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Beyond Octane

Overheard at the local track:

            “High octane fuels burn slower.”

            “My motor doesn’t need all that octane so that fuel won’t do me any good.”

            “I need the highest octane so I can max out my timing.”

Unfortunately, those statements are not always true.  As a matter of fact, those statements only have some merit in the street gas world where 93 octane fuel is king and 87 is used by most.  In the world of racing gasolines where higher octane choices abound, sharp engine builders and racers know they need to look beyond octane to find the right fuel.

But before we ignore octane, it’s important to look at how octane is measured in the first place.

Octane numbers are measured using single cylinder engines that look more like something out of a Model T than any modern engine.  These so-called “knock” engines are operated by trained technicians in labs under controlled conditions.  Two tests are used – one for Research Octane Number (RON) and another for Motor Octane Number (MON).  The MON test is not as “easy” as the RON test, so the MON is usually lower than the RON.

Many times you’ll hear that MON is more important than RON because the MON test is performed under higher temperature and engine speed conditions.  While this may be true, the laboratory test conditions are not indicative of what real race engines – heck, even mild street/strip motors for that matter – see at the track.  Also, some engines have shown a better correlation between horsepower and RON.  So, a word to the wise: don’t get hung up on octane numbers.

For a great example of why octane is not the only fuel parameter to ponder, consider the engines used in Formula 1.  These engines have compression ratios exceeding 18:1 and spin at RPMs pushing 20,000.  Sounds like a candidate for 116 octane race gas, right?  Nope - they use a 96 octane fuel!

It’s hard to find a wider variety of race cars than what shows up at your local drag strip.  On any given weekend you can see all sizes and shapes of engines ranging from raspy 4-cylinder motors to booming big blocks, and with all kinds of power adders thrown in the mix.  This is where looking beyond octane becomes real important.  What works for your buddy’s Pro Stock car may not be the best choice for your other buddy’s turbocharged import.

Naturally aspirated race motors with large combustion chambers spinning at high RPMs really like high-octane, fast burning fuels.  They need the octane to prevent uncontrolled combustion, and they need a fast-burning fuel so that the flame front can span the large bore of the combustion chamber quickly.  If you’re not sure which fuel burns faster than others, one indicator is specific gravity.  “Lighter” fuels – fuels with a lower specific gravity – tend to burn faster because fast burning hydrocarbons are themselves light.  Look for a specific gravity close to 0.70 and you’ll likely find a fast burning fuel.  Of course, consult with the fuel producer to verify your assumptions.  You might be surprised to learn that some of the highest octane fuels may also be some of the fastest burning fuels!

You might also be surprised to know that fast burning fuels may not need as much timing as their slower burning counterparts.  Many times we get calls from individuals who are dialing in new motors on a fast burning fuel but they’re using timing and jetting numbers from their old motor and fuel combo.  “Retard the timing a couple degrees and see what happens” is not the suggestion they expect to hear!  With high octane, fast burning fuels, it is easy to dial in too much timing.  In such cases, the engine is not detonating, but it is past the point of optimum spark advance, so it’s just heating things up and making less power.  A little less timing may really wake up the motor.

OK, so we’ve got the high winding naturally aspirated big blocks covered – what about those turbocharged, supercharged, and nitrous-fed motors?  Won’t the same fuel work for them too?  Isn’t it the best?  Well, yes and no…

Motors with lots of nitrous or boost don’t have to rely on fast burning fuels because the combustion chamber turbulence and pressure contributes greatly to the speed of combustion.  This is even more true for smaller and more efficient combustion chambers.

For these engines, “heavier” fuels – those with higher specific gravity values – are usually better suited.  Heavier hydrocarbon components in race fuels are very adept at holding off combustion until the spark plug says “bang”.  Interestingly, heavier hydrocarbons may not have the highest octane numbers, so be sure to consult with your fuel supplier for a good fuel recommendation.  In all likelihood you will be looking at fuels with specific gravity values of 0.72 and higher.

So do yourself a favor:  when selecting a race fuel, look beyond octane.  Sure, octane is important.  But if you want an edge on the guy in the other lane, consider the composition of the fuel too.  With a little homework – and a little luck – you’ll be coming back for the next round.

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