in the
know
WITH SUNOCO RACE FUELS
Words by Zachary Santner - Technical Specialist, Sunoco Race Fuels
BEYOND
OCTANE
Octane is the most talked about
property when it comes to
gasoline. It’s no surprise because
selecting octane of gasoline is
the only choice the consumer
has, except what station to buy it
from. In this article, Sunoco Race
Fuels focuses on octane stability
of regular and premium octane
pump fuels versus racing fuels
and what you can do to prevent
octane loss in your fuel.
You have probably 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 is 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
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.
technicians in labs under controlled conditions.
Two tests are used – one for Research Octane
Number (RON) and another for Motor Octane
Number (MON). The RON test results in a higher
octane value than the MON test.
If you average RON and MON, you get an octane
value called AKI, short for Anti-Knock Index. It is
often labelled (R+M)/2, reflecting the fact that it is
the average, or midpoint, between RON and MON.
AKI is used to rate retail pump fuels, and it is the
most commonly used octane value for race fuels
as well.
For example – BP Ultimate typically has a RON of 98
and a MON of 88, thus yielding an AKI of (98+88)/2
= 93. We provide all the octane numbers for all of
our race fuels – RON, MON, and AKI. So while we
may refer to fuels like SR18 as “118 octane” (that’s
the AKI value), we also show the RON and MON
numbers under “Technical Details”. For SR18, that
would be 120 and 116 for RON and MON, respectively.
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 four-cylinder
motors to booming big blocks, and with all kinds
of power adders thrown in the mix. This is where
looking beyond octane becomes really important.
What works for your buddy’s Pro Stock car may
not be the best choice for your other buddy’s
turbocharged import.