How to understand Fuel Octane ratings and how it relates to your motorcycle...
Optimal "ideal" gasoline is comprised of two parts,
n-heptane (seven carbon atoms on a 16-hydrogen-atom backbone) and
iso-octane (eight carbon atoms on an 18-hydrogen-atom backbone). Heptane
as a vapor in the right concentrations is very explosive, so much so,
that it tends to explode when subjected to a bit of over-pressurization
(predetonation) or ignite when heated beyond a certain point
(preignition). Octane on the other hand is much more stable, resisting
the tendency to predetonate/preignite much better. But, because of the
smaller molecule size of heptane, you can cram more of them into a
gallon; and because of their inherent explosiveness, you can get more
power out of that gallon than you can a gallon of pure octane.
Which leads us to modern gasoline motorcycle engines. The inside of a modern engine
compresses the fuel-air mixture quite a bit (see
inset, below). This is enough to cause pure heptane vapors to
combust many times over before a spark ever arrives at the cylinder. So
instead of pure heptane, fuel producers mix in octane to help retard
that reaction (to get the desired effect). And thus the percentage of
octane is the octane rating (i.e. - an ideal fuel of 13% heptane and 87%
octane would be labeled as having an octane rate of 87). Reduce the
heptane and up the octane, and the fuel can handle more compression &
heat without predetonating...
Typical Compression Ratios:
'98 - '06 Katana GSX600F: 11.3 times ambient (written 11.3:1)
'04 - '06 GSX-R600: 12.5:1 • '08 GSX-R600: 12.8:1
'98 - '06 Katana GSX750F: 10.5:1
2004 GSX-R1000: 12.0:1 • 05-08 GSX-R1000: 12.5:1
04-07 Hayabusa: 11.0:1 • 2008 Hayabusa: 12.5:1
This difference in compression ratios explains one of the
primary reasons why one 600 engine (such as the GSX-R600)
produces substantially more power than another type engine with
the same displacement (such as the GSX600F Katana). This
difference also means a significant difference in heat
production within the engine (and cooling system requirements),
plus how strong the individual components must be (and how fast
they'll wear out).
But nothing is ever quite that simple... And gasoline isn't usually made out of just n-heptane and
iso-octane, instead incorporating lots of other size -tanes and additives. So those figures are used as reference
numbers. When a manufacturer says the fuel is 87 octane, what he is really saying is that his
fuel, whatever it happens to be made out of, will behave, for all practical purposes, as if it was
composed of 13% heptane and 87% octane.
And that's not the end of the story yet either. Aircraft
engine designers discovered around 1915 that fuel octane numbers
behave differently under different load situations. For example,
what happens in neutral at low RPM's is not the same as what
happens in a hot engine under a heavy load while accelerating.
And thus, two different standard methods for testing octane
behavior were established - the Research Octane Number
(RON) and the Motor Octane Number (MON). The RON
represents a typical idle-type scenario. The MON better
represents a hot engine under load, and is usually about
10 points lower than the RON for the same fuel. In Europe, the
RON number is normally what is listed on the pump. In the USA,
the RON and MON numbers are averaged together to come up with a
Pump Octane Number (PON) (sometimes it's called by
other names, such as weight-averaged octane); normally on
the pump in the USA you will see a sticker that says something like
So, at least in the USA, we deal with PON numbers. Here, 87 octane
at the pump is actually an 87 PON. Now the problem with that is
that we don't know what the actual RON and MON numbers are for
that particular fuel formulation -- if the batch happened to
have a higher-than-usual RON value and a lower-than-usual MON
number, you might end up a fuel with the same 87 PON rating that suddenly
behaves like crap under heavy acceleration but is fine at idle... or visa-versa, if the RON number were
particularly low and the MON number particularly high, you might get pinging at idle.
Now to complicate matters a slight bit more:
Obviously, the higher the compression level of the engine, the
more the fuel needs to resist predetonation. With modern small
car engines and some motorcycle engines running as high as 13:1
compression ratios, these high-compression engines must generally use the highest
octane fuels to avoid predetonation. The size of the cylinder
and the RPMs of the engine play into the requirement heavily
also. If you think about it rationally, the time for one
cylinder of a 600cc 4 cylinder (149cc per cylinder) spinning at 10k RPM to be
able to predetonate is pretty short (about 1/320th of a second).
It also has a quite small surface area to produce heat
concentrations at, with fresh fuel-air charge being crammed in
(cooling down the cylinder) every 1/84th of a second at 10k RPM.
By comparison, a chevy 350 V-8 running at 3k RPM has a 716.9cc
cylinder, and is taking in a fresh charge every 1/25th of a
second. That's a lot of difference in heat exposure times, both
in terms of how long the detonation product is left in the
cylinder to heat the cylinder up, and in terms of how often fresh (cool)
charges are sucked in. Thus, in situations where the compression
ratio would dictate a nominal 93 octane rating, a much smaller cylinder
that cycles at a much higher rate may be able to get away with a
few points less, such as a 91 octane rating.
Now, remember, I said that compression ratios were a multiple
of the ambient air pressure (at least on
non-turbo/non-supercharged engines). As a result of the lower
air pressure at higher altitudes, less air is sucked into the
engine (and less fuel with it), reducing the actual pressure the
fuel vapor is exposed to. This means that a lower octane rating
can be used to get the same non-predetonating/non-preigniting effect in high altitude locations.
Thus, once you move away from sea level and up into serious
altitudes, gas stations stop selling the standard 87/89/92
PON and the PON numbers start falling to figures as low as
84/87/90 instead. This fuel generally contains more heptane (or
compounds which act like heptane) and thus is more explosive by
definition, which helps make up for the power loss the engine
suffers at altitude because of the smaller amount of fuel-air
mix it takes in.
1. If your engine has a large amount of carbon deposits in the cylinder (from poorly burned fuels), or
sulfated ash deposits (from sulfate-ash rich motor oils), then these deposits can retain sufficient heat to
pre-ignite the fresh fuel-air charge being sucked in, and/or produce sufficient of a change in the shape/size of the
cylinder to raise compression to undesirable levels. In these cases, use of a higher octane fuel may stop the
pre-ignition/pre-detonation from occurring because of the higher resistance values of octane. But that is a band-aide
over a serious problem that should be rectified before it causes additional or subsequent damages (large carbon
chunks falling off or building up too high can seize engines and break connecting rods & valves; small pieces will
score the #$&% out of your cylinder walls and ruin your rings' integrity).
2. Lower-compression engines which have been mechanically modified to alter the compression ratios through use of
WiseCo bore-over kits, thinner gaskets, head-work, turbo-chargers or super-chargers. All of these increase the compression
ratio sufficiently to require higher octane fuels.
Which brings us full circle back to the beginning:
As long as the engine on your Katana has not had any mechanical
changes that alter the compression levels AND is not a
cantankerous old carbon trap, then it requires nothing more than
87 PON (aka 91 RON) at sea level, and will not benefit from the
extra predetonation/preignition resistance. If you are buying
higher octane fuels, effectively, you are wasting your money on
It would be much wiser for you to buy low-octane
high-detergent-load fuel from a reliable dealer than to buy
high-octane fuel from anyone. I believe the detergent loads in
Chevron, Shell and Mobil are supposed to be superior in quality
to the no-name brands; in the case of Chevron and Shell, my
experience is that they are also generally superior in terms of
quantity per gallon.
Part 2: RPM's verses cylinder loading and how it relates to octane requirements.
The engine we'll use an example (Katana/oil-cooled GSXR
engines), like many high-revving motorcycle engines, utilizes
an overlap in exhaust and intake timing, and closes the intake
valves after the piston has already started rising up in it's
stroke. Effectively, it goes like this:
- Detonation pushes the piston down.
- End of the stroke, piston starts rising again.
- Exhaust valves open, exhaust gases start getting pushed out.
- Piston keeps rising, pushing the spent products out the exhaust valves.
- Intake valves open (piston still has a little bit of rise to it); the flow of the exhaust out the exhaust valves
has now set-up an air current which starts drawing fresh fuel-air in through the intake. The exhaust valves start to close.
- Piston tops out and starts back down, exhaust valves finish closing (any fresh charge that made it into the
exhaust now gets sucked backwards).
- Intake valve maximize their opening size, piston continues to drop, more charge is sucked in.
- Piston hits bottom and starts back upwards - Intake valves still open.
- From here on out, is where stuff changes depending on RPM:
- Low RPM: the rising piston shoves some of the fresh charge backwards through the carbs, then the intake valves close all the way, piston
continues it's compression stroke, spark happens 4 degrees before the piston gets to the top of it's stroke (9 degrees for those with
advancer kits, 9 to 13 degrees for most European models of the same engine).
- High RPM: the rising piston pushes upwards, but the speed of the fresh air coming in through the intake valves is so high that it
continues to load the cylinder in spite of the piston moving upwards, resulting in maximum loading. Then the intake valves close all the
way, piston continues it's compression stroke, spark happens 4 degrees before the piston gets to the top of it's stroke (9 degrees for those
with advancer kits, 9 to 13 degrees for most European models of the same engine).
This difference means that maximum compression ratios only occur at the RPM levels where this type of positive loading can happen in spite
of the piston rising (instead of being pushed backwards through the carbs). Thus, this is also the primary time that octane numbers are
really critical on a healthy engine (because this is the only time the maximum rated compression will actually occur).
This particular bit of engineering is why the torque curve of the Kat falls where it does -- 6800 RPM upwards on a typical Kat motor --
because that is the zone in which positive cylinder loading occurs in spite of the rising piston. Real easy to see on a dyno's output if you
look at the torque curve.
Below that 6800 RPM limit, the compression ratio on the engine is lower than the factory-listed compression ratio, because air (& fuel)
is being pushed back out of the cylinder... which means for those who baby their Kats and stay out of the primary torque power-peak, the
octane requirements would be even lower than the ones recommended by Suzuki...
Part 3: How fuel can have octane ratings higher than 100, and what's the deal with race fuels?
Well, that brings us to fuels that are rated above 100 octane (how can anything contain more than 100% of something else?). Fuels rated
above 100 octane contain compounds which behave as if they had more than 100% octane -- because there are other compounds that are even
more resistant to predetonation and preignition. These fuels also usually contain some form of oxygenator (TANE, MTBE, etc) which frees
extra oxygen from the fuel to mix into the reaction at time of detonation. This effectively does increase the combustion pressure levels,
which should result in *slight* increases in total power output -- not because of the extra predetonation or preignition protection, but
specifically because of the extra oxygen coming into the equation.
This is basically a weak version of the same thing that happens when you spray nitrous oxide (NOx) into your engine; the nitrogen is
neutral and the extra oxygen adds to the power output rate.
If you could design a filter to filter out only pure oxygen (O2) and ran that through your engine instead of ambient air, you would net
yourself about a 8 times (800%) increase in power out of the engine. It would also burn almost perfectly clean, with virtually no
by-products that commonly result from current combustion techniques. Creating such a filter system using modern twin-layer ceramic
filtration methods wouldn't be all that hard (although you'd have to use some form of serious over-pressure system like a supercharger to
force the gases to flow & separate through the filters, and you'd need a very large surface area to feed even a small cylinder). The problem
with this is that no modern engine can take the pressure and heat that would result (pressures that would split even pure titanium, heat
levels that would melt even the strongest metals); adding just a little extra oxygen in the form of NOx already strains modern engines
"The Oil Drum: Refining 101 - Winter Gasoline & Custom Fuel Blends" by Robert Rapier, September 15th, 2006.