What Octane Rating are you Burning?
#1
02-23-2005, 11:38 PM
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What Octane Rating are you Burning?
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02-23-2005, 11:44 PM
#3
02-23-2005, 11:57 PM
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#4
02-23-2005, 11:57 PM
Originally posted by Superchips_Distributor (snip)
it is NOT automatically & always true that using higher octane fuel is a complete waste of money on the stock tuning - though to be fair, I'm kinda-sorta "splitting hairs" here, as I'd be the first to say that I wouldn't spend the additional money on additional octane that the motor is not specifically tuned for, as you usually don't get enough additional mpg to *justify* that additional expense in these F-150's.
In reality, that actually depends on how the knock sensor system is set up (calibrated) in each individual vehicle's computer code (PCM revision/calibration), and that varies widely in these trucks. For example, some will allow as much as 8 more degrees of timing as long as the knock sensor does not detect detonation (and here we're talking about one specific form of detonation, "pre-ignition") - but the other catch is the actual *rate* at which that timing comes in. It's usually slow enough (say, only 1 degree every 2 seconds at the same TP & load) that it's just not worth spending the money as you don't get enough additional fuel mileage to offset that cost - UNLESS the truck is actually tuned for it's use. There are some F-150's (a good number) whose knock sensor system calibration cannot add any timing when detonation is not detected, while others can add only another 2 degrees of timing, but can do it pretty quickly - again, this varies widely in these F-150's. You can take 20 trucks of the exact same configuration & model year, and have 13 different knock sensor calibrations.
I generally *do* agree that as long as the engine is not specifically tuned for the higher octane in the borderline knock table, it's not going to be worth spending the additional 20 cents per gallon as compared to what can be had (both in terms of raw power and fuel mileage gains) if the higher octane is actually *tuned* for. These F-150's have more different PCM revisions each model year than virtually any other vehicle made - sometimes *hundreds* of different revisions each model year alone, and they vary widely in certain areas - such as how certain limiters are effected, how the knock sensor system is calibrated, etc. Just FYI........
When dealing with *quality* fuels, the actual BTU content per gallon (or per lb.) of fuel usually does not vary much regardless of octane (in the 87-100 octane range, R+M/2). The actual energy content of gasoline is expressed by the oil company/refiner in terms of the number of BTU's per gallon - say, 110,000 BTU's per gallon, or 98,000 BTU's per gallon, etc. - and that is not "automatically" increased or decreased (all else being equal) based solely on the octane rating.
Another point that comes up from time to time regarding fuels & higher octanes is the burn rate - once ignited, the flame front propagates itself across the combustion chamber at roughly the same speed regardless of octane (again, when we're talking about fuels in the 87-100 octane range R+M/2), generally speaking - but getting it *LIT* - actually INITIATING the burn of the A/F mixture takes more time - both in real time and in degrees of crankshaft rotation, as octane rating goes up. This is *precisely* why it's generally considered close to a waste of money to use more octane than the motor is tuned for, more often than not - and I'd be the first to say not to bother using more octane than the motor is tuned for, I do agree with you on that, certainly. Using significantly more octane than the engine is tuned for will actually *reduce* power and fuel mileage, and can also cause MIL's, etc. just as the owners manuals for most of these vehicles mentions.
With higher octane, it's not just that we can make more power & mpg *if* we add timing - proper tuning dictates that we *must* add more timing to the motor to compensate for now much longer it takes to *initiate* the burn of the higher octane A/F mixture. The spark plug needs to get it's current with the piston further down the bore (BTDC) so that peak cylinder pressure still occurs with the piston the same approximate number of degrees *down* the bore (ATDC) - otherwise the engine doesn't make the power it should, and won't get the fuel economy it should, either - this is why most of these vehicles owners manuals specifically instructs *not* to use premium gas, as they are originally tuned for 87 octane from the factory.
In tuning, the BTU content ("energy") of the fuel makes every bit as much difference, if not more, than just it's octane rating. A low energy 93 octane fuel can be out-performed by a higher-energy 92 octane fuel, for example. Or take methanol-based fuels (far different from gasoline) - now they have a *very* high octane rating, but have only about 50%-70% of the BTU content per gallon of gasoline.
Just some quick general FYI...................
it is NOT automatically & always true that using higher octane fuel is a complete waste of money on the stock tuning - though to be fair, I'm kinda-sorta "splitting hairs" here, as I'd be the first to say that I wouldn't spend the additional money on additional octane that the motor is not specifically tuned for, as you usually don't get enough additional mpg to *justify* that additional expense in these F-150's.
In reality, that actually depends on how the knock sensor system is set up (calibrated) in each individual vehicle's computer code (PCM revision/calibration), and that varies widely in these trucks. For example, some will allow as much as 8 more degrees of timing as long as the knock sensor does not detect detonation (and here we're talking about one specific form of detonation, "pre-ignition") - but the other catch is the actual *rate* at which that timing comes in. It's usually slow enough (say, only 1 degree every 2 seconds at the same TP & load) that it's just not worth spending the money as you don't get enough additional fuel mileage to offset that cost - UNLESS the truck is actually tuned for it's use. There are some F-150's (a good number) whose knock sensor system calibration cannot add any timing when detonation is not detected, while others can add only another 2 degrees of timing, but can do it pretty quickly - again, this varies widely in these F-150's. You can take 20 trucks of the exact same configuration & model year, and have 13 different knock sensor calibrations.
I generally *do* agree that as long as the engine is not specifically tuned for the higher octane in the borderline knock table, it's not going to be worth spending the additional 20 cents per gallon as compared to what can be had (both in terms of raw power and fuel mileage gains) if the higher octane is actually *tuned* for. These F-150's have more different PCM revisions each model year than virtually any other vehicle made - sometimes *hundreds* of different revisions each model year alone, and they vary widely in certain areas - such as how certain limiters are effected, how the knock sensor system is calibrated, etc. Just FYI........
When dealing with *quality* fuels, the actual BTU content per gallon (or per lb.) of fuel usually does not vary much regardless of octane (in the 87-100 octane range, R+M/2). The actual energy content of gasoline is expressed by the oil company/refiner in terms of the number of BTU's per gallon - say, 110,000 BTU's per gallon, or 98,000 BTU's per gallon, etc. - and that is not "automatically" increased or decreased (all else being equal) based solely on the octane rating.
Another point that comes up from time to time regarding fuels & higher octanes is the burn rate - once ignited, the flame front propagates itself across the combustion chamber at roughly the same speed regardless of octane (again, when we're talking about fuels in the 87-100 octane range R+M/2), generally speaking - but getting it *LIT* - actually INITIATING the burn of the A/F mixture takes more time - both in real time and in degrees of crankshaft rotation, as octane rating goes up. This is *precisely* why it's generally considered close to a waste of money to use more octane than the motor is tuned for, more often than not - and I'd be the first to say not to bother using more octane than the motor is tuned for, I do agree with you on that, certainly. Using significantly more octane than the engine is tuned for will actually *reduce* power and fuel mileage, and can also cause MIL's, etc. just as the owners manuals for most of these vehicles mentions.
With higher octane, it's not just that we can make more power & mpg *if* we add timing - proper tuning dictates that we *must* add more timing to the motor to compensate for now much longer it takes to *initiate* the burn of the higher octane A/F mixture. The spark plug needs to get it's current with the piston further down the bore (BTDC) so that peak cylinder pressure still occurs with the piston the same approximate number of degrees *down* the bore (ATDC) - otherwise the engine doesn't make the power it should, and won't get the fuel economy it should, either - this is why most of these vehicles owners manuals specifically instructs *not* to use premium gas, as they are originally tuned for 87 octane from the factory.
In tuning, the BTU content ("energy") of the fuel makes every bit as much difference, if not more, than just it's octane rating. A low energy 93 octane fuel can be out-performed by a higher-energy 92 octane fuel, for example. Or take methanol-based fuels (far different from gasoline) - now they have a *very* high octane rating, but have only about 50%-70% of the BTU content per gallon of gasoline.
Just some quick general FYI...................
#6
02-24-2005, 11:08 AM
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02-24-2005, 02:19 PM
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02-24-2005, 02:35 PM
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#11
02-24-2005, 03:26 PM
Interesting post's here. My 05 will rattle quite a bit on 87 but not at all on 89 octane. I have tried all of the local brands, Exxon Shell Diamond Shamrock with the same results. The truck does feel peppier on 89 octane also. I figure what the hell, not that much price difference and the detonation cant be good for it.
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02-24-2005, 03:32 PM
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02-24-2005, 04:07 PM
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02-24-2005, 05:28 PM
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02-24-2005, 07:36 PM
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