Marketing and the Octane Myth

Copyright © 2015

by Ralph F. Couey

I admit it.  I fell for it.  I believed it, that myth that was promulgated by gasoline retailers about octane.  I thought that the higher the octane rating, the more power I'd get out of the engine, and when you're a teen-aged boy with a brain bathed in testosterone, power is everything.

Even as an adult, I persisted in my ignorance.  Now, age doesn't always make you smarter, but in my case the facts finally caught up with me.

In the 1950's and 1960's, gasoline retailers used to duel with each other in advertisements about the octane rating of their product.  The way the adverts were worded, it was easy for the consumer to misinterpret the meaning.  Also, since high octane gas was more costly, it did handsome things to the company's bottom line.  In the '70's, (especially after the oil embargo) smaller cars with smaller, less powerful engines began to hit the roads.  Now, the tune changed to miles per gallon, and eventually the value of detergent additives.  Even to this day, however, if you were to ask the average Joe/Josephine on the street what octane is for, you'd likely get the wrong answer 9 out of 10 times.

Octane is a chemical which is added to gasoline for specific reasons.  It's sole purpose is to raise the compression ratio, and therefore the ignition point (when it catches fire), of the fuel.  Car and truck engines are not the same.  The game little 4-banger in a Dodge Neon, for example, is vastly different than the 12-cylinder monster that Lamborghini drops into their cars.  One of the most basic differences is in compression.

Now, car engines are known as "four cycle" engines.  This refers to the up-and-down cycle that the piston makes inside the cylinder.  The cylinder is a metal tube and depending on the car, an engine will have 4, 6, 8, and sometimes 12 of them.  At the top of the cylinder are two valves, or openings, and a spark plug.  This is the sequence of what goes on inside as you motor down the freeway.

In the first cycle, or stroke, the piston starts at the top and is drawn downward.  At the same time, one of the valves opens and the vacuum created by the dropping piston draws in a very fine vaporous mixture of gas and air which was either pre-mixed in the carburetor or in the fuel injection system.  This is called the intake stroke.

In the second stroke, the intake valve closes and the piston begins to travel upwards.  This compresses the gas-air mixture, which as a natural function of the laws of physics, also makes it hot.  This is called the compression stroke.

When the piston has gone all the way up, and when the vapor is fully compressed, the spark plug generates a jolt of electricity which jumps across that gap between the little nub and the L-shaped piece of steel on the plug.  This ignites the vapor and the resulting explosion drives the piston back down.  This is the power stroke and is really what makes the car go.  

In the last cycle, another valve opens and as the piston travels back upwards in the cylinder, it pushes the ash and smoke from the explosion out of the cylinder and eventually out of the tailpipe.  This is called the exhaust stroke.

Inside your engine, this is a carefully coordinated symphony of movement among the pistons.  The power from the third stroke rotates a drive shaft (with a heavy metal spinning disk called a flywheel to smooth out the jolts, which connects to the transmission (gears) and then the wheels.  Anything that alters this process even a little can have really bad consequences.

People try to cheat both ways, using regular gas in high-performance engines designed for high octane, and using high octane in small engines.  Both choices are bad ideas.

If you have an engine that requires higher octane (your owners manual will state this with perfect clarity) and you try to save money by using the cheaper stuff, you will create a condition that is familiarly known as "knocking."  What is happening inside the cylinder is as the piston pushes into the compression stroke, the lower octane gasoline will ignite before the piston gets all the way to the top.  That early explosion sends a jolt through the entire engine.  Imagine trying to climb a set of stairs, and every time you lift your leg to the next step, someone whacks the top of your knee with a baseball bat.  That's what premature ignition, or knocking, is doing to your engine's innards.

At the other extreme is what happens when using higher octane to increase a smaller engine's power.

The piston will reach its full compression, but the pressure that the fuel-air mixture reaches is far less than it's designed for, which means it's not going to burn completely.  So, during the power stroke, the explosion is less, so the power sent to the engine is less.  If that doesn't make sense to you, then what happens next should be easy to understand.

When the piston pushes into the exhaust stroke, it's not only pushing out debris from the explosion, but also a lot of gasoline that didn't ignite.  And that unburned gas goes right out the tailpipe.  You may as well throw dollar bills out of your window as you go down the road.

Oh yeah, and it doesn't do the environment any favors either.

The consequences for the engine are serious.  Some of the unburned fuel and smoke collects on the spark plug, and eventually gunks it up to the point where it won't make the spark, which means that none of the fuel in the power cycle will explode.  It will instead blow out behind you.  If you're really unlucky, it may explode in the tail pipe, which is called a backfire, which can be dangerous.  The gunk will also collect on the piston and on the rings and gasket that keep the fuel and air mixture and explosion separate from the oil which lubricates the cylinder.

Also, your car will smoke.  Smoke, any kind of smoke, is simply unburned fuel.  A perfect fire, whether in your engine our under your S'mores, won't generate any smoke, because everything is consumed by the flames and converted to heat.

If you're driving behind someone and you detect a strong smell of gasoline in their car's exhaust, chances are that's a sign that they're using the wrong octane.

Car and truck (and motorcycle) manufacturers put a lot of thought and work into their owner's manuals.  Even if you don't understand mechanical things all that well, there are certain things you really do need to know about your car.  Like the recommended octane rating.  And the recommended tire pressures.  And the maintenance intervals. Yeah, I know...blah, blah, blah. 

Still, the money you put into the purchase of a car represents a significant portion of your livelihood.  If you really want to make your car last as long as possible (or at least until you're bored with it), then try to make the effort to understand how important your knowledge is to the life of your car.