The C8H18 battle

C8H18, technically CH3(CH2)6 CH3. Octane. That's what we are talking about.

With gas prices the way they are, everybody is asking "what octane gas do I put in?" Why? Well the higher octane gas is more expensive. There are several websites and news agencies reporting that you can run your car on 87, the lowest octane available, without a problem. They state you waste your money buying higher octane gas.

Take this with a very large grain of salt. Technically they are correct. Your engine will run on 87 but some vehicles do highly recommend a higher grade of octane. We'll get into the whys in a little bit. For now we need to know what the heck this octane thing is all about.

Octane ratings, in number form, are an indication of gasoline's resistance to auto-ignition. What this boils down to is that higher octane gasoline is more resistant to burning. If you want to get technical we can talk about how the scale is measured by the equivilent mix of iso-octanes and heptanes, but we will leave that for another day.

Some common myths are that octane ratings somehow rate the power of gasoline or the speed of which it burns. This is NOT true. It also does NOT correlate to gas mileage increases or decreases due to the octane rating itself. Your style of driving a higher-performance car running low octane may result in MPG changes. Read on for more info.

Ok so what is "auto-ignition?" You may have heard the terms "pinging" or "knocking" used in conjunction with engines. When gasoline and air are mixed in the engine cylinder and compressed, the mixture's temperature rises. If it rises too much it will 'detonate' spontaneously instead of by the spark of a spark plug. It also may otherwise burn incorrectly. The timing of the explosion within the cylinder is quite precise. If you have gasoline exploding outside of this normal timing it creates a pressure/shockwave that works against the natural movements of the engine, which in turn creates the metallic 'ping' sound. This is of course not so good for the engine.

Keeping that whole timing/pressure/octane thing in mind, let's talk about the engine itself for a minute. Engines have a rating called a "compression ratio" which measures how much the air gets squished down as the cylinder compresses it. For instance, if you have an engine that normally lets in 1000 cubic centimeters of air and then compresses that to 100 cubic centimeters, this is a 10:1 compression ratio. Compression ratios vary from engine to engine. Normally higher performance engines have higher compression ratio. Why? The more you compress the fuel/air the bigger the explosion and the more power produced.

Higher octane ratings require higher activation energies. This correlates to the statement that the higher the octane rating, the more resistant to burning the gasoline is. (An activation energy is the amount of energy needed to start a reaction). Therefore you can compress higher octane gasoline more before it will spontaneously ignite or otherwise burn incorrectly.

Using the above few paragraphs we draw this conclusion. A regular non-performance oriented car does not have "lots of power" in mind with regards to engine design. The engine is built with a lower compression ratio because performance is not at the top of the list. This means the engine lasts longer, easier to design, etc, with the tradeoff of less horsepower. Therefore you can put a lower octane gasoline into the vehicle because the gasoline will burn normally at that lower compression ratio.

However a higher performance car demands more horsepower out of the engine. Said engines are usually designed with higher compression ratios. They also may be fitted with forced-air induction systems such as turbos or superchargers that effectively increase compression ratio. (the more air you stuff in to the cylinder, the more gas you can stuff in, the more power you make!) The higher compression ratio helps to produce more horsepower. It would also make lower octane gas ignite incorrectly, causing knocking. As we mentioned above the knocking is not good for engines.

Therefore, those higher compression engines require higher octane gasoline to run properly.

There is a wild card, however. Most engines today are fitted with a device called a "knock sensor". The knocking sound of engine timing being off produces a specific sound at a specific frequency. This little sensor listens for this sound. (Timing is defined as where the piston is in the up/down stroke when ignition of the gasoline occurs.)

When you're driving down the road this little sensor is always listening. Sometimes the engine will knock because of a myriad of different reasons: bad gas, too much load on the engine, wrong gasoline rating, etc... When the sensor hears the knocking sound it sends a message to the vehicle's computer saying "whoa buddy...adjust the timing to make the knocking stop!!"

When running normally most engines have what is called "advanced timing." The gasoline ignites a little before the piston reaches the top and starts on the way back down. Why? The gas takes a little time to burn so this 'advanced timing' allows the combustion to be finishing up as the piston starts its down-stroke (the "power" stroke) This produces more power.

If the timing is too far advanced then the fuel is completing its burning as the piston is still traveling up the cylinder. This also happens during if the gasoline burns too early due to overcompression of a low-octane gas. This burning explosion pushes down against the piston which is actually still on its way up. This creates the shock wave known as the knocking. Ah ha! We have a commonality between timing, octane, and knock! So too much timing produces knock!

So what does the computer do when this knock sensor starts freaking out? It adjusts the timing by retarding it. Retarding timing just means waiting longer to fire the spark plug and/or when to inject the gasoline into the cylinder. This produces less power. Also, if the timing has to be retarded too much the piston will be too far on its way down the cylinder to provide complete burning of the gas, thus producing far less power. It also results in high emissions and the possibility of unburnt gasoline. (Gasoline must be compressed to burn properly)

You can see that the adjustment of timing is finicky. Traditionally your car's computer will 'learn' what the maximum advance for timing is before it will knock. It keeps advancing the timing until it hears knock then kicks it back a notch. The most power you can get.

So you take your vehicle that the owner's manual is recommending you put 91 or 93 octane gasoline into and you instead put 87 octane into it because the media tells you your engine will run on it. Now the manufacturer of your car is recommending 91 or 93 most likely due to you owning a higher-performance or higher-compression ratio designed engine. Yes, it will run on this 87 you put into it. The knock sensor will retard the timing until the engine doesn't knock.

In the meantime, however, you are wondering why the engine seems to have lost power. Well, we just explained what happens when you retard timing to combat knock: less power. So what do you do? Mash the gas more. This uses more gas. It can also affect other systems such as automatic transmission shifts. The car is expecting xxx rpm to shift into xxx gear when the pedal is at xxx position, but that's not what the car sees so it shifts rough.

What caused all this? You put an octane in your engine that is lower than what the engine manufacturer designed the engine to run on. In extreme circumstances such as in super-duper tuned high-power cars you can retard the timing so much that it will cause expensive damage to engine components.

So what does all this boil down to? If your car recommends 87 octane you can put 87 in. The car will NOT run better with a higher octane unless you have some serious other mechanical issues with the engine. If you have a vehicle designed to run 93 octane, you should put in 93 octane. It WILL run on a lower octane but it will NOT run optimally. In the case of highly or custom tuned engines it could cause engine damage.

Keep this in mind when buying your next vehicle. Find out what octane the manufacturer recommends by looking in the book or on the gas flap. Should the vehicle you are purchasing have power-adding componentry such as a turbocharger you should expect it will require a higher octane gas.