Octane ratings and what they mean......

In reply to mad_machine:

My sho gets the same with either, the bike gets slightly worse with 87, but has noticeably more power.

So you guys that think your cars run better on higher octane than the factory has determined it needs, know more about your car than the people who made it, correct? In general I don't see why any manufacturer would specify an octane that the car doesn't run best on. I run my car on lower octane than it's rated for and it runs just fine and there is no change in gas mileage. There may be a reduction in power, but it's power I don't use, so it doesn't matter. But you can't base this on one vehicle, especially a vehicle with any significant miles on it, because no vehicle is exactly the same as another. And then there are other factors like ambient temperature, gas quality, altitude, cooling system condition, etc... I never understand why people somehow think its valid to apply their own single experience to the whole. I often read articles on this, not in car mags, but in professional engineering publications. Car companies, with few exceptions, specify an octane rating that works best in that vehicle. In the vast majority of circumstances, using anything more is a waste of money. As a car gets older, there may be some value in using a higher or lower octane than rated due to changes in the combustion chamber or a lowering of compression pressures from wear.

bravenrace wrote: So you guys that think your cars run better on higher octane than the factory has determined it needs, know more about your car than the people who made it, correct? In general I don't see why any manufacturer would specify an octane that the car doesn't run best on. I run my car on lower octane than it's rated for and it runs just fine and there is no change in gas mileage. There may be a reduction in power, but it's power I don't use, so it doesn't matter. But you can't base this on one vehicle, especially a vehicle with any significant miles on it, because no vehicle is exactly the same as another. And then there are other factors like ambient temperature, gas quality, altitude, cooling system condition, etc... I never understand why people somehow think its valid to apply their own single experience to the whole. I often read articles on this, not in car mags, but in professional engineering publications. Car companies, with few exceptions, specify an octane rating that works best in that vehicle. In the vast majority of circumstances, using anything more is a waste of money. As a car gets older, there may be some value in using a higher or lower octane than rated due to changes in the combustion chamber or a lowering of compression pressures from wear.

I'm not so sure car companies specify what works "best". For many cars they may specify what works acceptably. For people shopping for a car with economy in mind, they may shy away from a car that requires 91 due to cost. So if the car runs acceptably with 87 and not see a huge mpg difference but will make more power with 91 the auto manufacture may specify 87 because they believe they will sell more cars this way without incurring warranty costs.

I forget Volvo's language for my V70R but it is something like use premium for best performance. I don't see a statistically significant change in mpg between 87 and 91/93 so I often use 87 when I am just cruising on the interstate (not going heavily into boost).

wow.. this is worse than taking politics

bravenrace wrote: So you guys that think your cars run better on higher octane than the factory has determined it needs, know more about your car than the people who made it, correct?

If testing shows conclusively that there is an improvement, that's all that matters.

My RX-7 is quicker in acceleration on 93, even though Mazda famously was brewing up 80 octane for similar engines in their IMSA effort.

In reply to Knurled:
Octane is a measure of knock resistance, not energy content. The Mazda IMSA effort may have been a powerful fuel cocktail that had lower octane rating as a side effect. The Gurney built cars were also into mixing their own fuels. When one of the car went to Goodwood a few years ago the guy that had mixed the fuel back when was no longer available (or alive). The engine required a full remapping to use currently available fuel.

pappatho wrote:

bravenrace wrote: So you guys that think your cars run better on higher octane than the factory has determined it needs, know more about your car than the people who made it, correct? In general I don't see why any manufacturer would specify an octane that the car doesn't run best on. I run my car on lower octane than it's rated for and it runs just fine and there is no change in gas mileage. There may be a reduction in power, but it's power I don't use, so it doesn't matter. But you can't base this on one vehicle, especially a vehicle with any significant miles on it, because no vehicle is exactly the same as another. And then there are other factors like ambient temperature, gas quality, altitude, cooling system condition, etc... I never understand why people somehow think its valid to apply their own single experience to the whole. I often read articles on this, not in car mags, but in professional engineering publications. Car companies, with few exceptions, specify an octane rating that works best in that vehicle. In the vast majority of circumstances, using anything more is a waste of money. As a car gets older, there may be some value in using a higher or lower octane than rated due to changes in the combustion chamber or a lowering of compression pressures from wear.

I'm not so sure car companies specify what works "best". For many cars they may specify what works acceptably. For people shopping for a car with economy in mind, they may shy away from a car that requires 91 due to cost. So if the car runs acceptably with 87 and not see a huge mpg difference but will make more power with 91 the auto manufacture may specify 87 because they believe they will sell more cars this way without incurring warranty costs. I forget Volvo's language for my V70R but it is something like use premium for best performance. I don't see a statistically significant change in mpg between 87 and 91/93 so I often use 87 when I am just cruising on the interstate (not going heavily into boost).

Correct. I will tell you without a doubt that pretty much any GM made after the late 90's will run at least 4° more timing advance on 93 than they will with 87 with the factory computer, and unless they NEED higher than 87 to not knock themselves to death at a decent mount of timing they specify 87 in the owners manual. Like an ecotec cavalier will make more power and get better mileage on 93 than 87, but if GM put that that hateful piece of E36 M3 required 93 they'd hardly sell a single one. Conversely a cobalt SS/SC needs to run stupidly retarded timing to keep from knocking on 87 which is why 89 (or 91, I forget) is recommended.

Leafy wrote:

pappatho wrote:

bravenrace wrote: So you guys that think your cars run better on higher octane than the factory has determined it needs, know more about your car than the people who made it, correct? In general I don't see why any manufacturer would specify an octane that the car doesn't run best on. I run my car on lower octane than it's rated for and it runs just fine and there is no change in gas mileage. There may be a reduction in power, but it's power I don't use, so it doesn't matter. But you can't base this on one vehicle, especially a vehicle with any significant miles on it, because no vehicle is exactly the same as another. And then there are other factors like ambient temperature, gas quality, altitude, cooling system condition, etc... I never understand why people somehow think its valid to apply their own single experience to the whole. I often read articles on this, not in car mags, but in professional engineering publications. Car companies, with few exceptions, specify an octane rating that works best in that vehicle. In the vast majority of circumstances, using anything more is a waste of money. As a car gets older, there may be some value in using a higher or lower octane than rated due to changes in the combustion chamber or a lowering of compression pressures from wear.

I'm not so sure car companies specify what works "best". For many cars they may specify what works acceptably. For people shopping for a car with economy in mind, they may shy away from a car that requires 91 due to cost. So if the car runs acceptably with 87 and not see a huge mpg difference but will make more power with 91 the auto manufacture may specify 87 because they believe they will sell more cars this way without incurring warranty costs. I forget Volvo's language for my V70R but it is something like use premium for best performance. I don't see a statistically significant change in mpg between 87 and 91/93 so I often use 87 when I am just cruising on the interstate (not going heavily into boost).

Correct. I will tell you without a doubt that pretty much any GM made after the late 90's will run at least 4° more timing advance on 93 than they will with 87 with the factory computer, and unless they NEED higher than 87 to not knock themselves to death at a decent mount of timing they specify 87 in the owners manual. Like an ecotec cavalier will make more power and get better mileage on 93 than 87, but if GM put that that hateful piece of E36 M3 required 93 they'd hardly sell a single one. Conversely a cobalt SS/SC needs to run stupidly retarded timing to keep from knocking on 87 which is why 89 (or 91, I forget) is recommended.

You just countered what you think you are agreeing with.

GM could have advanced the spark 4 degrees and gotten better performance with 93, but, instead, since they know better with their engine, kept it retared so that it would run with 89. That very much seems as if they know the engine very well.

Very few ordinary cars are sold as premium only, or even premium recommended. And the amount of time the engine spends on a dyno would suggest that as released, the car is very well known by the OEM.

I'm not saying that an engine could not be easily modified to run better on premium, but that in the form that the OEM releases a car, they do know what the fuel will do in the car.

Best is very relative. Your best and an OEM's best may be very different things. Actually, I'm sure they are.

alfadriver wrote:

Leafy wrote:

pappatho wrote:

bravenrace wrote: So you guys that think your cars run better on higher octane than the factory has determined it needs, know more about your car than the people who made it, correct? In general I don't see why any manufacturer would specify an octane that the car doesn't run best on. I run my car on lower octane than it's rated for and it runs just fine and there is no change in gas mileage. There may be a reduction in power, but it's power I don't use, so it doesn't matter. But you can't base this on one vehicle, especially a vehicle with any significant miles on it, because no vehicle is exactly the same as another. And then there are other factors like ambient temperature, gas quality, altitude, cooling system condition, etc... I never understand why people somehow think its valid to apply their own single experience to the whole. I often read articles on this, not in car mags, but in professional engineering publications. Car companies, with few exceptions, specify an octane rating that works best in that vehicle. In the vast majority of circumstances, using anything more is a waste of money. As a car gets older, there may be some value in using a higher or lower octane than rated due to changes in the combustion chamber or a lowering of compression pressures from wear.

I'm not so sure car companies specify what works "best". For many cars they may specify what works acceptably. For people shopping for a car with economy in mind, they may shy away from a car that requires 91 due to cost. So if the car runs acceptably with 87 and not see a huge mpg difference but will make more power with 91 the auto manufacture may specify 87 because they believe they will sell more cars this way without incurring warranty costs. I forget Volvo's language for my V70R but it is something like use premium for best performance. I don't see a statistically significant change in mpg between 87 and 91/93 so I often use 87 when I am just cruising on the interstate (not going heavily into boost).

Correct. I will tell you without a doubt that pretty much any GM made after the late 90's will run at least 4° more timing advance on 93 than they will with 87 with the factory computer, and unless they NEED higher than 87 to not knock themselves to death at a decent mount of timing they specify 87 in the owners manual. Like an ecotec cavalier will make more power and get better mileage on 93 than 87, but if GM put that that hateful piece of E36 M3 required 93 they'd hardly sell a single one. Conversely a cobalt SS/SC needs to run stupidly retarded timing to keep from knocking on 87 which is why 89 (or 91, I forget) is recommended.

You just countered what you think you are agreeing with. GM *could* have advanced the spark 4 degrees and gotten better performance with 93, but, instead, since they know better with their engine, kept it retared so that it would run with 89. That very much seems as if they know the engine very well. Very few ordinary cars are sold as premium only, or even premium recommended. And the amount of time the engine spends on a dyno would suggest that *as released*, the car is very well known by the OEM. I'm not saying that an engine could not be easily modified to run better on premium, but that in the form that the OEM releases a car, they do know what the fuel will do in the car. Best is very relative. Your best and an OEM's best may be very different things. Actually, I'm sure they are.

No. I dont have HP Tuners installed on this computer to show you but the modern GM computers have a low and high octane spark table, the high octane table is basically what you would do for a conservative 93 octane tune on the engine and the low octane table is designed to work on horse piss. The ecu attempts to run the 93 octane table at first after being reset and slowly interpolates the the low octane table in based of an interpolation factor that it determines from how much timing it has to pull to not know based on feedback from the knock sensor. The engine will continue to attempt to advance the spark and check for knock at some interval or if it sense the fuel level to have changed noticeably.

stafford1500 wrote: In reply to Knurled: Octane is a measure of knock resistance, not energy content. The Mazda IMSA effort may have been a powerful fuel cocktail that had lower octane rating as a side effect.

Exactly.

IIRC the "80 octane" that Mazda was making was toluene and kerosene. But I have no evidence of that.

I do not claim or believe that octane has anything to do with energy content OR flame speed. You can have slow burning low octane fuels and fast burning high octane fuels. It's just resistance to autoignition when subjected to heat and pressure, nothing more.

So... like anything else, statements without testing behind them are just words.

alfadriver wrote: You just countered what you think you are agreeing with. GM *could* have advanced the spark 4 degrees and gotten better performance with 93, but, instead, since they know better with their engine, kept it retared so that it would run with 89. That very much seems as if they know the engine very well.

There are high octane maps even every GM computer I've peeked into, even on models that have a minimum 87 (RON+MON)/2.

edit: what Leafy said. HPT is kind of an eye opener to show what they are doing and what they are looking at.

Knurled wrote:

alfadriver wrote: You just countered what you think you are agreeing with. GM *could* have advanced the spark 4 degrees and gotten better performance with 93, but, instead, since they know better with their engine, kept it retared so that it would run with 89. That very much seems as if they know the engine very well.

There are high octane maps even every GM computer I've peeked into, even on models that have a minimum 87 (RON+MON)/2. edit: what Leafy said. HPT is kind of an eye opener to show what they are doing and what they are looking at.

And? Again, what this board thinks is "best" is probably very different than what GM thinks. They need to sell a car that will run well on low octane fuel to not upset customers.

The fact that they have spark data that tells them what MBT is just says that they know the engine. We have the same maps- best spark and borderline spark- most of the time they are not the same.

The ignition map is laid out to meet the specs that the average customer expects. HPT tunes the car for their customers, too.

Plus, in the 90's, knock detection may have been possible, but it was not cheap. Especially for a basic car.

alfadriver wrote: > You just countered what you think you are agreeing with. GM *could* have advanced the spark 4 degrees and gotten better performance with 93, but, instead, since they know better with their engine, kept it retared so that it would run with 89. That very much seems as if they know the engine very well. Very few ordinary cars are sold as premium only, or even premium recommended. And the amount of time the engine spends on a dyno would suggest that *as released*, the car is very well known by the OEM. I'm not saying that an engine could not be easily modified to run better on premium, but that in the form that the OEM releases a car, they do know what the fuel will do in the car.

I am going to disagree that GM tuned the engine to run on 87 octane because they know what is best for the engine. I am going to say they tuned it to run on 87 octane because they know how their customers will treat the engine. Most people do not want to spend more for a little bit more performance.. GM knows that just because they may recommend 93 octane that a good majority of their owners will never put that grade of gas into the car

mad_machine wrote:

alfadriver wrote: > You just countered what you think you are agreeing with. GM *could* have advanced the spark 4 degrees and gotten better performance with 93, but, instead, since they know better with their engine, kept it retared so that it would run with 89. That very much seems as if they know the engine very well. Very few ordinary cars are sold as premium only, or even premium recommended. And the amount of time the engine spends on a dyno would suggest that *as released*, the car is very well known by the OEM. I'm not saying that an engine could not be easily modified to run better on premium, but that in the form that the OEM releases a car, they do know what the fuel will do in the car.

I am going to disagree that GM tuned the engine to run on 87 octane because they know what is best for the engine. I am going to say they tuned it to run on 87 octane because they know how their customers will treat the engine. Most people do not want to spend more for a little bit more performance.. GM knows that just because they may recommend 93 octane that a good majority of their owners will never put that grade of gas into the car

How is that statement different? GM knows what's best for the engine BECAUSE they know what the customer wants and expects. The spark map is accurate for low octane fuel. Again, just because the engine can run high octane fuel and run better with it- that does not mean that it's the best thing to do.

edit- they also make the engine start on a wide band of fuels, even though it will start and run best with a single fuel. Again- it's that GM has a good idea how the engine will be used, so the engine is tuned that way.

I think y'all are arguing two different points and actually agreeing.