselves the $$$ folks. Buy regular. I know so many folks who are paying that extra $0.20 it is silly.

Ironforge: If you run the higher octane in a low octane engine it won't damage the engine. However... if the engine runs cool to begin with.. running higher octane fuel may be detrimental to performance and mileage. Especially if your ignition system is on the weak side. Like I said, high octane wants high temps.

If you run low octane in an engine designed for high octane fuel you may be asking for trouble. Pinging and knocking happens when the gas explodes before it is supposed to. The piston is only 2/3's of the way up when WHAM this explosion tries to push it down as it is still going up. That is very hard on an engine... the shock plus it makes it run hotter increasing temps and compounding the problem. Now all engines these days have sensors that detect that knocking and retard the timing. This kills performance and mileage. Should that sensor fail or not work up to snuff it will allow the engine to face the additional wear. Either way... if the engine is designed for mid or high grade fuel you had better use it.

**HfdWolfPack** · July 19th, 2002, 01:47 PM

Thank you for elaborating Gomer

My response was more of a "In a nutshell"

**EndobioticChaos** · July 19th, 2002, 01:47 PM

It's really very very simple. Higher octane = higher ponies. One of my friends had an old Honda Accord. One day he was driving along and he happened past a gas station that had fuel somewhere around 125 octane. He was curious, and had to try it.

That ol' Honda with a lil 4-banger smoked rubber fit to kill with that high-octane gas.

Edit: I always use 93 or 94 octane gas in my '89 Accord around here, because if I don't it really struggles to climb the hills. It does make a difference, believe it or not.

shahani · July 19th, 2002, 01:59 PM

Endo's answer is the closest I think. Octane number refers to the heat/energy output in oxidation of a unit of fuel. Ergo, higher the octane number the higher the energy output per unit of fuel.

I buy the cheapest gas for my 1990 Accord DX because I don't need the extra kick since I am mostly alone in the car and its pretty flat around here.

But, yeah, if you want performance, use a higher octane. Unleaded gas has nothing to do with octane #. Nowadays almost all gas is unleaded.

**Cruez** · July 19th, 2002, 02:44 PM

Originally posted by EndobioticChaos
It's really very very simple. Higher octane = higher ponies. One of my friends had an old Honda Accord. One day he was driving along and he happened past a gas station that had fuel somewhere around 125 octane. He was curious, and had to try it. That ol' Honda with a lil 4-banger smoked rubber fit to kill with that high-octane gas.

Edit: I always use 93 or 94 octane gas in my '89 Accord around here, because if I don't it really struggles to climb the hills. It does make a difference, believe it or not.

Possibly because there was so much buildup on the pistons to raise the compression.....

NO NO NO....higher octane does not = higher performance... exerpts included...

High octane fuel is specifically designed for higher compression engines, which need slower burning fuel. For the rest of the cars on the road, it's just a waste of money and a source of extra pollution.

In many high-performance situation, riders clamor for higher octane fuels, thinking this will give them additional horsepower and, thus, an advantage over the competition. But this is not the case--adding higher-octane race fuel to your motorcycle (vehicle) may actually produce less horsepower. Here's why: Octane, an arbitrary number which is calculated as the average of the Research Octane Number (RON) and the Motor Octane Number (MON), and is only an indication of a fuel's sensitivity to knock, which is typically pressure-induced self-ignition. (Of these two ratings, MON is more applicable to racing fuels as it is measured under high load and high speed conditions.)
Octane, as you can see, is not a measure of how much power--or, more correctly, specific energy--is contained in a fuel. And remember that leaded high-octane race fuels burn slower than most unleaded fuels, and may reduce performance in stock or lightly modified motorcycles (vehicles). A high octane rating itself, however, does not mean that the fuel is slow burning. Hence, it has no direct bearing on the power characteristics of the fuel.

The bottom line here is that, in a given engine, a fuel that doesn't knock will produce the same power as most expensive racing gasolines.

I rebuilt my 280Z engine and raised the compression from 8.3 to 10.75. If I used the same fuel as I originally did, It would sound like a Mack truck due to the knocking....eventually fouling the spark plugs also. It is all a matter of the compression that the engine has.
Higher compression = higher octane and needs good ignition system.
Lower compression = lower octane, also good for turboed and supercharged engines.

**Gomer** · July 19th, 2002, 03:11 PM

Since there are still doubters and those who believe high octane=performance...

Better read up guys (so as not to continue the myth)

http://www.techweasel.com/articles/octane.htm

The difference in total energy between high octane and regular gas is insignificant. In fact, race gas often has slightly less energy by weight than pump gas! High octane fuels are generally harder to ignite, and burn more slowly than ones with low octane. In some cases, an engine optimized to run on 87 octane gas will produce less power when fed racing gasoline due to the slower ignition and combustion characteristics of the high octane fuel.

The number we most often associate with octane is the "Anti-Knock Index", or the "Pump Octane" number. This rating is an average of two different measurements; the Motor Octane Number, and the Research Octane Number. Both of these measurements are taken using a special single cylinder test engine that has a variable compression ratio. The RON measures the knock resistance of a fuel during low RPM, light load conditions, while the MON is representative of high-speed, high load operation. As a result the MON will always be lower than the RON, but for our purposes the Motor Octane Number is more significant because it represents the way racing engines are operated.

all you ever want to know about gas http://www.repairfaq.org/filipg/AUTO/F_Gasoline.html

The Low-Down on High Octane Gasoline (from the ftc)
http://www.ftc.gov/bcp/conline/pubs/autos/octane.htm

http://www.motorcycle.com/mo/mcnuts/rt-fuel1.html

In many high-performance situation, riders clamor for higher octane fuels, thinking this will give them additional horsepower and, thus, an advantage over the competition. But this is not the case--adding higher-octane race fuel to your motorcycle may actually produce less horsepower. Here's why: Octane, an arbitrary number which is calculated as the average of the Research Octane Number (RON) and the Motor Octane Number (MON), and is only an indic ation of a fuel's sensitivity to knock, which is typically pressure-induced self-ignition. (Of these two ratings, MON is more applicable to racing fuels as it is measured under high load and high speed conditions.)

Octane, as you can see, is not a measure of how much power--or, more correctly, specific energy--is contained in a fuel. And remember that leaded high-octane race fuels burn slower than most unleaded fuels, and may reduce performance in stock or lightly modified motorcycles. A high octane rating itself, however, does not mean that the fuel is slow burning. Hence, it has no direct bearing on the power characteristics of the fuel.

The knock tendency (and hence, the Octane rating) of a fuel is a function of the amount of free radicals present in the fuel prior to ignition and can be reduced by the addition of tetra ethyl lead, aromatics and other additives.

Although some racing organizations still use maximum octane number as the discriminating factor for fuel legality, it is really not appropriate for racing purposes.

Instead one should look at the amount of energy (heat) released in the burning of a particular fuel. This is described by the specific energy of the fuel. This quantity describes the amount of power one can obtain from the fuel much more accurately. The specific energy of the fuel is the product of the lower heating value (LHV) of the fuel and molecular weight of air (MW) divided by the air-fuel ratio (AF):

Specific Energy = LHV*MW/AF

For example, for gasoline LHV= 43 MJ/kg and AF=14.6, while for methanol LHV= 21.1MJ/kg (less "heat" than gasoline) and AF=6.46 (much richer jetting than gasoline). Using the above formula we see that methanol only has a 10% higher specific energy than g asoline! This means that the power increase obtained by running methanol, with no other changes except jetting, is only 10%. Comparing the specific energy of racing and premium pump gas you can see that there is not much, if any, difference. Only alcohol s (such as methanol or ethanol) have a slightly higher specific energy than racing or pump gas.

The real advantage of racing gasolines comes from the fact that they will tolerate higher compression ratios (due to their higher octane rating) and thus indirectly will produce more power since you can now build an engine with a higher compression rati o. Also, alcohols burn cooler than gasoline, meaning even higher compression ratios are possible with them, for even more power.

The bottom line here is that, in a given engine, a fuel that doesn't knock will produce the same power as most expensive racing gasolines.

Also important is the flame speed: Power is maximized the faster the fuel burns because the combustion pressure rises more quickly and can do more useful work on the piston. Flame speed is typically between 35 and 50 cm/sec. This is rather low compared to the speed of sound, at which pressure waves travel, or even the average piston speed. It is important to note that the flame propagation is greatly enhanced by turbulence (as in a motor with a squish band combustion chamber). (remember high octane='s slow flame speed)

By the way, pump gas has specific energies which are no better or worse than most racing gasolines. The power obtained from pump gas is therefore often identical to that of racing fuels, and the only reason to run racing fuels would be detonation probl ems, or, since racing fuels are often more consistent than pump gas--which racers call "chemical soup"--a consistent reading of the spark plugs and exhaust pipe.

additional info on octane http://theserviceadvisor.com/octane.htm

from cartalk.com (click and clack radio show... auto gurus)

Lots of drivers think that they're benefiting their engine or the environment by occasionally "treating" their car to premium gas.

That's hogwash.

Modern engines don't require premium gas and don't benefit from it in any way. And, in fact, most experts say excess octane creates more pollution. So unless your owner's manual specifically calls for the use of premium gas, don't bother. Regular gas is less expensive and less harmful to the environment.

Don't believe us? Fine. Here's what the Environmental Protection Agency has to say: "Unless your car needs high-octane gasoline, use of 'premium' gas will not improve performance or emissions--it will just cost you more."

FTC lawsuit info on a case where Unocal was making claims that higher octane fuel increases performance
http://www.ftc.gov/opa/predawn/F93/unocal-leo.htm

According to the FTC's complaint detailing the charges, the
challenged ads included statements such as:
-- "...you can't trust your investment to just any gasoline.
That's why 76 developed our 92 unleaded. It's the highest
level octane gasoline you can buy to help your car run
better, longer"; and
- more -
Unocal/Leo Burnett--12/30/93)
-- "Compared to regular unleaded, our 89 octane will give
your car smoother starts and stops...."
The FTC alleged that, through these and other statements,
Unocal, Union Oil and Leo Burnett have represented that Unocal's
89 and 92 octane gasolines provide superior engine performance and
longevity as compared to regular unleaded gasoline. According to
the complaint, the respondents did not have a reasonable basis to
substantiate these representations.
The proposed consent agreement to settle these charges,
announced today for public comment, would require all three
respondents to have competent and reliable scientific evidence to
support any future representations about:
-- the superiority of Unocal 92 or 89 in providing engine
power or acceleration for any car;
-- the superiority of Unocal 92 or 89 in prolonging engine
longevity; or
-- the attributes or performance of any gasoline with respect
to engine power, acceleration, longevity or any other
performance characteristic.

that should do for now... anyway, qualifying is on now =)

Enjoy... AND SAVE SOME $$$ NOW!!!!

**osprey4** · July 19th, 2002, 03:20 PM

Originally posted by shahani
Octane number refers to the heat/energy output in oxidation of a unit of fuel.

You're not a chemist, I take it. "Octane", to put it simply, is the ability to resist combustion.

The components that cause pre-ignition (aka knocking) tend to be lighter, aliphatic, linear compounds. More sophisticated, expensive refining processes take these out. The flip side is that the most polluting components, polynucleated aromatics, are not removed and are in greater proportion in higher octane fuels, so they pollute more.

**desmocat** · July 19th, 2002, 03:58 PM

Yep, and gas don't smell like it use ta'...

**Agent_Embryo** · July 19th, 2002, 04:07 PM

In Sweden we have 95, 96 & 98 Octane.

If I got it right, the higher the Octane, the faster(or slower....not sure) it burns.

But then again I know nothing about this stuff really...