Understand Motorcycles & Fuel Octane Numbers

8/7/2019 Understand Motorcycles & Fuel Octane Numbers

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How to understand Fuel Octane ratings

and how it relates to your motorcycle...

Optimal "ideal" gasoline is comprised of two parts, n-heptane (sevencarbon atoms on a 16-hydrogen-atom backbone) and iso-octane(eight carbon atoms on an 18-hydrogen-atom backbone). Heptaneas 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 acertain point (preignition). Octane on the other hand is much morestable, resisting the tendency to predetonate/preignite much better.But, because of the smaller molecule size of heptane, you can crammore of them into a gallon; and because of their inherentexplosiveness, you can get more power out of that gallon than youcan a gallon of pure octane.

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: 10.5:12004 GSX-R1000: 12.0:1 • 05-08 GSX-R1000: 12.5:104-07 Hayabusa: 11.0:1 • 2008 Hayabusa: 12.5:1

This difference in compression ratios explains one of theprimary reasons why one 600 engine (such as the GSX-R600)produces substantially more power than another type enginewith the same displacement (such as the GSX600F Katana).This difference also means a significant difference in heatproduction within the engine (and cooling systemrequirements), plus how strong the individual components

Which leads us to modern gasoline motorcycle engines. The insideof a modern engine compresses the fuel-air mixture quite a bit (seeinset, below) . This is enough to cause pure heptane vapors tocombust many times over before a spark ever arrives at the cylinder.So instead of pure heptane, fuel producers mix in octane to helpretard that reaction (to get the desired effect). And thus thepercentage of octane is the octane rating (i.e. - an ideal fuel of 13%heptane and 87% octane would be labeled as having an octane rateof 87). Reduce the heptane and up the octane, and the fuel canhandle more compression & heat without predetonating...

Understand Motorcycles & Fuel Octane NumbersFriday 18 February 20111:11 AM

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must be (and how fast they'll wear out).

But nothing is ever quite that simple... And gasoline isn't usuallymade out of just n-heptane and iso-octane, instead incorporatinglots of other size -tanes and additives. So those figures are used asreference numbers. When a manufacturer says the fuel is 87 octane,what he is really saying is that his fuel, whatever it happens to bemade 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 enginedesigners discovered around 1915 that fuel octane numbers behavedifferently under different load situations. For example, whathappens in neutral at low RPM's is not the same as what happens ina hot engine under a heavy load while accelerating. And thus, twodifferent standard methods for testing octane behavior wereestablished - the Research Octane Number ( RON) and the MotorOctane Number ( MON ). The RON represents a typical idle-typescenario. The MON better represents a hot engine under load, and isusually about 10 points lower than the RON for the same fuel. InEurope, the RON number is normally what is listed on the pump. Inthe USA, the RON and MON numbers are averaged together tocome up with a Pump Octane Number ( PON ) (sometimes it's called by other names, such as weight-averaged octane) ; normally on thepump in the USA you will see a sticker that says something likePON=(R+M)/2.So, at least in the USA, we deal with PON numbers. Here, 87 octaneat the pump is actually an 87 PON. Now the problem with that isthat we don't know what the actual RON and MON numbers are forthat particular fuel formulation -- if the batch happened to have ahigher-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... orvisa-versa, if the RON number were particularly low and the MONnumber 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, themore the fuel needs to resist predetonation. With modern small carengines and some motorcycle engines running as high as 13:1compression ratios, these high-compression engines must generallyuse the highest octane fuels to avoid predetonation. The size of thecylinder and the RPMs of the engine play into the requirementheavily 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 beable to predetonate is pretty short (about 1/320th of a second). Italso has a quite small surface area to produce heat concentrationsat, with fresh fuel-air charge being crammed in (cooling down thecylinder) every 1/84th of a second at 10k RPM. By comparison, achevy 350 V-8 running at 3k RPM has a 716.9cc cylinder, and istaking 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 thedetonation product is left in the cylinder to heat the cylinder up, andin terms of how often fresh (cool) charges are sucked in. Thus, insituations where the compression ratio would dictate a nominal 93

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octane rating, a much smaller cylinder that cycles at a much higherrate may be able to get away with a few points less, such as a 91octane rating.Now, remember, I said that compression ratios were a multiple of the ambient air pressure (at least on non-turbo/non-superchargedengines). As a result of the lower air pressure at higher altitudes,less air is sucked into the engine (and less fuel with it), reducing theactual 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 moveaway from sea level and up into serious altitudes, gas stations stopselling the standard 87/89/92 PON and the PON numbers startfalling to figures as low as 84/87/90 instead. This fuel generallycontains more heptane (or compounds which act like heptane) andthus is more explosive by definition, which helps make up for thepower loss the engine suffers at altitude because of the smalleramount of fuel-air mix it takes in.Special Exceptions:1. If your engine has a large amount of carbon deposits in thecylinder (from poorly burned fuels), or sulfated ash deposits (fromsulfate-ash rich motor oils), then these deposits can retain sufficientheat to pre-ignite the fresh fuel-air charge being sucked in, and/orproduce sufficient of a change in the shape/size of the cylinder toraise compression to undesirable levels. In these cases, use of ahigher octane fuel may stop the pre-ignition/pre-detonation fromoccurring because of the higher resistance values of octane. But thatis a band-aide over a serious problem that should be rectified beforeit causes additional or subsequent damages (large carbon chunksfalling off or building up too high can seize engines and breakconnecting rods & valves; small pieces will score the #$&% out of your cylinder walls and ruin your rings' integrity).

2. Lower-compression engines which have been mechanicallymodified to alter the compression ratios through use of WiseCobore-over kits, thinner gaskets, head-work, turbo-chargers or super-chargers. All of these increase the compression ratio sufficiently torequire higher octane fuels.Which brings us full circle back to the beginning:

As long as the engine on your Katana has not had any mechanicalchanges that alter the compression levels AND is not a cantankerousold carbon trap, then it requires nothing more than 87 PON (aka 91RON) at sea level, and will not benefit from the extrapredetonation/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 fromanyone. I believe the detergent loads in Chevron, Shell and Mobilare supposed to be superior in quality to the no-name brands; in thecase of Chevron and Shell, my experience is that they are alsogenerally superior in terms of quantity per gallon.

Part 2: RPM's verses cylinder loading and how

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it relates to octane requirements.

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 anair current which starts drawing fresh fuel-air in through theintake. The exhaust valves start to close.

6. Piston tops out and starts back down, exhaust valves finishclosing (any fresh charge that made it into the exhaust nowgets 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.

○ Low RPM: the rising piston shoves some of the freshcharge backwards through the carbs, then the intakevalves close all the way, piston continues it'scompression stroke, spark happens 4 degrees before thepiston gets to the top of it's stroke (9 degrees for thosewith advancer kits, 9 to 13 degrees for most Europeanmodels of the same engine).

○ High RPM: the rising piston pushes upwards, but thespeed of the fresh air coming in through the intakevalves is so high that it continues to load the cylinder inspite of the piston moving upwards, resulting inmaximum loading. Then the intake valves close all theway, piston continues it's compression stroke, sparkhappens 4 degrees before the piston gets to the top of it's stroke (9 degrees for those with advancer kits, 9 to13 degrees for most European models of the sameengine).

9. From here on out, is where stuff changes depending on RPM:

The engine we'll use an example (Katana/oil-cooled GSXR engines),like many high-revving motorcycle engines, utilizes an overlap inexhaust and intake timing, and closes the intake valves after thepiston has already started rising up in it's stroke. Effectively, it goeslike this:

This difference means that maximum compression ratios only occurat the RPM levels where this type of positive loading can happen in

spite of the piston rising (instead of being pushed backwardsthrough the carbs). Thus, this is also the primary time that octanenumbers are really critical on a healthy engine (because this is theonly time the maximum rated compression will actually occur).This particular bit of engineering is why the torque curve of the Katfalls where it does -- 6800 RPM upwards on a typical Kat motor --because that is the zone in which positive cylinder loading occurs inspite of the rising piston. Real easy to see on a dyno's output if youlook at the torque curve.Below that 6800 RPM limit, the compression ratio on the engine islower than the factory-listed compression ratio, because air (& fuel)

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is being pushed back out of the cylinder... which means for thosewho baby their Kats and stay out of the primary torque power-peak,the octane requirements would be even lower than the onesrecommended by Suzuki...

Part 3: How fuel can have octane ratingshigher than 100, and what's the deal with

race fuels?Well, that brings us to fuels that are rated above 100 octane (howcan anything contain more than 100% of something else?). Fuelsrated above 100 octane contain compounds which behave as if theyhad more than 100% octane -- because there are other compoundsthat 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 thereaction at time of detonation. This effectively does increase thecombustion pressure levels, which should result in *slight* increasesin total power output -- not because of the extra predetonation or

preignition protection, but specifically because of the extra oxygencoming into the equation.This is basically a weak version of the same thing that happens whenyou spray nitrous oxide (NOx) into your engine; the nitrogen isneutral and the extra oxygen adds to the power output rate.If you could design a filter to filter out only pure oxygen (O2) and ranthat through your engine instead of ambient air, you would netyourself 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 combustiontechniques. Creating such a filter system using modern twin-layerceramic filtration methods wouldn't be all that hard (although you'dhave to use some form of serious over-pressure system like asupercharger to force the gases to flow & separate through thefilters, and you'd need a very large surface area to feed even a smallcylinder). The problem with this is that no modern engine can takethe pressure and heat that would result (pressures that would spliteven pure titanium, heat levels that would melt even the strongestmetals); adding just a little extra oxygen in the form of NOx alreadystrains modern engines heavily.

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

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Understand Motorcycles & Fuel Octane Numbers

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