Octane Ratings
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Octane Ratings
I wanted to shed some light on the Octane rating questions because I have seen them around lately and heard them in person several times lately. This is information only, but it is the correct information.
Octane is NOT a measure of power capability by any means, ever!
Octane simply gauges a fuel's ability to resist knocking. That's it. The end.
A fuel's ability to produce power is based on it's flash rate, or burn rate, or burn speed, or fuel sequence, or any fun name you have for how fast the fuel actually burns. This number is not on a pump because the government is not concerned with your extreme power output. They only care about Octane, which is the number on the pump, but does not reflect the fuel's burn rate.
Fuel that is formulated to have a high burn rate is USUALLY coupled with a high Octane rating of 100 or higher. They know that the only people spending that kind of money on gas have spent some money on their motor. High compression race engines, 15:1 compression or higher, actually need the knock resistance of the higher Octane fuel. There are still variances in the burn rate among these fuels the same as there are variances in each producer's fuel mix. Shell v Sunoco, for example.
Because the higher Octane gas is only for "race" use, there are some chemicals in it which you may or may not like that may or may not be legal for street use. These chemicals can severely damage a street motor because the parts and service of a street motor are drastically different than that of a race motor. By running +100 Octane gas, you have "modifiers" in your gas that allow it to exceed 100 percent Octane (it's an actual measurement, not just a name). The chemicals that allow fuel to resist knocking beyond that of pure Octane can damage injectors, spark plugs, piston rings, O2 sensors, catalytic converters, and exhaust parts.
The people that make the gas assume that anyone buying it has no exhaust, race plugs, and a motor that gets new rings and injectors on a fairly regular basis. The concern is for a fuel that will not rob power by "knocking" or pre-ignition. Maximum power comes with maximum cylinder pressure at about 15 degreees past TDC. Ignition before the optimal point will push up the point of maximum compression and cause a loss of power.
Once you have achieved a no-knock status, you no longer need to increase your Octane rating. For most everyone in the world, that would be about 91 or so. Even less in cars with low compression engines.
Now where it gets tricky. The lower the Octane number, yes LOWER, the faster the fuel will burn once lit. This is important. In high heat, high compression motors, the resistance to knock is more important than the faster burn. Motorcycles without internal work done are neither high heat nor high compression (in race terms). Thus, they would theoretically make more power with the lowest possible Octane that prevents knocking.
The higher Octane gasolines, as mentioned above, are made specifically for racing and as such have chemicals that increase burn rates as well as Octane ratings, but NOT IN ANY RELATION. E85, which I'm sure you've all heard of, has an Octane rating of 105. Put that in your bike and smoke it, LOL. E85 has a MUCH lower burn rate because it is formulated to work in street cars. Pure ethanol has an Octane rating of 129, but will severely damage your motor and cause catastrophic losses of power. These are just examples of how higher Octane fuels do not always have increased burn rates.
For those that wish to use high burn rate fuels, you have some other things that should be considered. The Octane rating is NOT the burn rating. Be careful what
Octane is NOT a measure of power capability by any means, ever!
Octane simply gauges a fuel's ability to resist knocking. That's it. The end.
A fuel's ability to produce power is based on it's flash rate, or burn rate, or burn speed, or fuel sequence, or any fun name you have for how fast the fuel actually burns. This number is not on a pump because the government is not concerned with your extreme power output. They only care about Octane, which is the number on the pump, but does not reflect the fuel's burn rate.
Fuel that is formulated to have a high burn rate is USUALLY coupled with a high Octane rating of 100 or higher. They know that the only people spending that kind of money on gas have spent some money on their motor. High compression race engines, 15:1 compression or higher, actually need the knock resistance of the higher Octane fuel. There are still variances in the burn rate among these fuels the same as there are variances in each producer's fuel mix. Shell v Sunoco, for example.
Because the higher Octane gas is only for "race" use, there are some chemicals in it which you may or may not like that may or may not be legal for street use. These chemicals can severely damage a street motor because the parts and service of a street motor are drastically different than that of a race motor. By running +100 Octane gas, you have "modifiers" in your gas that allow it to exceed 100 percent Octane (it's an actual measurement, not just a name). The chemicals that allow fuel to resist knocking beyond that of pure Octane can damage injectors, spark plugs, piston rings, O2 sensors, catalytic converters, and exhaust parts.
The people that make the gas assume that anyone buying it has no exhaust, race plugs, and a motor that gets new rings and injectors on a fairly regular basis. The concern is for a fuel that will not rob power by "knocking" or pre-ignition. Maximum power comes with maximum cylinder pressure at about 15 degreees past TDC. Ignition before the optimal point will push up the point of maximum compression and cause a loss of power.
Once you have achieved a no-knock status, you no longer need to increase your Octane rating. For most everyone in the world, that would be about 91 or so. Even less in cars with low compression engines.
Now where it gets tricky. The lower the Octane number, yes LOWER, the faster the fuel will burn once lit. This is important. In high heat, high compression motors, the resistance to knock is more important than the faster burn. Motorcycles without internal work done are neither high heat nor high compression (in race terms). Thus, they would theoretically make more power with the lowest possible Octane that prevents knocking.
The higher Octane gasolines, as mentioned above, are made specifically for racing and as such have chemicals that increase burn rates as well as Octane ratings, but NOT IN ANY RELATION. E85, which I'm sure you've all heard of, has an Octane rating of 105. Put that in your bike and smoke it, LOL. E85 has a MUCH lower burn rate because it is formulated to work in street cars. Pure ethanol has an Octane rating of 129, but will severely damage your motor and cause catastrophic losses of power. These are just examples of how higher Octane fuels do not always have increased burn rates.
For those that wish to use high burn rate fuels, you have some other things that should be considered. The Octane rating is NOT the burn rating. Be careful what
Senior Member
Joined: Nov 2006
Posts: 1,495
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From: Los Angeles, California
RE: Octane Ratings
Do not worry. You can post factual evidence and people will still argue how much more power they make on 105 octane fuel. I am so sick of telling my friends that their cars are not made to work with high octane and therefore it is doing nothing but hurting them.
I only have one friend that understands octane ratings and he is running a 600 RWHP 4 cylinder engine.
I only have one friend that understands octane ratings and he is running a 600 RWHP 4 cylinder engine.
Senior Member
Joined: Aug 2007
Posts: 549
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From: stockbridge,ga
RE: Octane Ratings
ORIGINAL: demonchild
Do not worry. You can post factual evidence and people will still argue how much more power they make on 105 octane fuel. I am so sick of telling my friends that their cars are not made to work with high octane and therefore it is doing nothing but hurting them.
I only have one friend that understands octane ratings and he is running a 600 RWHP 4 cylinder engine.
Do not worry. You can post factual evidence and people will still argue how much more power they make on 105 octane fuel. I am so sick of telling my friends that their cars are not made to work with high octane and therefore it is doing nothing but hurting them.
I only have one friend that understands octane ratings and he is running a 600 RWHP 4 cylinder engine.
Senior Member
Joined: Nov 2006
Posts: 1,495
Likes: 0
From: Los Angeles, California
RE: Octane Ratings
Aint fair. You guys have aluminum heads. My steal headed 14:1 compression 396 big block with 43 degress of static timing will only run on 120 octane and only pushed ~650-700 RWHP (never dynoed, based on weight vs trap times). I am currently building a ls2 twin ball bearing turbo engine that will push close to 900 RWHP on pump fuel and still be streetable. Amazing what technology can do.
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