I’ve never driven a car that’s needed anything but the cheapest gas available.  My family drove a car that ran on regular when it meant leaded well into the 80s.  I’m not sure how to fill up a tank with anything but the cheapy under 90 octane stuff.  I have friends who drive nicer cars than mine, not hard to do given I’m still rolling my windows down by hand, who fill up their cars with the good stuff.  Some places call it super, or supra, and others call it plus or premium.  Whatever it’s name, it’s got more octane, and apparently some cars need more octane.

How many of us actually even know what octane is, or what the difference between 87 octane and 93 octane? The octane referred to at gas stations is technically 2,2,4-trimethylpentane (isooctane), one of the isomers of CH3(CH2)6CH3.  For those who haven’t take organic chemistry, an isomer is a particular configuration for a particular molecule.

Isooctane looks like this (that is if you could actually see it):

Another isomer, 2-methylheptane - (CH3)2CH(CH2)4CH3, meaning is has the same atoms – the same number of hydrogens and carbons but configured differently looks like this:

So what does this all have to with your car? In the octane rating scale which goes from 0 to 100, the rating is actually a ratio of Octane to n-heptane, CH3(CH2)5CH3.  Apparently octane which has one more carbon atom is harder to combust and as a result is key to preventing “knocks.” Knocks are caused when fuel prematurely combusts in the compression cycle of engine operation.

Higher performance cars generally speaking have higher performance engines that have higher compression ratios.  The compression ratio is a measure of how much a piston compresses during the downstroke.  A bigger ratio implies that a piston compresses the air more “tightly” during each stroke.  Gasoline is intended to combust at the bottom of the stroke when there’s the most energy.  A lower octane fuel with its higher ratio of n-heptane is more likely to combust before the bottom of the stroke.  The resultant premature explosion causes both knocks which is effectively a mistimed piston upstroke and delivers lower performance as the resultant movement back of the piston is less than the intended full length.

So the question is do you need to use the recommended higher octane fuels for your car? Generally speaking, engines run at maximum compression when they are required to deliver top performance like climbing a hill.  The fact is most of the time an engine is not running at the top of their performance chart, and most engines do not run at high enough compression ever to require higher octane fuel.

I would generally think that if the owner’s manual requires a premium fuel then use premium fuel.  However, if it only recommends premium higher octane fuel than just stick the regular cheap stuff.  Chris Brown has been filling up his Maxima with regular for 8 years despite his owner’s manual recommending premium without problems.  Using a higher octane fuel does not improve performance nor is it better for an engine that does not require it.  If the compression ratio is such that a lower octane fuel does not prematurely detonate, using a higher octane fuel does nothing to improve the running of the engine.  Here are some facts and myths from the Minnesota Department of Commerce. The short answer is that if your engine doesn’t require premium high octane fuel, using high octane fuel is a waste of money.  However, I’ve never owned a car that recommended higher octane fuels, let alone one that required them,  so I don’t have any personal experience to back it up.  If anyone does, please comment.