Thread: What is octane? KR? Explained here.

Since so many people still have yet to understand what Octane is....and how octane ratings work, I thought I'd bring to light a little information about octane, Knock Retard, and the misconception of putting Race Fuel in a car that does NOT require it.
I've pulled several sources from the Web into one thread.
Now, Granted....this is a lengthy read, but please take the time to read this and understand what they are saying. You will get a better grasp on what your car needs relating to Knock and Detonation.

There's always a lot of confusion regarding octane, octane-boosters and how they work. Typical misconceptions are evident in blank-statements like:

"Higher octane fuels burn slower, thus their higher octane number"
"Higher octane fuels burn hotter, therefore more power is generated"
"Higher octane fuels explodes with more force, thus their higher power"

All three of which are untrue and are coincidental in effect, rather than causal. In actual practice, an engine has to be tuned specifically for high-octane fuels to generate extra power. If you have an engine fully-tuned and optimized for 91-octane pump gas, adding 100-octane race-gas into it will yield little if any increase. However, if you were to take that engine and increase the compression, advance the timing and/or increase the boost, then you can take advantage of the higher-octane fuel. But this precludes going back to the previous lower-octane fuels.

There are Three Kinds of Octane Boosters

1. ORGANO-METALLICS

There are three primary octane-boosting additives mixed in with gasoline: organo-metallics, ethers/alcohols and aromatics. Each one has distinct chemical properties and results (along with side-effects) on octane-boosting. Some people get these families of compounds and their effects mixed up.

First, let's look at organo-metallics which is used in the little bottles of over-the-counter octane boosters, what makes them work and how they compare. By far and large, these work on the same principle as TEL-Tetra Ethyl Lead which is the principle octane-boosting component of AvGas. For automotive OTC use, a slightly less carcinogenic MMT compound is used (methylcyclopentadienyl manganese tricarbonyl); it has pretty much the same structure as TEL, but with manganese substituted for lead. These compounds have a non-linear octane-boosting curve. The initial amounts give the most boost and adding more gives decreasing benefits. Typically you get 3-4 'points' increase with these types of additives; going from 91-octane to 91.4 octane max.

As you can imagine from the metallic content, these boosters create nasty deposits in your engine. That's why they typically include a solvent such as mineral spirits to try and dissolve the deposits. Then a lubricant such as ATF or Marvel Mystery Oil is typically added to the cocktail to help your rings slide over the deposits easier and minimize the damage. If you dyno-test a car using organo-metallics (with straight-through exhaust), you can actually collect metallic pellets coming out your tailpipe. Not a good thing to be putting into your combustion chambers no less...

2. OXYGENATES

The next group of octane-boosting compounds are oxygenates: ethers & alcohols which also serves an emissions purpose by bundling their own oxygen along with the fuel. The best compound here is ethanol (CH3CH2OH) with a 115-octane (R+M)/2 rating and containing 34.73% oxygen by weight. However, its high volatility with a RVP of 18 makes it unsuitable for use in warm climates for emissions control. In which case, MTBE (CH3OC(CH3)3), ETBE (CH3CH2OC(CH3)3)__ and TAME ((CH)3CCH2OCH3) are used which has more favorable RVPs of 1.5-8.0. But they also have correspondingly lower octane of 105-110 along with lower oxygen content of 15.66-18.15 by weight.

Ethers & alcohols are basically hydrocarbons fuels with an extra hydroxyl -OH group added to one end. These fuel-additives reduce your gas-mileage due to the displacement of hydrogen and carbon atoms by the larger oxygen molecules. The increased molecular-mass of the compounds with the attahced -OH is what gives the octane-boosting effect. The -OH group also makes the compound polar, water-soluble and highly reactive chemically. They will dissolve rubber and plastic fuel lines and thus their concentration in fuels is fairly limited. Thus their octane-boosting power is also reduced. Ethers and alcohols are starting to be banned in a lot of areas because their water-solubility makes tank leaks and dispersion by ground-water a big problem.

3. AROMATICS

The final group of octane-boosting compounds, aromatics show the most promise. Due to their stable benzene-ring structures, the compounds are non-polar and chemically stable (non-reactive). In fact, they are less volatile and less reactive than most other hydrocarbons in gasoline. This stability is what gives aromatics their octane-boosting powers. Normal gasoline typically contain around 25-30% aromatics, primarily toulene and xylene. Adding more will simply increase the octane rating and bring their concentrations up to what you find in higher-octane European gas (40-45% aromatics): Gasoline composition.

So by using aromatic toluene and xylene as octane-boosters, you get none of the bad side-effects of using organo-metallics (cancer and engine-deposits) or ethers & alcohols (low gas-mileage and rotting fuel-lines). By using just two gallons of xylene in a 15-gal tank of 91-octane pump gas, you've brought the octane-rating up to 94.5 and have roughly the same aromatic content as German or French gasoline. You may also notice in the Octane Booster Comparison article above, that the best octane-boosting solution was to use unleaded race-gas; the primary octane-boosting components used are toluene and xylene.

"Doesn't higher octane fuel have higher energy content and makes more power?"

Well, it's not so simple. Really depends upon what you mean by 'higher' and 'energy content'. 'Octane' does not directly relate to 'energy content' or 'power' in anyway. There are many, many components and properties of gasoline that is custom-tailored by the refinery, such as specific-gravity, octane, oxygenates (ethers & alcohols), RVP-reid vapor pressure (volatility), D86-distillation curve, combustion-temperature, sensitivity, flame-front speed, VL-vapor/liquid ratio, etc. Just about each and every one of these properties can be tailored and are sometimes dependent, and sometimes independent upon each other.

One of the basic measures of energy-content is BTU/gallon or Calories/gal. The amount of heat released by any given volume of fuel is directly related to the number of Hydrogen and Carbon atoms in that gallon. Oxygenated fuels that use MTBE or alcohols to have extra Oxygen onboard deliver much less energy per gallon because the oxygen atoms are simply HUGE compared to a hydrogen or carbon. Such fuels tend to deliver less mileage per gallon than non-oxygenated fuels. BUT, they do not deliver less power, because that's more of a function of air-mass ingested into the engine per 4-stroke cycle than fuel (air is tough to cram in, fuel is simple to inject).

Compared to gasoline's specfic-gravity of 0.751-g/cc, toluene is 0.881-g/cc and xylene (most likely a mixture of m-xylene; o-xylene; p-xylene) is around 0.871-g/cc. This means they have more hydrogens and carbons to combust per gallon with the O2 in the air that's being pumped through the engine. The results of using large-percentage mixtures of these aromatics in your fuel is a richer mixture than before with just pure pump gas (without re-jetting). This will be safer than using the other common additive, 100LL AvGas which is lighter than gasoline and will result in lean mixtures and melted catalytics and O2-sensors. (LowLead AvGas is still contains several times more organometallics than leaded auto gasoline). I've known of several people that have destroyed some very expensive engines because they ran a large amount AvGas without re-tuning their air-fuel ratios. Besides, 100LL AvGas is only about 98-MON anyway, so it's not as effective as toluene or xylene.

Octane Doesn't Predict...

Another factor that octane doesn't predict is combustion temperature which may or may not relate to the power produced. It's possible to blend two mixtures of branched-chain paraffins along with aromatics to create two concoctions both of which have higher octane than pump gas, and one of them will have higher combustion temps than pump-gas, and yet the other will have lower combustion temps.

A lot of people also confuse octane with flame-front propagation speed which is yet another independent factor.

Octane Does Predict...

In the end, all that octane predicts is AKI-Anti Knock Index as measured on a knock engine. These are variable-compression single-cylinder engines that can vary their compression between about 7.0:1 to 15.0:1. There's a highly-sensitive and accurate knock-sensor and computer hooked up to this engine that gives a readout of knock. The engine is run with the mystery fuel and starts at a low-compression. Then the compression is increased gradually while knock is monitored. Various levels of compression-ratios are used and the corresponding knock measured. This is looked-up on standardized tables and the MON-octane rating of the fuel is then determined. In the end, that's ALL that the octane predicts, is how much resistance the fuel has to knock.

Edit: Here is a little more information to add from a website that I found that sheds some light on octane ratings...I think LOL


Whitfield Oil Company: 100 Octane Racing Gasoline vs. Octane Boosters
What are octane boosters? Do they work? Why should I buy high octane gasoline when I can buy octane boosters?

Octane boosters usually contain one active ingredient, sometimes diluted in a solvent (like toluene). Typical active ingredients for octane improvers are alcohols, ethers, manganese (MMT), or tetraethyl lead (TEL).

ALCOHOLS: Methanol and ethanol are alcohols which have been used as octane boosters. They work since both have a higher octane number than typical street gasoline. They are more effective in low octane gasolines than in high octane gasolines. Alcohol have an affinity for water. This means that if there is a slight amount of water in the bottom of your gas tank, the alcohol can grab hold of the water and separate from the gasoline, leaving you with a water/alcohol mix at the bottom of your tank with gasoline floating on top. This is not good. And the last thing, even if you mix octane improvers containing alcohols with your gasoline, you will still not know what octane you end up with.

ETHERS: MTBE, TAME, and ETBE are the most common ethers available for gasoline use. They have higher octane values than typical gasoline, so like the alochols they will increase the octane quality of street gasoline. Ethers do not hae an affinity for water, will not seperate from gasoline, and blend like a hydrocarbon. When ethers are used as additives, the enthusiast still does not know what his final octane number is.

MANGANESE (MMT): Sometimes referred to as manganese, or more correctly Methyl Cyclopentadienyl Manganese Tricarbonyl (MMT). This can be an effective octane improver at very low concentrations YOu can gain one or two octane numbers using the recommended treat rate. Problems with emissions, injectors, spark plugs, oxygen sensors, and catalytic converters have all been traced to the use of MMT, which is why it is not legal to use by U.S. Refiners in Reformulated Gasoline. As indicated above with the alcohols and the ethers, it is tough to know what octane number you have attained.

TEL (Lead): Lead, Tetraethyl Lead, or TEL is known to be a very effective octane improver used in many racing gasolines and aviation gasoline. It is extremely toxic in its pur eform, and is illegal to use in any street driven vehicle in the U.S. since January 1996. It will poison oxygen sensors and catalytic converters. It is sold in a very diluted form by at least one supplier but not in California because of restrictions on metallic additives. Again, one still does not know the octane number of the final blend.

STREET GASOLINE FACTS: The 87, 89, 91, 92, or 93 octane that you buy at the local gasoline retailer is a good gasoline to satisfy the government's requirement for improved fuel economy and reduced exhaust emissions. It is a poor choice to make real power at high RPM. Spending money to enhance street gasoline with octane boosters is a waste unless your are more interested in satisfying octane needs than performance needs. Read on for another (and better) way to enhance performance with racing gasoline technology.

100 OCTANE UNLEADED RACING GASOLINE: An alternate solution to using additives is to use 100 octane Unleaded Racing Gasoline. This is a street legal 100 octane unleaded gasoline that can be used in its pure form, or it can be blended with any street gasoline. You will always know what octane you end up with because we can provide you with a blending chart that helps you to determine that. In addition, if the 100 octane gasoline is used in its pure form, the engine will make additional power due to the "improved burn" characteristics. This is because 100 Unleaded Racing Gasoline contains a very select group of hydrocarbons that vaporize and burn much more readily than those found in conventional street gasoline. When more of the gasoline is burned in the combustion chamber, the engine makes more horsepower. This phenomenon is known as improved combustion efficiency.

Engines equipped with nitrous oxide systems, turbochargers or superchargers develop higher cylinder pressures than normally aspirated engines and therefore need a higher octane gasoline. Higher cylinder pressures mean more horsepower. More horsepower (cylinder pressure) can translate into a destroyed engine if the octane quality is not satisfactory. 100 octane Unleaded Racing Gasoline can be a significant benefit for these applications.

Knock sensors are used on some engines to detect detonation (also referred to as ping). When the knock sensor is activated by detonation, it sends a signal to the engine control computer which electronically retards the spark timing until detonation ceases. This spark retard reduces engine efficiency which reduces horsepower an fuel economy. 100 octane Unleaded Racing Gasoline can provide you with higher octance and therefore solve the horsepower and fuel economy deficiencies.

Some racers and/or tuners think that they need a slow burning gasoline to make good horsepower. This is far form the truth. What we need is a good fast burn gasoline to be able to complete the burn in the time available. An added benefit is that less spark timing is required with a fast burn gasoline. The reason is that peak cylinder pressure occurs sooner with a fast burn gasoline. Too soon is not good, so the spark timing can be slightly reduced to take advantage of the fast burn, still allowing peak power to occur at the correct crankshaft position.

Keep in mind that at 6000 RPM, each spark plug fires 50 times per second. At this rate, there is very little time to draw the intake charge into the cylinder, compress it, burn it, expand it, and exhaust it. A good fast burn gasoline is very important in making as much horsepower as possible at this engine speed. The most horsepower is developed when the gasoline is burned completely.

Reptile...would you mind if I made this a sticky? that is VERY good information

Indeed, this is good stuff, probably should be added to the How To Section as well. Great stuff, thank you Reptile.

I like what you've written Reptile but whenever I've read about the whole 'flame front' thing with regards to KR I'm a little foggy. Another way I think about KR and timing is that fuel takes time to burn in the cylinder. Once combustion begins and starts to spread you will reach a point of peak combustion. Ideally you want this to occur slightly after the cylinder reaches TDC. If peak combustion happens before TDC you get Knock. Imagine your piston racing towards the top of the cylinder and before it gets there it runs into the meat of the fuel burn explosion. All of a sudden the force of peak combustion is trying to force the cylinder down and yet the mechanical makeup of the engine keeps forcing the piston up. At some point things are going to break. Anyway, that is why timing is retarded as one way to combat Knock. You start the combustion later to allow peak combustion to happen after TDC.

That is a picture I can get in my head. It helps me understand timing with relation to what I want to achieve with tuning but also how I would want to use it to battle KR. KR is actually the PCM retarding timing to accomplish this.

I need to try to get my head around the part about higher octane not burning slower, though. My understanding has always been that higher octane fuel has a more controlled (hence slower) combustion. That's why it helps combat knock. It allows the peak combustion to occur after TDC. Just a different method to get to the same place as retarding timing. I'll need to re-read your section on octane and burn rates and why higher octane reduces Knock a few more times.

Thanks for doing this.

What is octane?
If you've read How Car Engines Work, you know that almost all cars use four-stroke gasoline engines. One of the strokes is the compression stroke, where the engine compresses a cylinder-full of air and gas into a much smaller volume before igniting it with a spark plug. The amount of compression is called the compression ratio of the engine. A typical engine might have a compression ratio of 8-to-1. (See How Car Engines Work for details.)
The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.

The compression ratio of your engine determines the octane rating of the gas you must use in the car. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a "high-performance engine" has a higher compression ratio and requires higher-octane fuel. The advantage of a high compression ratio is that it gives your engine a higher horsepower rating for a given engine weight -- that is what makes the engine "high performance." The disadvantage is that the gasoline for your engine costs more.

The name "octane" comes from the following fact: When you take crude oil and "crack" it in a refinery, you end up getting hydrocarbon chains of different lengths. These different chain lengths can then be separated from each other and blended to form different fuels. For example, methane, propane and butane are all hydrocarbons. Methane has a single carbon atom. Propane has three carbon atoms chained together. Butane has four carbon atoms chained together. Pentane has five, hexane has six, heptane has seven and octane has eight carbons chained together.

It turns out that heptane handles compression very poorly. Compress it just a little and it ignites spontaneously. Octane handles compression very well -- you can compress it a lot and nothing happens. Eighty-seven-octane gasoline is gasoline that contains 87-percent octane and 13-percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane). It spontaneously ignites at a given compression level, and can only be used in engines that do not exceed that compression ratio.

I would love to see that myself. Anyone have that, post it up or send it to me an I'll host it.

I remember seeing that now. You need to download that thing and host it or save it somehow.

That is a great video. This has me doubting some claims of special plug configurations.

Originally Posted by IndeedSS

That is a great video. This has me doubting some claims of special plug configurations.

Lee when you say "special plug configurations?" What exactly are you referring to? The Gapping, the brand of sparkplug, or the heat range of the sparkplug?

Just curious.

Originally Posted by archemedes

I was guessing the multi electrode things (which make no sense to me as the whole purpose of indexing plugs was to get the electrode out of the freaking way of the flame)

I was thinking the whole index thing as well. Not too sure.

Originally Posted by GeddyLee

Lee when you say "special plug configurations?" What exactly are you referring to? The Gapping, the brand of sparkplug, or the heat range of the sparkplug?

Just curious.

Side gapped, cutback, muli directional spark spark plugs, etc., indexing...all of the stuff.

sorry /off topic

Originally Posted by archemedes

I was guessing the multi electrode things (which make no sense to me as the whole purpose of indexing plugs was to get the electrode out of the freaking way of the flame)

The point of indexing plugs is to point the plug to the far side of the chamber. Unless you have a hemi design where the plug sits in the center of the chamber.

On our configuration, the plug is on one side of the chamber and the plug can potentially thread in where the L of the plug points at the short side of the chamber and can actually inhibit flame front propagation. Using the indexing washers adjusts the tightening point of the plug so that it points to the far side so that flame front propagation better takes place.

Originally Posted by Mike N.

I ran 87 all the time, all of my GPs have been fine.

Define "FINE"
Sounds like your the reason for Variable timing, knock retard an Bad Fuel Spark tables all rolled up into one.

Sure the car will run. Doesn't mean it didn't suffer.

Originally Posted by IndeedSS

What is octane?

It turns out that heptane handles compression very poorly. Compress it just a little and it ignites spontaneously. Octane handles compression very well -- you can compress it a lot and nothing happens. Eighty-seven-octane gasoline is gasoline that contains 87-percent octane and 13-percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane). It spontaneously ignites at a given compression level, and can only be used in engines that do not exceed that compression ratio.

Not to call you out, but if octane rating were a percentage, It would be called octane percentage wouldn't it? and how would 115 octane gas fit into this theory.....

sorry if I stepped on any toes, just want to make sure the info in this sticky is accurate