Octane ratings

[Eagle 5]

This question may not belogn in this section but I'm not sure anyway,

100 octane is the highest octane you can have. however, certain chemicals/ race gas etc have higher octane levels. Toulene is i think 114, propan is 110 i believe etc. so my question is, what scale is being used above an octane rating of 100? You cant have an octane higher than 100. However the other chemicals are rated higher. Does anyone have an idea of what scale is being used above 100? Above 100 the scale is made up. does anyone know what scale is used to decided what octane level something is?

 

[awd4g63]

100 octane may be the highest octane you can get for unleaded gas. Then you get into leaded gas, like 110 octane, etc.

 

[BoostinAWD]

I can get 107 unleaded.
The octane rating is defined by how quickly the fuel burns, dont think of it as a 0-100 scale, 100 is not the ceiling.

 

[98spydert]

The octane rating is resistance to detonation and it's not a percentage. 100 octane is not 100%.

 

[Eagle 5]

100 ocatane is the highest of its kind, then obvisouly you can get higher numbers from diff chemicals. I had a discussion about it with my manager at work. we were wondering how you go above 100. i know its not a percentage, i already said its not a linear scale. so my question would be, does anyone know how they scale it above 100? yes ther eis 100+ unleaded octane... but it is not pure octane. 100 is the highest pure octane level, and its not linear from around 85ish to 100, below 85 though it is fairly linear. so, does anyone know how they scale the level?

propane has an octane level of about 110... but its not 110 octane. toulene has a level of 114 i think, but its not octane. so, how do they actually measure this level, that is my question.

 

[Eagle 5]

The name "octane" comes from the following fact: When you take crude oil and "crack" it in a refinery, you end up getting hydrocarbon chains of different lengths. These different chain lengths can then be separated from each other and blended to form different fuels. For example, you may have heard of methane, propane and butane. All three of them are hydrocarbons. Methane has just a single carbon atom. Propane has three carbon atoms chained together. Butane has four carbon atoms chained together. Pentane has five, hexane has six, heptane has seven and octane has eight carbons chained together.

It turns out that heptane handles compression very poorly. Compress it just a little and it ignites spontaneously. Octane handles compression very well -- you can compress it a lot and nothing happens. Eighty-seven-octane gasoline is gasoline that contains 87-percent octane and 13-percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane). It spontaneously ignites at a given compression level, and can only be used in engines that do not exceed that compression ratio.

During WWI, it was discovered that you can add a chemical called tetraethyl lead to gasoline and significantly improve its octane rating. Cheaper grades of gasoline could be made usable by adding this chemical. This led to the widespread use of "ethyl" or "leaded" gasoline.

from howstuffworks.com, a good site

so as i said, 100 octane is the highest octane, but obvsiouly we have ratings higher than 100. so, does anyone know how the scale works above 100 as it is made up? or is merely a linear scale, but if its linear than how is it defined.

 

[1fast97gsx]

this thread got me thinking .. if octane is the resistance to detonation, then obviously the higher the octane the more boost you can run. Now why is it that places ( roadrace for example ) tell you 650cc injectors are good for 21-22 psi, for 23-25 psi you need 720cc or larger. Bigger injectors don't raise the octane level they just spray more fuel. It doesn't seem like 650s would even be close to being maxed out at only 21-22 psi on a decent sized turbo. This thread kinda confused me ... at what point do you run out of "octane" no matter how big the injectors to where you need higher octane fuel? I've heard of people running 24 psi or so on 93 - 94 octane pump gas, but then others having a hard time running 20 psi with the same gas. Is it just tuning?

 

[98spydert]

Because adding extra fuel to the combustion process cools the mixture. Same reason you need an efficent intercooler to safly run higher boost levels, to keep the air fuel mixture cool.

 

[Eagle 5]

^^^ what he said. more fuel cools everything going in down some, so if the mixture goign into the chamber is cooler, the gas is less likely to detonate or pre ignite i guess, whichever its called. also if you have a more efficient intercooler the mixture will be colder, also depends on outside temp. you may be able to run say, 22 psi on pump in winter but maybe only 20 in the summer. apparently there are some people running up to around 25 on pump gas. which i assume if your compression ratio is lower, you could. say 8:1 instead of 8.5:1. a car with 8:1 comp ratio could run more boost than a 9:1 comp ratio... ASSuMEing that each was was tuned the same. still you can run the same amount of boost just in general its easier on lower compression, since its less likely to detonate or preignite.

this is also intersting to me as jet engines run lean and not rich, where when they add more fuel its gets hotter instead of colder... but this is of course, a completely diff engine and a different cycle

 

[kpt4321]

The octane rating is defined by how quickly the fuel burns

Almost, but not quite.

Octane is a rating based on the detonation resistance of a fuel, compared to the detonation resistance of pure octane. Two tests (research and motor) are conducted, and factors are changed until the fuel detonates. Depending on when this happens, compared to when it happens with pure octane, you get an octane rating.

Therefore, if a fuel with extra additives (such as lead, etc) lasts longer in the test without detonating than octane does, it gets a higher rating.

 

[kpt4321]

100 is the highest pure octane level, and its not linear from around 85ish to 100, below 85 though it is fairly linear. so, does anyone know how they scale the level?

It's not linear? Where did you find that informaton?

propane has an octane level of about 110... but its not 110 octane. toulene has a level of 114 i think, but its not octane. so, how do they actually measure this level, that is my question.

This a pretty contradictory, and it doesn't really make sense.

Propane has an octane rating of about 110 (R and M will be different, etc). Toulene has an octane rating of about 114. That's really all there is to it.

 

[kpt4321]

this thread got me thinking .. if octane is the resistance to detonation, then obviously the higher the octane the more boost you can run. Now why is it that places ( roadrace for example ) tell you 650cc injectors are good for 21-22 psi, for 23-25 psi you need 720cc or larger. Bigger injectors don't raise the octane level they just spray more fuel. It doesn't seem like 650s would even be close to being maxed out at only 21-22 psi on a decent sized turbo. This thread kinda confused me ... at what point do you run out of "octane" no matter how big the injectors to where you need higher octane fuel? I've heard of people running 24 psi or so on 93 - 94 octane pump gas, but then others having a hard time running 20 psi with the same gas. Is it just tuning?

There is a lot more at hand here than simply having enough fuel and having "enough" octane.

Road race cars have a number of conditions which make it crucial that you have extra fuel; they get hotter, they run longer, and they usually don't run the very expensive race fuels. The heat is the main killer, assuming the same fuel is used.

A drag car runs at WOT for 10-14 seconds. A road race car runs at WOT for several minutes, to several hours. To a point, the longer you run the hotter everything will get; intercooler outlet temperatures, coolant temperatures, the temperature of the physical motor itself, etc.

A richer A/F ratio will help with detonation, for two reasons. These reasons will be in my next post. Road race cars ned to run richer, so they ned more injector.

 

[kpt4321]

^^^ what he said. more fuel cools everything going in down some, so if the mixture goign into the chamber is cooler, the gas is less likely to detonate or pre ignite i guess, whichever its called.

Well, they are two different things.

Running a richer AFR will help keep the car from detonation for two main reasons. One of those reasons you have mentioned, the effects of the fuel cooling the mixture. However, that is the second reason that adding fuel stops detonation, the other is more important.

The other reason is that a richer AFR will slow down the flame front speed. The flame front will propigate faster as you lean the mixture out, due to the abundance, or overabundance, of oxygen. As you add fuel, you simply concentrate the mixture and slow the flame front speed down. A slower flame propigation will help to keep the car from detonating, and will lessen the effects of detonation when it does occur.

this is also intersting to me as jet engines run lean and not rich, where when they add more fuel its gets hotter instead of colder... but this is of course, a completely diff engine and a different cycle

This makes perfect sense; if you're running leaner than stoich, some of the air in the motor doesn't get ignited. As you approach stoich, you're creating more and more combustion for a given volume, so the temps go up. Then, as you start to get richer than stoich, you're no longer using the full volume for combustion again, so it cools back down.

 

[Eagle 5]

i should know that since jet engines run at less than stoich... i believe, i forget at the moment. something else about jet engines is sometimes you will inject more water than you are fuel. this lowers th eemissions and temperatures by a fairly large amount.

what i meant byt propan and toulene was what the octane ratings were, i confused myself when writing that.

also the part about it being linear came up when my manager and I were talking about it.

as he put it, its not very linear from around 85-100 i believe, meaning its not a perfectly straight line, there is some variance of which is the reason why i beleive some people see such a difference between using 92 and 94 octane. ill have to ask him again what he meant by it. also i may have the range wrong.