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08-14-2007
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http://auto.howstuffworks.com/gasoline.htm/printable
What is octane?
If you've read How Car Engines Work, you know that almost all cars use four-stroke gasoline engines. One of the strokes is the compression stroke, where the engine compresses a cylinder-full of air and gas into a much smaller volume before igniting it with a spark plug. The amount of compression is called the compression ratio of the engine. A typical engine might have a compression ratio of 8-to-1. (See How Car Engines Work for details.)
The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.
The compression ratio of your engine determines the octane rating of the gas you must use in the car. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a "high-performance engine" has a higher compression ratio and requires higher-octane fuel. The advantage of a high compression ratio is that it gives your engine a higher horsepower rating for a given engine weight -- that is what makes the engine "high performance." The disadvantage is that the gasoline for your engine costs more.
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, methane, propane and butane are all hydrocarbons. Methane has 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.
If you've read How Car Engines Work, you know that almost all cars use four-stroke gasoline engines. One of the strokes is the compression stroke, where the engine compresses a cylinder-full of air and gas into a much smaller volume before igniting it with a spark plug. The amount of compression is called the compression ratio of the engine. A typical engine might have a compression ratio of 8-to-1. (See How Car Engines Work for details.)
The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.
The compression ratio of your engine determines the octane rating of the gas you must use in the car. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a "high-performance engine" has a higher compression ratio and requires higher-octane fuel. The advantage of a high compression ratio is that it gives your engine a higher horsepower rating for a given engine weight -- that is what makes the engine "high performance." The disadvantage is that the gasoline for your engine costs more.
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, methane, propane and butane are all hydrocarbons. Methane has 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.
#4
08-16-2007
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I found runing mid-grade improved mpg by almost 1/4 tank on hiway but not much in performance. I am at 6000-10000feet of altitude. Sat down to read my Subaru manual one day and it recomended 87 octane which is midgrade up here. Regular is 85oct, mid is 87, and premium is 89.
When I started using 87 inthe Subaru I saw an immediate difference in mpg and performance. I have not tried premium, but now that prices are down a bit, I may try it.
When I started using 87 inthe Subaru I saw an immediate difference in mpg and performance. I have not tried premium, but now that prices are down a bit, I may try it.
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08-17-2007
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#6
08-17-2007
i stand by what i posted above.......your driving habits have changed i would bet......the ONLY way to optimize the higher octane fuel is with a tuner (X2)......you are absolutely wasteing your money on the 91 octane fuel.....
if your truck is truely "running better" (which i believe is in your head), then you have fuel delivery problems and need to get it fixed.....that way you can go back to 87/89 octane fuel.......
Ranger engines along with 90% of car engines today, are designed/engineered to run with peek fuel milage/performance on 87 octane; unless you get into the high performance engines (ie: corvette, Lambo, Ferrari, etc)
if your truck is truely "running better" (which i believe is in your head), then you have fuel delivery problems and need to get it fixed.....that way you can go back to 87/89 octane fuel.......
Ranger engines along with 90% of car engines today, are designed/engineered to run with peek fuel milage/performance on 87 octane; unless you get into the high performance engines (ie: corvette, Lambo, Ferrari, etc)
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08-17-2007
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#9
08-17-2007
Originally Posted by Du510
Exactly - running 91 or 93 octane in an engine designed for 87 wont help a bit. If your engine compression is designed for 87, then the only benefit to running 93 is a lighter wallet.
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#11
08-28-2007
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Ok, alll very interesting information. Guess I'll see how the Subaru does on it. I have a feeling sticking to midgrade will do just fine. Interestingly enough, that newish Subaru SUV, Tribeca, calls for premium according to Subaru website. Well, now I can't findit.
Anyway, time will tell, as with all things...
Myles
Anyway, time will tell, as with all things...
Myles
#12
09-11-2007
Member
Originally Posted by redbeard
I found runing mid-grade improved mpg by almost 1/4 tank on hiway but not much in performance. I am at 6000-10000feet of altitude. Sat down to read my Subaru manual one day and it recomended 87 octane which is midgrade up here. Regular is 85oct, mid is 87, and premium is 89.
When I started using 87 inthe Subaru I saw an immediate difference in mpg and performance. I have not tried premium, but now that prices are down a bit, I may try it.
When I started using 87 inthe Subaru I saw an immediate difference in mpg and performance. I have not tried premium, but now that prices are down a bit, I may try it.
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Originally Posted by greygooseranger
Thats f*cked up right there.... I thought the minimum octane was 87, I have never seen anything lower. In any event, your "midgrade" is regular unleaded for most of us, so that is what you should run in your vehicle....
~HJ
#14
09-11-2007
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#15
09-11-2007
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if your getting spark knock on 87 then you may want to think about upper end cleaning. try running some seafoam through it or have it profesionaly done. carbon builds up on the top of the pistons and the back of the intake valves and intake manifold causing "pinging"
Last edited by 04 EDGE; 09-11-2007 at 09:36 AM.
#16
09-11-2007
Newer Ford knock sensor strategies can add spark advance as well as remove it. So it is sometimes possible to gain a modest fuel economy benefit by running a higher octane fuel than recommended. The question then is whether the benefit outweighs the added cost of the higher octane fuel.
Originally Posted by rwenzing
A knock sensor monitors vibration in the engine and reports this information to the PCM. The PCM is calibrated to react to a range of vibration that is associated with the frequency and intensity of the ringing in the engine assembly associated with audible detonation. The physical location of a knock sensor on the engine should be chosen to allow, as nearly as possible, equal sensitivity to knock in all cylinders. The PCM calibration is carefully adjusted to ensure detection of detonation while ignoring other background engine noise. Inclusion of a knock sensor and its calibration strategy helps to insure that the timing will be optimized despite variations in fuel octane, changes in ambient temperature, pressure or humidity, and compression changes caused by high mileage engine wear or the formation of combustion chamber deposits.
In late model Ford engines, the knock sensor is allowed a range of influence, both above and below the corrected base timing value. A typical number for this range is plus or minus 6 degrees for a total range of 12 degrees.
The base timing table is calibrated using 87-octane fuel. When the engine begins running above idle, the ignition timing is initialized at a number equal to the current 87-octane base table value plus any spark adders from associated scalars or tables (positive or negative). An example of a spark adder would be a table of values that adds spark at different speed/load cells while the EGR is active. Other adjustments to spark timing are available in the calibration for corrections based on intake air temperature, coolant temperature, barometric pressure, etc. As in the case of the IAT sensor on late model engines, these tables or adders may be 'zeroed out' and not used, at the discretion of the calibrator. (Note: Spark tables are defined in terms of RPM and 'load'. Although load is a composite number based on several parameters, it can be thought of as proportional to throttle opening for purposes of this discussion.)
If knock is detected while running above idle at the corrected base value, the timing jumps back immediately by a calibratable amount, typically -3 degrees. If knock continues to be present at this new advance value, the timing will jump back 3 more degrees. The total negative deviation from the "correct" laboratory value is limited to 6 degrees.
If, on the other hand, the engine at its current base spark value shows no knock, the timing is steadily advanced by a calibratable rate (typically 1 degree/second) until knock is detected. Then the immediate 3-degree jumpback occurs as many times as necessary to eliminate the knock, again limited to -6 degrees from base.
The ignition advance remains in this continuous tug-o'-war between this slow 1 degree advancing and immediate 3 degree jumpback. The goal is to provide an engine free of audible knock while maintaining the highest possible spark advance to provide maximized power and fuel economy. Use of a fuel with octane higher than 87 may provide a small but noticeable increase in power and fuel mileage.
There are certain RPM/loads in the table where the full 12 degrees of ignition adjustment by the knock sensor strategy is not available. At high load and/or high RPMs, gasoline engines typically have spark advance values that are detonation-limited requiring that they run a base value that is below optimum. In these table cells, there is an advantage to allowing the spark to advance when possible.
At lower RPM/load points, the base value provides the optimum spark advance and any additional advance would reduce power rather than add it. Therefore, there is a second clipping table that overrides the other calculations to define the maximum spark advance allowable at any given RPM/load point. So, at lower RPMs and engine loads, the range of influence for the knock sensor may be limited to as few as 6 degrees, all negative.
The above strategy applies to conditions above idle. When the engine is idling and there is normally no possibility of spark knock, the ignition timing goes into a different mode that does not involve the knock sensor. At idle, the timing advance is used to help control the idle speed. The Idle Air Controller (IAC) provides the slow, coarse adjustment and the ignition timing is the fine adjuster, reacting quickly to correct small RPM errors from the desired idle speed.
In late model Ford engines, the knock sensor is allowed a range of influence, both above and below the corrected base timing value. A typical number for this range is plus or minus 6 degrees for a total range of 12 degrees.
The base timing table is calibrated using 87-octane fuel. When the engine begins running above idle, the ignition timing is initialized at a number equal to the current 87-octane base table value plus any spark adders from associated scalars or tables (positive or negative). An example of a spark adder would be a table of values that adds spark at different speed/load cells while the EGR is active. Other adjustments to spark timing are available in the calibration for corrections based on intake air temperature, coolant temperature, barometric pressure, etc. As in the case of the IAT sensor on late model engines, these tables or adders may be 'zeroed out' and not used, at the discretion of the calibrator. (Note: Spark tables are defined in terms of RPM and 'load'. Although load is a composite number based on several parameters, it can be thought of as proportional to throttle opening for purposes of this discussion.)
If knock is detected while running above idle at the corrected base value, the timing jumps back immediately by a calibratable amount, typically -3 degrees. If knock continues to be present at this new advance value, the timing will jump back 3 more degrees. The total negative deviation from the "correct" laboratory value is limited to 6 degrees.
If, on the other hand, the engine at its current base spark value shows no knock, the timing is steadily advanced by a calibratable rate (typically 1 degree/second) until knock is detected. Then the immediate 3-degree jumpback occurs as many times as necessary to eliminate the knock, again limited to -6 degrees from base.
The ignition advance remains in this continuous tug-o'-war between this slow 1 degree advancing and immediate 3 degree jumpback. The goal is to provide an engine free of audible knock while maintaining the highest possible spark advance to provide maximized power and fuel economy. Use of a fuel with octane higher than 87 may provide a small but noticeable increase in power and fuel mileage.
There are certain RPM/loads in the table where the full 12 degrees of ignition adjustment by the knock sensor strategy is not available. At high load and/or high RPMs, gasoline engines typically have spark advance values that are detonation-limited requiring that they run a base value that is below optimum. In these table cells, there is an advantage to allowing the spark to advance when possible.
At lower RPM/load points, the base value provides the optimum spark advance and any additional advance would reduce power rather than add it. Therefore, there is a second clipping table that overrides the other calculations to define the maximum spark advance allowable at any given RPM/load point. So, at lower RPMs and engine loads, the range of influence for the knock sensor may be limited to as few as 6 degrees, all negative.
The above strategy applies to conditions above idle. When the engine is idling and there is normally no possibility of spark knock, the ignition timing goes into a different mode that does not involve the knock sensor. At idle, the timing advance is used to help control the idle speed. The Idle Air Controller (IAC) provides the slow, coarse adjustment and the ignition timing is the fine adjuster, reacting quickly to correct small RPM errors from the desired idle speed.
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09-11-2007
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Greygooseranger:
1. So, according to your post, since you have never seen it, it doesn't exist.
2 Maybe I'm reading things wrong, but I do believe, sir, you are calling me a liar. I grew up on the East Coast, 87 was reg, 89 mid, and 91-93 premium. Back when Sunoco was around, you could blend your own octane. Each pump had a dial on it
that you turned to pick your blend, 87-93octane. Matter of fact, where I lived, premium was 2 except for Sunoco with their 93. I was taught unless you are pushing your vehicle to maximum performance and or had a high performance engine that pinged like a bb shoot at the county carnival on anything less than premium, that one should us the lowest octane that gave optimum performance.
Gee, there must be a reason that BMW K series motorcycles came with a high altitude jumper that you just plugged in to the fuel injection computer ciruit for consistant riding over 9000 feet above sea level. This jumper would close a ciruit that would automatically adjust for alttude to a finer degree than just assuming the FI would adjust it self, which it would. The operational word in this system is "conitnuously". Going up and down a pass such as Monarch Pass here in Colorado, just leave everything the same and all is right. Monarch Pass is over 11,000 feet elevation.
Now, let's say you want to ride the Colorado Trail, which is almost entirely above 10,000feet. Over 10,000feet, rejetting of carburetors was necessary to obtain best performance. FI has made that unneccessary in most cases. Switching to hotter spark plugs is a easy alternative to rejetting. Oh yes, FAA requires all pilots and copilots flying above 10,000feet to where oxygen masks unless in a pressurized cabin. Hmm, wonder why that is... Back to the original thread, yes, 85 UPHERE is equivalent to sea level regular of 87 octane. Once agian for those not listening (reading), the operator's manual for my 2003 Subaru Forester reads use regular unleaded gas of 87 octane and if at higher altitudes, in to use a 'higher' rated gasoline that is equivalent to 87 octane. HMMM, that must be MIDGRADE to us running around with clouds all around us, as opposed to over us.
3. I'm not sure what part of my post was f*cked up right there, but yours is f*cked up by saying 'I thought minimum octane was 87, I have never seen anything lower." You then go on to tell me what rating gas I should use. Did it occur to you the reason for the difference in octane rated gas? hmm, could it be that there is less oxygen up here.In order to keep the fuel/air ratio correct for 'most' vehicles in 'thinner air',the lower octane performs that function. The Subaru Forester maay only be a 2.5L horizontal or 'boxer' engine, it is considerred to be almost a high performance engine. It just lacks the polished ports, tuned FI, etc. Subaru worked out the details for these engines by not only competing, but winning most of the 'BAJA 1000' races in Baja Mexico. I will admit to not knowing the actual number of wins or losses. All I know is what I read in articles that made the decision on a Subaru easy.
Wait, we're talking Rangers here, anyway since Midgrade improved the mpg and perforemance in my Subaru, I tried it in my Ranger. It improved the Ranger's mpg and performance enough to be noticed easily.
I will report that the premium gas experiment is over. There was not improvement over midgrade.
And now I am done. I will not stand for anyone calling my posts "f*cked up because they have never seen it". It is thinking like that the so many people over the ages fought against. What people? hmm, Einstein, Robert Goddard, Howard Hughes, Columbus, Magellan, Martin Luther King, JFK, FDR and probably a few others at least.
Now, good night to all.
M.
1. So, according to your post, since you have never seen it, it doesn't exist.
2 Maybe I'm reading things wrong, but I do believe, sir, you are calling me a liar. I grew up on the East Coast, 87 was reg, 89 mid, and 91-93 premium. Back when Sunoco was around, you could blend your own octane. Each pump had a dial on it
that you turned to pick your blend, 87-93octane. Matter of fact, where I lived, premium was 2 except for Sunoco with their 93. I was taught unless you are pushing your vehicle to maximum performance and or had a high performance engine that pinged like a bb shoot at the county carnival on anything less than premium, that one should us the lowest octane that gave optimum performance.
Gee, there must be a reason that BMW K series motorcycles came with a high altitude jumper that you just plugged in to the fuel injection computer ciruit for consistant riding over 9000 feet above sea level. This jumper would close a ciruit that would automatically adjust for alttude to a finer degree than just assuming the FI would adjust it self, which it would. The operational word in this system is "conitnuously". Going up and down a pass such as Monarch Pass here in Colorado, just leave everything the same and all is right. Monarch Pass is over 11,000 feet elevation.
Now, let's say you want to ride the Colorado Trail, which is almost entirely above 10,000feet. Over 10,000feet, rejetting of carburetors was necessary to obtain best performance. FI has made that unneccessary in most cases. Switching to hotter spark plugs is a easy alternative to rejetting. Oh yes, FAA requires all pilots and copilots flying above 10,000feet to where oxygen masks unless in a pressurized cabin. Hmm, wonder why that is... Back to the original thread, yes, 85 UPHERE is equivalent to sea level regular of 87 octane. Once agian for those not listening (reading), the operator's manual for my 2003 Subaru Forester reads use regular unleaded gas of 87 octane and if at higher altitudes, in to use a 'higher' rated gasoline that is equivalent to 87 octane. HMMM, that must be MIDGRADE to us running around with clouds all around us, as opposed to over us.
3. I'm not sure what part of my post was f*cked up right there, but yours is f*cked up by saying 'I thought minimum octane was 87, I have never seen anything lower." You then go on to tell me what rating gas I should use. Did it occur to you the reason for the difference in octane rated gas? hmm, could it be that there is less oxygen up here.In order to keep the fuel/air ratio correct for 'most' vehicles in 'thinner air',the lower octane performs that function. The Subaru Forester maay only be a 2.5L horizontal or 'boxer' engine, it is considerred to be almost a high performance engine. It just lacks the polished ports, tuned FI, etc. Subaru worked out the details for these engines by not only competing, but winning most of the 'BAJA 1000' races in Baja Mexico. I will admit to not knowing the actual number of wins or losses. All I know is what I read in articles that made the decision on a Subaru easy.
Wait, we're talking Rangers here, anyway since Midgrade improved the mpg and perforemance in my Subaru, I tried it in my Ranger. It improved the Ranger's mpg and performance enough to be noticed easily.
I will report that the premium gas experiment is over. There was not improvement over midgrade.
And now I am done. I will not stand for anyone calling my posts "f*cked up because they have never seen it". It is thinking like that the so many people over the ages fought against. What people? hmm, Einstein, Robert Goddard, Howard Hughes, Columbus, Magellan, Martin Luther King, JFK, FDR and probably a few others at least.
Now, good night to all.
M.
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