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How to understand Fuel Octane ratings and how it relates to your motorcycle...

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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 PON=(R+M)/2.

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.

Special Exceptions:
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 a non-benefit.
   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:

  1. Detonation pushes the piston down.
  2. End of the stroke, piston starts rising again.
  3. Exhaust valves open, exhaust gases start getting pushed out.
  4. Piston keeps rising, pushing the spent products out the exhaust valves.
  5. 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.
  6. Piston tops out and starts back down, exhaust valves finish closing (any fresh charge that made it into the exhaust now gets sucked backwards).
  7. Intake valve maximize their opening size, piston continues to drop, more charge is sucked in.
  8. Piston hits bottom and starts back upwards - Intake valves still open.
  9. 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 heavily.

 


Additional readings:

"The Oil Drum: Refining 101 - Winter Gasoline & Custom Fuel Blends" by Robert Rapier, September 15th, 2006.

 
 

 
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