Octane rating
Seasoned HDF Member
Joined: Sep 2009
Posts: 17,134
Likes: 6,156
From: SE Michigan
Which weighs more, a pound of feathers, or a pound of bricks!? Octane is octane!
Seasoned HDF Member
Joined: Mar 2007
Posts: 66,948
Likes: 4,573
From: TOAK western branch
Yep. Run the lowest octane possible!
Grand HDF Member
Joined: Jan 2012
Posts: 4,438
Likes: 2,782
From: Oklahoma
In a properly running engine, the spark plug determines when combustion starts and a flame front moves from the spark plug across the combustion chamber hopefully generating peak pressure at just the right time to generate maximum power. Spark knock is when the fuel charge is so close to the self ignition temperature that when the plug fires, the entire charge burns at once. Even though combustion happened at the right time, it was so fast that peak pressure occurred too early and made the noise. An extremely hot engine can "preheat" the fuel charge so that pinging happens with fuel that is working properly at lower engine temperatures. Higher octane fuel continues to wait for the spark plug and burns with a controlled flame front even at higher temps and pressures, but even the higher temperature can be exceeded in some engines. Supercharges/turbocharges increase the air charge to the cylinders thus raising the compression temperature rise. That is why aviation fuel used to be supplied in the 120 octane range.
Buying higher octane fuel than you need guarantees that the spark plug will light the charge and with the fuel at a temperature well below the self ignition temperature, the flame front may advance a bit more slowly than in a high compression engine made for high octane fuel thus developing less than optimum power with the delayed maximum combustion pressure, much like slightly retarding the spark.
The technique of mixing in lower octane fuel helps to get the flame front to advance at the optimum rate to develop maximum combustion pressure at just the right time. Fiddling with timing would do about the same thing. Simply put, octane is a measure of resistance to dieseling in a gasoline engine. It is NOT, by itself, a claim for better detergency or having more or less power per gallon, or of the fuel being cleaner or of higher quality.
Two similar engines (bore and stroke the same) but one with higher compression pistons and or heads will "inhale" the same amount of gas/air mixture with each intake stroke. When the mixture is ignited, the heated air will be in a smaller space in the high compression engine thus exerting more combustion pressure on the piston which exerts more force on the crankshaft. That is why a high compression engine makes more power for a given size. The trade off is more expensive fuel and heavier duty engine components making the high compression version more expensive to fuel and buy. Often when an engine is "improved" for higher compression, other changes are made also to improve breathing, but that is outside the topic at the moment.
Last edited by btsom; Oct 10, 2013 at 12:46 PM.
Grand HDF Member
Joined: May 2008
Posts: 4,579
Likes: 668
From: Rolla
My OCD won't let this continue. All gasoline has pretty much the same energy content (pure gas, not corn adulterated junk). Octane is a way of expressing a higher self ignition temperature, the temp at which a combustible mixture will "explode" without an outside source of ignition. Compressing any gas causes its temperature to increase, that is the way diesels work. If the temperature increase during compression reaches the self igniting temp of the fuel/air charge, it explodes causing the ping or knock you hear.
In a properly running engine, the spark plug determines when combustion starts and a flame front moves from the spark plug across the combustion chamber hopefully generating peak pressure at just the right time to generate maximum power. Spark knock is when the fuel charge is so close to the self ignition temperature that when the plug fires, the entire charge burns at once. Even though combustion happened at the right time, it was so fast that peak pressure occurred too early and made the noise. An extremely hot engine can "preheat" the fuel charge so that pinging happens with fuel that is working properly at lower engine temperatures. Higher octane fuel continues to wait for the spark plug and burns with a controlled flame front even at higher temps and pressures, but even the higher temperature can be exceeded in some engines. Supercharges/turbocharges increase the air charge to the cylinders thus raising the compression temperature rise. That is why aviation fuel used to be supplied in the 120 octane range.
Buying higher octane fuel than you need guarantees that the spark plug will light the charge and with the fuel at a temperature well below the self ignition temperature, the flame front may advance a bit more slowly than in a high compression engine made for high octane fuel thus developing less than optimum power with the delayed maximum combustion pressure, much like slightly retarding the spark.
The technique of mixing in lower octane fuel helps to get the flame front to advance at the optimum rate to develop maximum combustion pressure at just the right time. Fiddling with timing would do about the same thing. Simply put, octane is a measure of resistance to dieseling in a gasoline engine. It is NOT, by itself, a claim for better detergency or having more or less power per gallon, or of the fuel being cleaner or of higher quality.
Two similar engines (bore and stroke the same) but one with higher compression pistons and or heads will "inhale" the same amount of gas/air mixture with each intake stroke. When the mixture is ignited, the heated air will be in a smaller space in the high compression engine thus exerting more combustion pressure on the piston which exerts more force on the crankshaft. That is why a high compression engine makes more power for a given size. The trade off is more expensive fuel and heavier duty engine components making the high compression version more expensive to fuel and buy. Often when an engine is "improved" for higher compression, other changes are made also to improve breathing, but that is outside the topic at the moment.
In a properly running engine, the spark plug determines when combustion starts and a flame front moves from the spark plug across the combustion chamber hopefully generating peak pressure at just the right time to generate maximum power. Spark knock is when the fuel charge is so close to the self ignition temperature that when the plug fires, the entire charge burns at once. Even though combustion happened at the right time, it was so fast that peak pressure occurred too early and made the noise. An extremely hot engine can "preheat" the fuel charge so that pinging happens with fuel that is working properly at lower engine temperatures. Higher octane fuel continues to wait for the spark plug and burns with a controlled flame front even at higher temps and pressures, but even the higher temperature can be exceeded in some engines. Supercharges/turbocharges increase the air charge to the cylinders thus raising the compression temperature rise. That is why aviation fuel used to be supplied in the 120 octane range.
Buying higher octane fuel than you need guarantees that the spark plug will light the charge and with the fuel at a temperature well below the self ignition temperature, the flame front may advance a bit more slowly than in a high compression engine made for high octane fuel thus developing less than optimum power with the delayed maximum combustion pressure, much like slightly retarding the spark.
The technique of mixing in lower octane fuel helps to get the flame front to advance at the optimum rate to develop maximum combustion pressure at just the right time. Fiddling with timing would do about the same thing. Simply put, octane is a measure of resistance to dieseling in a gasoline engine. It is NOT, by itself, a claim for better detergency or having more or less power per gallon, or of the fuel being cleaner or of higher quality.
Two similar engines (bore and stroke the same) but one with higher compression pistons and or heads will "inhale" the same amount of gas/air mixture with each intake stroke. When the mixture is ignited, the heated air will be in a smaller space in the high compression engine thus exerting more combustion pressure on the piston which exerts more force on the crankshaft. That is why a high compression engine makes more power for a given size. The trade off is more expensive fuel and heavier duty engine components making the high compression version more expensive to fuel and buy. Often when an engine is "improved" for higher compression, other changes are made also to improve breathing, but that is outside the topic at the moment.
1st Gear
Joined: Oct 2013
Posts: 5
Likes: 0
From: NW OKlahoma
Great analogy btsom-that using higher-than-necessary octane is much like retarding the ignition timing.
If I might add to your excellent post:
Lower octane fuel is more volatile than higher octane fuel thus in colder weather a high octane fuel will not vaporize as well and can cost you power, mileage and may even cause hard-starting.
My owner's manual and the service manual both say to use 87 octane or higher. I've tried them all (87, 89; 91 in my area) and the bike runs fine on all with no pinging on any and best economy on 87. It's a box-stock (no mods whatsoever) Evo with the M-M fuel injection.
I also feel that any modded engine is the equivalent of an experiment and as test-pilots we need to try different things to see what functions best. IMO use the lowest octane fuel that doesn't cause pinging unless there is evidence (dyno or fuel economy) that your particular experiment prefers one over another.
If I might add to your excellent post:
Lower octane fuel is more volatile than higher octane fuel thus in colder weather a high octane fuel will not vaporize as well and can cost you power, mileage and may even cause hard-starting.
My owner's manual and the service manual both say to use 87 octane or higher. I've tried them all (87, 89; 91 in my area) and the bike runs fine on all with no pinging on any and best economy on 87. It's a box-stock (no mods whatsoever) Evo with the M-M fuel injection.
I also feel that any modded engine is the equivalent of an experiment and as test-pilots we need to try different things to see what functions best. IMO use the lowest octane fuel that doesn't cause pinging unless there is evidence (dyno or fuel economy) that your particular experiment prefers one over another.
Road Captain
Joined: Dec 2009
Posts: 570
Likes: 4
From: New Mexico
There are several ways to measure octane, the ones we prolly need to know are research, which means very little, and motor. There is an adjustable compression motor that is run and slowly compression added till it knocks and that is the true octane of the fuel. Pumps are usually marked "motor" or "rm2" I think. Motor is the true number. If you mix gas and you know the motor numbers you can take an average and that will be you octane. If you mix a gallon of 86 and a gallon of 90 you wind up with 88. btsom is right on the money also.
Grand HDF Member
Joined: Sep 2013
Posts: 3,820
Likes: 587
From: Michigan
Lots of info & mis-info getting posted here. For the record, "pure" gasoline has a MAXIMUM Research Octane number of 100 (referred to as iso-octane) and is 2,2,4-trimethylpentane. The other end of the scale (0) is normal heptane (C7H16).
RON is determined using a prescribed single cylinder engine with a variable compression ratio (capable of changing CR while running) under a very controlled & defined set of environmental variables. This is a research engine (developed to research the science of combustion in engines). Blending iso-octane and n-pentane in specific ratios, then observing the combustion characteristics of that blend (specifically noting when detonation occurs) provides the RON values that can then be used to quantify the RON of unknown fuels. A 90% iso-octane/10% n-heptane blend defines the research engine detonation level assigned to a 90 RON fuel, etc.
Motor octane uses the same research engine, but a different set of environmental parameters (higher rpm, hotter charge premix, etc). Motor octane numbers (MON) are typically 8/10 numbers lower than RON values for the same fuel.
Typical manufacturer specs for fuel octane are referenced in RON. Pump labels for gasoline in the US are (R+M)/2 - that is, an average of the fuel's RON & MON values.
Additionally (and this is where things become complicated), fuels are blended for specific markets and seasons. A fuel sold in FL will have significantly different properties than a fuel sold in CO. And fuels designed for summer use in TX will be completely different than fuel sold in ND in the winter.
My source of info? Retired tech w/35+ yrs working for GMR Fuels & Lubes.
RON is determined using a prescribed single cylinder engine with a variable compression ratio (capable of changing CR while running) under a very controlled & defined set of environmental variables. This is a research engine (developed to research the science of combustion in engines). Blending iso-octane and n-pentane in specific ratios, then observing the combustion characteristics of that blend (specifically noting when detonation occurs) provides the RON values that can then be used to quantify the RON of unknown fuels. A 90% iso-octane/10% n-heptane blend defines the research engine detonation level assigned to a 90 RON fuel, etc.
Motor octane uses the same research engine, but a different set of environmental parameters (higher rpm, hotter charge premix, etc). Motor octane numbers (MON) are typically 8/10 numbers lower than RON values for the same fuel.
Typical manufacturer specs for fuel octane are referenced in RON. Pump labels for gasoline in the US are (R+M)/2 - that is, an average of the fuel's RON & MON values.
Additionally (and this is where things become complicated), fuels are blended for specific markets and seasons. A fuel sold in FL will have significantly different properties than a fuel sold in CO. And fuels designed for summer use in TX will be completely different than fuel sold in ND in the winter.
My source of info? Retired tech w/35+ yrs working for GMR Fuels & Lubes.