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Octane Ratings
by Matthew Kramer

Misconceptions
Do you have that friend with the Dodge Neon who claims he can beat 'Vettes down the straight when he fills up with 112 octane racing fuel? Or that buddy who works for the local airport that claims the AvGas he can get there makes his 15 year old Ranger pick-up into a virtual rocket ship? If so, maybe you should consult your friends to seek drug abuse treatment.

Despite what you may hear over and over to the contrary, a higher octane number does not mean a fuel will make more power. In fact, the two things have nothing to do with each other. Octane ratings are used as an anti-knock index, and mean little other than that.

Knock
"Anti-knock? Huh?" Well, anti-knock index refers to how well a fuel resists knocking. But what's knocking? Maybe you've heard of it referred to as detonation or pinging (and sometimes erroneously called pre-ignition). It's that dreadful "ping ping ping ping" noise your engine might make sometimes when it's struggling. Hopefully you've never heard this noise, but most of us have at some point.

What happens is this: In a 4-stroke internal combustion engine, air and fuel are sucked into the cylinders during the intake stroke. Then the valves are closed and this mixture is compressed as the piston rises in the compression stroke (appropriately named, no?). But in case you didn't already know, engines get hot. And on compression, the mixture in the cylinder gets even hotter. This might cause little bits of your fuel to ignite at the incorrect time (ie, some time other than when the spark plug fires). The uneven pressures from these little pockets of fuel igniting makes a pinging sound inside the cylinder. Hence the term "pinging" or "knock."

Knock is bad for more reasons that just the obnoxious noise it makes. The premature ignition of your fuel stresses components of the engine, leading to eventual failure. It increases temperature levels, which isn't good either. And for those people out there who care only for performance and couldn't give a damn about longevity (I envy you), it robs the engine of power.

Why Your Engine Knocks
So we know the underlying cause of knock, but what is the cause of the cause? Knowing that heat and high pressure inside the cylinder lead to knock, consider what can raise the temperature or pressure. Outside air temperature and barometic air pressure are givens. Making your engine work harder (hill-climbing or towing) have quite an affect. Other things are forced induction (turbo and supercharging), higher compression ratios, advanced ignition timing, and leaner fuel mixtures.

Looking at those "other things," notice that they all lead to performance increases. Ah, the bane of engine tuners. Turn up your turbocharger's boost, or shave a few mils off the cylinder head, and you get more power. That is, if you can keep the engine from knocking, which just got a hell of a lot more likely.

Fighting Knock
Obviously, if you're building a high-performance engine, much of what you do to the engine will increase its tendency to knock. Pretty counter-productive. So a way to prevent knock is greatly needed. If knock is caused ultimately by the fuel igniting too easily, a way to prevent it would seem to be by changing the nature of the fuel itself.

It was found that by mixing certain substances in with the gasoline, it became more resistant to knock. For the longest time, the common compound used was lead alkyl. This is where the term "leaded fuel" comes from. As you probably know, though, in the U.S. leaded fuel has been banned from the pumps by the EPA. European countries have begun banning its use now as well. Today's most common compounds are toulene, methyl tertiary butyl ether (MTBE) and tertiary amyl methyl ether (TAME). However, as seems to happen quite often these days, MTBE has been identified as a cancer-causing substance, and gasoline manfuacturers are under pressure to eliminate it from their fuels.

RON and MON
So that's great... We know that our engines are prone to knocking, and that by mixing in some substances with the fuel, the tendency to knock is reduced. But the older our engines get, or the more performance modifications we do to them, the higher that tendency becomes. How do we know that stuff we put in our tanks will resist knock in our particular engine. This is where the anti-knock index I mentioned earlier comes in.

The anti-knock index is how resistant a fuel is to knocking. This is also referred to as the octane rating. A higher octane fuel is less likely to knock than a lower one. Unfortunately, so many little things go into knock tendency, that there is no concrete way to determine knock resistance. The two most common measurement methods are the Reasearch Method and the Motor Octane Method.

It was found that a compound referred to as iso-octane had a high resistance to knocking, and that heptane was extremely prone to knocking. Studies were then performed to rate automotive fuels knock tendency against these compounds.
Subsequent tests of fuels in those days found that more reliable results were found with a slightly modified version of the Research Method's engine. The Motor Octane Method specified the same type of measurements using a faster engine and hotter mixtures. The results of these tests are specified as Motor Octane Number, or MON.

As fuels changed over time, it was found that neither RON nor MON accurately determined knock tendency over the full range of conditions. In the U.S., we adopted the use of their average as an anti-knock index. This is why when you read the labels on a fuel pump, you'll see soemthing like, "89 Octane (calculated using (R+M)/2)."

Your Car - From the Manufacturer
An automobile manufacturer designs a car's engine with certain parameters. They have to decide what the compression ratio will be, how high it will rev, whether or not it will have a turbo or supercharger, how they can keep it cool, and all sorts of goodies. There are trade-offs in every decision they make; considerations for longevity, cost-effectiveness, gas mileage, available fuels, environmental friendliness, etc.. In the end they give you an engine that will make X amount of power, and that you better always use Y octane fuel or higher.

Your Car - Using Lower Octane
As a fuel's octane number goes up, so does its cost. Why not just use the lowest octane number available to save money? By now, you should realize that by using an octane rating lower than what the factory determines is safe, you're risking damange and performance loss. Granted, they do specify these requirements with a bit of a buffer, as from refinery to refinery or even batch to batch, fuels may vary some. But while it is designed to run on a bad batch of 89 octane gas (that might as well be 87 octane), if you fill up with 87 octane, you might be getting a bad batch of it, and problems will ensue. You should always use the proper octane rating in your car, unless it's just plain not available where you're at (like Mexico).

Your Car - Using Higher Octane
I started this whole thing with the typical non-sense you hear from others who think higher octane fuel will turn their car into a land rocket. But face it, they're morons. In fact, using a higher-rated fuel than what your car requires will net you nothing in the power department. Your engine is made with certain parameters. Its compression ratio is fixed and its timing is set at its max setting (often it can be retarded by the engine control computer, but not advanced). As long as its not knocking, it's making as much power as it was designed to make. Increasing the octane of your fuel does not increase your compression ratio or advance your timing. Actually, if anything, the compounds mixed into the fuel to increase its octane rating will lower the stored energy of the fuel, decreasing power. However, this affect is negligible.

Your Car - Increasing Power
They make race fuel high-octane for a reason, though. It's not because the higher octane rating makes the engines more powerful. It's because the more powerful engines are more prone to knocking than a road car's engine, and a fuel with a higher anti-knock index must be used to prevent power loss and engine destruction.

So yes, your car can actually benefit from high octane fuel. But not when it's exactly as designed by the manufacturer. Only when you start making performance mods will the engine start getting hotter. Modifications like turbochargers (or increased boost if you've already got a turbo), shaved heads, advanced timing, or high-compression pistons are the real devils. When you make these, the tendency to knock will begin to increase. Then you too will need to follow the path of high-performance cars, and use a higher octane. But don't go straight for the 103 at your local 76 station, or the 112 at the race track. Increase by little bits. If you find your engine originally required 87 octane, see how it behaves on 89. If it still knocks at times, go up to 92.

Don't waste your money on $3.50/gal fuel that's not going to do a damn thing for you. Let your moronic friends do that. It depletes their budget for nice performance mods while you save the bucks.

 

JB,

That's a good article that you posted. It drives home the point that Octane has a lot more to do with pinging than it does with increasing power and performance.

Another very good article on the same subject is this one from Car & Driver which was brought to my attention by MickeyP a few months ago in one of his posts: Regular or Premium?

 

Great job on the post jbabbler, I just want to point out one thing. You mentioned 'little pockets of fuel burning at the wrong time', that is exactly what is happening. What you did not mention is those pockets are actually causing the piston to rock on the piston pin and the noise you hear is the piston skirt smacking the cylinder wall. In the average scenario that leads to rapid wear on the cylinder walls, and in real bad cases it can break the skirt off of the piston! That will sure put a knot in your tail! Thanks again for educating the rest of us on the 'hows and whys' of octane ratings. Well done!

 

its a good article, but its just by passing the problem with out fixing it. going to a higer octane doesnot fix the orginal problem because the 3.0L (its just a grenal article though) is ment to run on 87. so the next thing is how to fix the nock with out going to a higher octane.