Question: What’s the difference between the various octane ratings for gasoline?
Answer: It isn’t a stretch to say our daily lives depend on gasoline; we rely on it for transportation and, for many AM employees and customers, for recreation as well. So it would seem logical that we would know a bit about the smelly stuff that supports our modern “get up and go” lifestyles, but gasoline octane is one of the top 10 things we get questions about.
It’s all In the Numbers…
Depending on where you live, you may see anything from 85 to 93 octanes when you pull up to the pump, but what do those numbers really mean for your Mustang fuel system? Octane is the measure of a fuel’s resistance to knock, aka detonation. The number on the pump is known as the “Anti-Knock Index” or AKI number, and is the average of the RON (Research Octane Number) and MON (Motor Octane Number). The research octane number is obtained by running the fuel in a test engine under “low load”, controlled conditions and is typically a few points higher than MON. Motor octane is found by running a preheated sample of the same fuel in a similar test engine at higher speeds with variable ignition timing, pushing the fuel to its limits. These results are compared to mixtures of iso-octane (100 octane) and n-heptane (0 octane), giving them their octane rating.
…but what octane should I use in my Mustang?
If you’re running a naturally aspirated engine with bolt-on mods that don’t significantly affect compression ratio (like CAI, exhaust, throttle body, pulleys, etc) then the OE specified octane is your best bet. Using a higher octane won’t give you more horsepower or better mpg’s – this is one of the biggest myths out there. So when is the right time to start splurging on higher octane gas?
- If you’re tuned – you can always run a higher octane than you’re tuned for, but never EVER run a lower octane – this can severely damage your engine.
- If you’re spraying – nitrous raises compression ratio and combustion chamber temperature (sorry about the confusion, guys). Because of this, lower octane fuels are likely to cause knocking or pinging (detonation).
- If you’re blown – don’t laugh! Running a low octane fuel in your turbo- or supercharged motor can cause premature catastrophic failure. (Ok, now you can laugh)
Also – never run leaded gas (like race gas) in a car with catalytic converters. Tetra-ethyl lead (the additive found in “leaded” and race fuels) is a popular octane booster that was replaced with MTBE and ethanol for environmental and health concerns in the 90’s. Leaded fuels will result in an untimely death for your cats and your oxygen sensors.
Knowledge is the Key…
Many Mustangs are given early death sentences because of uneducated drivers, but hopefully this will educate and answer an all too common question. Running low-octane fuel in a car that should be running 91 or 93 will put it in the ground well before its time. Conversely, spending $40-$50 for a tank of 93 is just wasting money if your owner’s manual specifies 87. Now that you’ve been enlightened in the ways of octane – go share the Gospel of the Gas!