Octane Rating, What It Is and What Octane Does for Your Car

When people refer to octane.  They normally mean the octane rating of gasoline.  Every gas pump you pull up to has at least two grades of gasoline.  Normal people will call it cheap and expensive or maybe standard and premium.  Every now and then you’ll get someone who calls it low octane and high octane.  Which is closer to the truth than the other labels.  The question is what is octane rating and why does it matter, or does it?

Octane rating is the ability to avoid pre-detonation.  Pre-detonation is when fuel vapors ignite in your engine before your spark plug ignites.  This causes fuel to burn before the piston is the right position for detonation.  Which causes a knock in your engine, as your piston is now being forced down, while it’s being forced up by the crankshaft.

Most new engines have knock sensors that tell the computer if the engine is pre-detonating and retards the timing to prevent it.  However, you can only retard the timing so much before it causes running problems.  Thus the need for octane in fuels.  To allow the engine to run without knocking.

What does Octane Rating mean

Octane rating is typically a measurement of a fuel’s tendency to burn in a controlled manner, instead of exploding.  Octane is a family of hydrocarbons.  They boil around 260 degrees F.  Isooctane is used as the reference standard for gasoline’s resistance to self-ignite.  Better known as it’s anti-knock properties.

Various fuels are tested in a test engine, to see what their pre-denotation properties are.  Then they are compared to a fuel with a mixture of isooctane and heptane that has the same properties.  If the fuel has the same properties as a mixture of 87% isooctane and 13% heptane then the fuels Octane rating is called 87 octane.  Of course, no fuel uses just iso-octane and heptane.  In fact, fuels use all sorts of octane boosters and other fuel additives

History of Fuel Additives for Boosting Octane Rating

In the early 1920’s automotive manufactures searched for a way to keep engines from knocking.  GM engineers looked at all sorts of additives but settled on LEAD(tetraethyl lead), because it was cheap to produce.  Even though they knew there were problems with using lead in the 1960’s.  Gasoline was leaded until the clean air act of 1970.  With this act, lead was phased out of all gas.  It wasn’t until 1996 until lead was completely banned.

To replace Lead gasoline either MTBE(Methyl Tertiary Butyl Ether) or Ethanol was used.  By 1998 MTBE started to be phased out due to concerns of it contaminating the groundwater. By 2000  the USDA called for an increase in the use of Ethanol to replace MTBE, and by 2005 17 states had banned it.

Another additive used as an alternative to Lead was BTEX.  This was mainly used to raise the octane rating of low octane fuels.  However, due to health risks it has been being phased out and is currently limited to 0.62% in gasoline.

This really leaves Ethanol as the main octane booster in gasoline.  Whereas pure ethanol has an Octane rating of over 100.  This means when you mix 10% ethanol into an 84 octane fuel you get 87 octane gasoline.

Why you need Octane

When the fuel-air mixture is compressed in your engine it’s important that it ignites at the right time.  When it ignites too early the piston gets pushed down while it’s still trying to travel up.  If it ignites to late the piston is already going down thus making your engine less efficient.

What does make your engine run more efficiently, is higher compression.  With the right timing.  The problem is with compression comes extra heat.  That, in turn, can cause your fuel to pre-ignite.  Thus the need for a high enough octane rating to control the fuel/air mixture burn rate.

The Amount of Octane You Need

The amount of octane you need depends on the car you drive and the engine it has in it.  Better yet it really depends on the compression ratio of your engine.  Since most people don’t build engines and have not control over the compression ratio of their engine. Let’s assume some things.  Most basic modern cars will run off a standard octane fuel.  Depending on where you live that is anywhere from 85-87 octane.  People who live in higher altitudes need less octane due to thinner air.

If you drive a performance car or a car with a turbo you’ll want a higher octane fuel. You’ll need it to take advantage of the engine.  Otherwise, the knock sensor will keep your engine from running at peak performance.  Granted your sports car may run ok on standard octane but it can run better on higher octane.

If you have any questions on what octane fuel you should run in your engine consult your owners manual.  For those who don’t have an owner’s manual run a few tanks of regular then a few tanks of premium and see which one performs better and which one gets you better fuel mileage.

As with any time you check your fuel mileage make sure you do the calculations yourself.  Don’t trust anything your car may say.  Fill your tank.  Track the miles then fill it again.  Divide the number of miles you drove by the number of gallons it took to fill your tank again.  That will be true gas mileage.  Anything else is an estimate and won’t tell you enough about how the fuel is truly performing.

Conclusion

The Octane rating doesn’t really matter to the normal driver.  You can fill up with whatever gas you want and not have a problem.  If your car starts knocking.  Then consider a higher octane fuel.  Of course, if you drive a performance car then, by all means, use high octane fuel in your car.  Just don’t concern yourself on what octane is and why every fuel pump has a rating on it.  In the grand scheme of things it really doesn’t matter a whole lot for most cars.

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