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Re: Re: 85 vs 87 Octane
On Sun, 2 Sep 2007 20:05:19 -0500, "DougW"
<I.only.read.usenet@invalid.address> wrote: >First, do not confuse power with octane rating, they arn't related. Actually they are a LOT more related than you think in some engine that have knock sensing timing control >The higher the octane number the harder the fuel is to ignite. No it will light just as easy, the differnece is that it has a higher auto ignite tempature and a slightly slower flame speed but it is no harder to light its fuse with a spark >And higher octane fuel has no more power than lower octane fuel. Actually it has a few less BTU in it but in a higher compression engine it allows for a more favorable ignition and expansion cycle than low octane fuel does (which has to use a retarded curve to prevent to rapid of a pressure build up and auto ignition or detenation) > >What your experiencing is lower air pressure at altitude. The same >thing that makes you tire more easily at higher elevations also means >your engine is not getting the same amount of oxygen with each intake. >Less oxygen means less fuel can be burned. Less fuel of course means >less power unless you can push more air into the engine with a >supercharger or turbocharger. Mostly true. In the old days you would advance base line timing about 2 degrees for each 1000 feet above 1000 MSL up to a max of 12 degrees or so. It did not restore all power but it does help some. Some modern cars can advance timing a bit more automatically as well as lean mixture out. At start up it reads MAP sensor just before engine cranks to set elevation data. The problem with this is if you start engine at 4000 feet and drive to 9000 feet the computer does not know it is at 9000 feet until you shut it out and restart it. >You want to use the lowest octane fuel that does not cause ping. This is not cut and dried as it seems. This works with old fixed timing engine but not with some modern engine with knock sensor because many of them (especailly GM's ) will never knock no matter what, performance will just suffer. BTW, octane requirements do decrease generally with elevation and this is why above 4000 MSL or so you start seeing 85 or 85.5 octane fuel for regular, 88 or so for plus and 91 for premium. You do not want a tank of 85 when you are headed to lower elevations especailly in hot weather (ambent temp effects octane needs too) Generally if your engine has a CR of 9 to 1 or higher you cannot burn 87 at sea level without some spark timing trickery as 9 to 1 and above really needs 89 or more (especailly in warm weather) for optimal timing curving. At 10 to one and above you really need 91 or better unless you run a compromised timing curve which also reduces efficency and power too. ----------------- TheSnoMan.com |
Re: Re: 85 vs 87 Octane
On Sun, 2 Sep 2007 20:05:19 -0500, "DougW"
<I.only.read.usenet@invalid.address> wrote: >First, do not confuse power with octane rating, they arn't related. Actually they are a LOT more related than you think in some engine that have knock sensing timing control >The higher the octane number the harder the fuel is to ignite. No it will light just as easy, the differnece is that it has a higher auto ignite tempature and a slightly slower flame speed but it is no harder to light its fuse with a spark >And higher octane fuel has no more power than lower octane fuel. Actually it has a few less BTU in it but in a higher compression engine it allows for a more favorable ignition and expansion cycle than low octane fuel does (which has to use a retarded curve to prevent to rapid of a pressure build up and auto ignition or detenation) > >What your experiencing is lower air pressure at altitude. The same >thing that makes you tire more easily at higher elevations also means >your engine is not getting the same amount of oxygen with each intake. >Less oxygen means less fuel can be burned. Less fuel of course means >less power unless you can push more air into the engine with a >supercharger or turbocharger. Mostly true. In the old days you would advance base line timing about 2 degrees for each 1000 feet above 1000 MSL up to a max of 12 degrees or so. It did not restore all power but it does help some. Some modern cars can advance timing a bit more automatically as well as lean mixture out. At start up it reads MAP sensor just before engine cranks to set elevation data. The problem with this is if you start engine at 4000 feet and drive to 9000 feet the computer does not know it is at 9000 feet until you shut it out and restart it. >You want to use the lowest octane fuel that does not cause ping. This is not cut and dried as it seems. This works with old fixed timing engine but not with some modern engine with knock sensor because many of them (especailly GM's ) will never knock no matter what, performance will just suffer. BTW, octane requirements do decrease generally with elevation and this is why above 4000 MSL or so you start seeing 85 or 85.5 octane fuel for regular, 88 or so for plus and 91 for premium. You do not want a tank of 85 when you are headed to lower elevations especailly in hot weather (ambent temp effects octane needs too) Generally if your engine has a CR of 9 to 1 or higher you cannot burn 87 at sea level without some spark timing trickery as 9 to 1 and above really needs 89 or more (especailly in warm weather) for optimal timing curving. At 10 to one and above you really need 91 or better unless you run a compromised timing curve which also reduces efficency and power too. ----------------- TheSnoMan.com |
Re: Re: 85 vs 87 Octane
On Sun, 2 Sep 2007 20:05:19 -0500, "DougW"
<I.only.read.usenet@invalid.address> wrote: >First, do not confuse power with octane rating, they arn't related. Actually they are a LOT more related than you think in some engine that have knock sensing timing control >The higher the octane number the harder the fuel is to ignite. No it will light just as easy, the differnece is that it has a higher auto ignite tempature and a slightly slower flame speed but it is no harder to light its fuse with a spark >And higher octane fuel has no more power than lower octane fuel. Actually it has a few less BTU in it but in a higher compression engine it allows for a more favorable ignition and expansion cycle than low octane fuel does (which has to use a retarded curve to prevent to rapid of a pressure build up and auto ignition or detenation) > >What your experiencing is lower air pressure at altitude. The same >thing that makes you tire more easily at higher elevations also means >your engine is not getting the same amount of oxygen with each intake. >Less oxygen means less fuel can be burned. Less fuel of course means >less power unless you can push more air into the engine with a >supercharger or turbocharger. Mostly true. In the old days you would advance base line timing about 2 degrees for each 1000 feet above 1000 MSL up to a max of 12 degrees or so. It did not restore all power but it does help some. Some modern cars can advance timing a bit more automatically as well as lean mixture out. At start up it reads MAP sensor just before engine cranks to set elevation data. The problem with this is if you start engine at 4000 feet and drive to 9000 feet the computer does not know it is at 9000 feet until you shut it out and restart it. >You want to use the lowest octane fuel that does not cause ping. This is not cut and dried as it seems. This works with old fixed timing engine but not with some modern engine with knock sensor because many of them (especailly GM's ) will never knock no matter what, performance will just suffer. BTW, octane requirements do decrease generally with elevation and this is why above 4000 MSL or so you start seeing 85 or 85.5 octane fuel for regular, 88 or so for plus and 91 for premium. You do not want a tank of 85 when you are headed to lower elevations especailly in hot weather (ambent temp effects octane needs too) Generally if your engine has a CR of 9 to 1 or higher you cannot burn 87 at sea level without some spark timing trickery as 9 to 1 and above really needs 89 or more (especailly in warm weather) for optimal timing curving. At 10 to one and above you really need 91 or better unless you run a compromised timing curve which also reduces efficency and power too. ----------------- TheSnoMan.com |
Re: Re: 85 vs 87 Octane
sooo, if I increase my octane to 87 at say 9,000 or so altitude, I will gain
some power back ?? my jeep is a 2001 grand with the straight 6, 4.0 engine... "SnoMan" <admin@snoman.com> wrote in message news:nlrmd3lugankvqmfpcps6k2rmlqmivcfk0@4ax.com... > On Sun, 2 Sep 2007 20:05:19 -0500, "DougW" > <I.only.read.usenet@invalid.address> wrote: > >>First, do not confuse power with octane rating, they arn't related. > > Actually they are a LOT more related than you think in some engine > that have knock sensing timing control > > >>The higher the octane number the harder the fuel is to ignite. > > No it will light just as easy, the differnece is that it has a higher > auto ignite tempature and a slightly slower flame speed but it is no > harder to light its fuse with a spark > >>And higher octane fuel has no more power than lower octane fuel. > > Actually it has a few less BTU in it but in a higher compression > engine it allows for a more favorable ignition and expansion cycle > than low octane fuel does (which has to use a retarded curve to > prevent to rapid of a pressure build up and auto ignition or > detenation) > >> >>What your experiencing is lower air pressure at altitude. The same >>thing that makes you tire more easily at higher elevations also means >>your engine is not getting the same amount of oxygen with each intake. >>Less oxygen means less fuel can be burned. Less fuel of course means >>less power unless you can push more air into the engine with a >>supercharger or turbocharger. > > Mostly true. In the old days you would advance base line timing about > 2 degrees for each 1000 feet above 1000 MSL up to a max of 12 degrees > or so. It did not restore all power but it does help some. Some modern > cars can advance timing a bit more automatically as well as lean > mixture out. At start up it reads MAP sensor just before engine cranks > to set elevation data. The problem with this is if you start engine at > 4000 feet and drive to 9000 feet the computer does not know it is at > 9000 feet until you shut it out and restart it. > >>You want to use the lowest octane fuel that does not cause ping. > > This is not cut and dried as it seems. This works with old fixed > timing engine but not with some modern engine with knock sensor > because many of them (especailly GM's ) will never knock no matter > what, performance will just suffer. BTW, octane requirements do > decrease generally with elevation and this is why above 4000 MSL or so > you start seeing 85 or 85.5 octane fuel for regular, 88 or so for plus > and 91 for premium. You do not want a tank of 85 when you are headed > to lower elevations especailly in hot weather (ambent temp effects > octane needs too) Generally if your engine has a CR of 9 to 1 or > higher you cannot burn 87 at sea level without some spark timing > trickery as 9 to 1 and above really needs 89 or more (especailly in > warm weather) for optimal timing curving. At 10 to one and above you > really need 91 or better unless you run a compromised timing curve > which also reduces efficency and power too. > > ----------------- > TheSnoMan.com |
Re: Re: 85 vs 87 Octane
sooo, if I increase my octane to 87 at say 9,000 or so altitude, I will gain
some power back ?? my jeep is a 2001 grand with the straight 6, 4.0 engine... "SnoMan" <admin@snoman.com> wrote in message news:nlrmd3lugankvqmfpcps6k2rmlqmivcfk0@4ax.com... > On Sun, 2 Sep 2007 20:05:19 -0500, "DougW" > <I.only.read.usenet@invalid.address> wrote: > >>First, do not confuse power with octane rating, they arn't related. > > Actually they are a LOT more related than you think in some engine > that have knock sensing timing control > > >>The higher the octane number the harder the fuel is to ignite. > > No it will light just as easy, the differnece is that it has a higher > auto ignite tempature and a slightly slower flame speed but it is no > harder to light its fuse with a spark > >>And higher octane fuel has no more power than lower octane fuel. > > Actually it has a few less BTU in it but in a higher compression > engine it allows for a more favorable ignition and expansion cycle > than low octane fuel does (which has to use a retarded curve to > prevent to rapid of a pressure build up and auto ignition or > detenation) > >> >>What your experiencing is lower air pressure at altitude. The same >>thing that makes you tire more easily at higher elevations also means >>your engine is not getting the same amount of oxygen with each intake. >>Less oxygen means less fuel can be burned. Less fuel of course means >>less power unless you can push more air into the engine with a >>supercharger or turbocharger. > > Mostly true. In the old days you would advance base line timing about > 2 degrees for each 1000 feet above 1000 MSL up to a max of 12 degrees > or so. It did not restore all power but it does help some. Some modern > cars can advance timing a bit more automatically as well as lean > mixture out. At start up it reads MAP sensor just before engine cranks > to set elevation data. The problem with this is if you start engine at > 4000 feet and drive to 9000 feet the computer does not know it is at > 9000 feet until you shut it out and restart it. > >>You want to use the lowest octane fuel that does not cause ping. > > This is not cut and dried as it seems. This works with old fixed > timing engine but not with some modern engine with knock sensor > because many of them (especailly GM's ) will never knock no matter > what, performance will just suffer. BTW, octane requirements do > decrease generally with elevation and this is why above 4000 MSL or so > you start seeing 85 or 85.5 octane fuel for regular, 88 or so for plus > and 91 for premium. You do not want a tank of 85 when you are headed > to lower elevations especailly in hot weather (ambent temp effects > octane needs too) Generally if your engine has a CR of 9 to 1 or > higher you cannot burn 87 at sea level without some spark timing > trickery as 9 to 1 and above really needs 89 or more (especailly in > warm weather) for optimal timing curving. At 10 to one and above you > really need 91 or better unless you run a compromised timing curve > which also reduces efficency and power too. > > ----------------- > TheSnoMan.com |
Re: Re: 85 vs 87 Octane
sooo, if I increase my octane to 87 at say 9,000 or so altitude, I will gain
some power back ?? my jeep is a 2001 grand with the straight 6, 4.0 engine... "SnoMan" <admin@snoman.com> wrote in message news:nlrmd3lugankvqmfpcps6k2rmlqmivcfk0@4ax.com... > On Sun, 2 Sep 2007 20:05:19 -0500, "DougW" > <I.only.read.usenet@invalid.address> wrote: > >>First, do not confuse power with octane rating, they arn't related. > > Actually they are a LOT more related than you think in some engine > that have knock sensing timing control > > >>The higher the octane number the harder the fuel is to ignite. > > No it will light just as easy, the differnece is that it has a higher > auto ignite tempature and a slightly slower flame speed but it is no > harder to light its fuse with a spark > >>And higher octane fuel has no more power than lower octane fuel. > > Actually it has a few less BTU in it but in a higher compression > engine it allows for a more favorable ignition and expansion cycle > than low octane fuel does (which has to use a retarded curve to > prevent to rapid of a pressure build up and auto ignition or > detenation) > >> >>What your experiencing is lower air pressure at altitude. The same >>thing that makes you tire more easily at higher elevations also means >>your engine is not getting the same amount of oxygen with each intake. >>Less oxygen means less fuel can be burned. Less fuel of course means >>less power unless you can push more air into the engine with a >>supercharger or turbocharger. > > Mostly true. In the old days you would advance base line timing about > 2 degrees for each 1000 feet above 1000 MSL up to a max of 12 degrees > or so. It did not restore all power but it does help some. Some modern > cars can advance timing a bit more automatically as well as lean > mixture out. At start up it reads MAP sensor just before engine cranks > to set elevation data. The problem with this is if you start engine at > 4000 feet and drive to 9000 feet the computer does not know it is at > 9000 feet until you shut it out and restart it. > >>You want to use the lowest octane fuel that does not cause ping. > > This is not cut and dried as it seems. This works with old fixed > timing engine but not with some modern engine with knock sensor > because many of them (especailly GM's ) will never knock no matter > what, performance will just suffer. BTW, octane requirements do > decrease generally with elevation and this is why above 4000 MSL or so > you start seeing 85 or 85.5 octane fuel for regular, 88 or so for plus > and 91 for premium. You do not want a tank of 85 when you are headed > to lower elevations especailly in hot weather (ambent temp effects > octane needs too) Generally if your engine has a CR of 9 to 1 or > higher you cannot burn 87 at sea level without some spark timing > trickery as 9 to 1 and above really needs 89 or more (especailly in > warm weather) for optimal timing curving. At 10 to one and above you > really need 91 or better unless you run a compromised timing curve > which also reduces efficency and power too. > > ----------------- > TheSnoMan.com |
Re: Re: 85 vs 87 Octane
sooo, if I increase my octane to 87 at say 9,000 or so altitude, I will gain
some power back ?? my jeep is a 2001 grand with the straight 6, 4.0 engine... "SnoMan" <admin@snoman.com> wrote in message news:nlrmd3lugankvqmfpcps6k2rmlqmivcfk0@4ax.com... > On Sun, 2 Sep 2007 20:05:19 -0500, "DougW" > <I.only.read.usenet@invalid.address> wrote: > >>First, do not confuse power with octane rating, they arn't related. > > Actually they are a LOT more related than you think in some engine > that have knock sensing timing control > > >>The higher the octane number the harder the fuel is to ignite. > > No it will light just as easy, the differnece is that it has a higher > auto ignite tempature and a slightly slower flame speed but it is no > harder to light its fuse with a spark > >>And higher octane fuel has no more power than lower octane fuel. > > Actually it has a few less BTU in it but in a higher compression > engine it allows for a more favorable ignition and expansion cycle > than low octane fuel does (which has to use a retarded curve to > prevent to rapid of a pressure build up and auto ignition or > detenation) > >> >>What your experiencing is lower air pressure at altitude. The same >>thing that makes you tire more easily at higher elevations also means >>your engine is not getting the same amount of oxygen with each intake. >>Less oxygen means less fuel can be burned. Less fuel of course means >>less power unless you can push more air into the engine with a >>supercharger or turbocharger. > > Mostly true. In the old days you would advance base line timing about > 2 degrees for each 1000 feet above 1000 MSL up to a max of 12 degrees > or so. It did not restore all power but it does help some. Some modern > cars can advance timing a bit more automatically as well as lean > mixture out. At start up it reads MAP sensor just before engine cranks > to set elevation data. The problem with this is if you start engine at > 4000 feet and drive to 9000 feet the computer does not know it is at > 9000 feet until you shut it out and restart it. > >>You want to use the lowest octane fuel that does not cause ping. > > This is not cut and dried as it seems. This works with old fixed > timing engine but not with some modern engine with knock sensor > because many of them (especailly GM's ) will never knock no matter > what, performance will just suffer. BTW, octane requirements do > decrease generally with elevation and this is why above 4000 MSL or so > you start seeing 85 or 85.5 octane fuel for regular, 88 or so for plus > and 91 for premium. You do not want a tank of 85 when you are headed > to lower elevations especailly in hot weather (ambent temp effects > octane needs too) Generally if your engine has a CR of 9 to 1 or > higher you cannot burn 87 at sea level without some spark timing > trickery as 9 to 1 and above really needs 89 or more (especailly in > warm weather) for optimal timing curving. At 10 to one and above you > really need 91 or better unless you run a compromised timing curve > which also reduces efficency and power too. > > ----------------- > TheSnoMan.com |
Re: 85 vs 87 Octane
SnoMan wrote:
> On Sun, 2 Sep 2007 20:05:19 -0500, "DougW" > <I.only.read.usenet@invalid.address> wrote: > >> First, do not confuse power with octane rating, they arn't related. > > Actually they are a LOT more related than you think in some engine > that have knock sensing timing control > > >> The higher the octane number the harder the fuel is to ignite. > > No it will light just as easy, the differnece is that it has a higher > auto ignite tempature and a slightly slower flame speed but it is no > harder to light its fuse with a spark > >> And higher octane fuel has no more power than lower octane fuel. > > Actually it has a few less BTU in it but in a higher compression > engine it allows for a more favorable ignition and expansion cycle > than low octane fuel does (which has to use a retarded curve to > prevent to rapid of a pressure build up and auto ignition or > detenation) > >> What your experiencing is lower air pressure at altitude. The same >> thing that makes you tire more easily at higher elevations also means >> your engine is not getting the same amount of oxygen with each intake. >> Less oxygen means less fuel can be burned. Less fuel of course means >> less power unless you can push more air into the engine with a >> supercharger or turbocharger. > > Mostly true. In the old days you would advance base line timing about > 2 degrees for each 1000 feet above 1000 MSL up to a max of 12 degrees > or so. It did not restore all power but it does help some. Some modern > cars can advance timing a bit more automatically as well as lean > mixture out. At start up it reads MAP sensor just before engine cranks > to set elevation data. The problem with this is if you start engine at > 4000 feet and drive to 9000 feet the computer does not know it is at > 9000 feet until you shut it out and restart it. > >> You want to use the lowest octane fuel that does not cause ping. > > This is not cut and dried as it seems. This works with old fixed > timing engine but not with some modern engine with knock sensor > because many of them (especailly GM's ) will never knock no matter > what, performance will just suffer. BTW, octane requirements do > decrease generally with elevation and this is why above 4000 MSL or so > you start seeing 85 or 85.5 octane fuel for regular, 88 or so for plus > and 91 for premium. You do not want a tank of 85 when you are headed > to lower elevations especailly in hot weather (ambent temp effects > octane needs too) Generally if your engine has a CR of 9 to 1 or > higher you cannot burn 87 at sea level without some spark timing > trickery as 9 to 1 and above really needs 89 or more (especailly in > warm weather) for optimal timing curving. At 10 to one and above you > really need 91 or better unless you run a compromised timing curve > which also reduces efficency and power too. > > ----------------- > TheSnoMan.com Compression ratio is not necessarily the only factor involved. Engines with aluminum heads can run higher compression ratios than engines with cast iron heads due to thermal conductivity of the metal. Also, the valve timing influences octane requirements. More overlap on the valve timing reduces cylinder pressure, which is the true determining factor for octane requirements. Other factors that affect octane requirements are the efficiency of the combustion chamber design and the quench area around the outer diameter of the combustion chamber. I have a 350 in my S10 truck that runs on 89 octane with a true blueprinted 10.7 compression ratio and cast iron heads, but it also has a cam with quite a bit of overlap and the combustion chambers are highly polished. The quench distance between the top of the piston and the head is .038" which is the tightest recommended distance, but also the best for reducing detonation. I do not have to compromise my ignition timing with this setup. I am running 12 degrees initial timing and 24 degrees mechanical. Chris |
Re: 85 vs 87 Octane
SnoMan wrote:
> On Sun, 2 Sep 2007 20:05:19 -0500, "DougW" > <I.only.read.usenet@invalid.address> wrote: > >> First, do not confuse power with octane rating, they arn't related. > > Actually they are a LOT more related than you think in some engine > that have knock sensing timing control > > >> The higher the octane number the harder the fuel is to ignite. > > No it will light just as easy, the differnece is that it has a higher > auto ignite tempature and a slightly slower flame speed but it is no > harder to light its fuse with a spark > >> And higher octane fuel has no more power than lower octane fuel. > > Actually it has a few less BTU in it but in a higher compression > engine it allows for a more favorable ignition and expansion cycle > than low octane fuel does (which has to use a retarded curve to > prevent to rapid of a pressure build up and auto ignition or > detenation) > >> What your experiencing is lower air pressure at altitude. The same >> thing that makes you tire more easily at higher elevations also means >> your engine is not getting the same amount of oxygen with each intake. >> Less oxygen means less fuel can be burned. Less fuel of course means >> less power unless you can push more air into the engine with a >> supercharger or turbocharger. > > Mostly true. In the old days you would advance base line timing about > 2 degrees for each 1000 feet above 1000 MSL up to a max of 12 degrees > or so. It did not restore all power but it does help some. Some modern > cars can advance timing a bit more automatically as well as lean > mixture out. At start up it reads MAP sensor just before engine cranks > to set elevation data. The problem with this is if you start engine at > 4000 feet and drive to 9000 feet the computer does not know it is at > 9000 feet until you shut it out and restart it. > >> You want to use the lowest octane fuel that does not cause ping. > > This is not cut and dried as it seems. This works with old fixed > timing engine but not with some modern engine with knock sensor > because many of them (especailly GM's ) will never knock no matter > what, performance will just suffer. BTW, octane requirements do > decrease generally with elevation and this is why above 4000 MSL or so > you start seeing 85 or 85.5 octane fuel for regular, 88 or so for plus > and 91 for premium. You do not want a tank of 85 when you are headed > to lower elevations especailly in hot weather (ambent temp effects > octane needs too) Generally if your engine has a CR of 9 to 1 or > higher you cannot burn 87 at sea level without some spark timing > trickery as 9 to 1 and above really needs 89 or more (especailly in > warm weather) for optimal timing curving. At 10 to one and above you > really need 91 or better unless you run a compromised timing curve > which also reduces efficency and power too. > > ----------------- > TheSnoMan.com Compression ratio is not necessarily the only factor involved. Engines with aluminum heads can run higher compression ratios than engines with cast iron heads due to thermal conductivity of the metal. Also, the valve timing influences octane requirements. More overlap on the valve timing reduces cylinder pressure, which is the true determining factor for octane requirements. Other factors that affect octane requirements are the efficiency of the combustion chamber design and the quench area around the outer diameter of the combustion chamber. I have a 350 in my S10 truck that runs on 89 octane with a true blueprinted 10.7 compression ratio and cast iron heads, but it also has a cam with quite a bit of overlap and the combustion chambers are highly polished. The quench distance between the top of the piston and the head is .038" which is the tightest recommended distance, but also the best for reducing detonation. I do not have to compromise my ignition timing with this setup. I am running 12 degrees initial timing and 24 degrees mechanical. Chris |
Re: 85 vs 87 Octane
SnoMan wrote:
> On Sun, 2 Sep 2007 20:05:19 -0500, "DougW" > <I.only.read.usenet@invalid.address> wrote: > >> First, do not confuse power with octane rating, they arn't related. > > Actually they are a LOT more related than you think in some engine > that have knock sensing timing control > > >> The higher the octane number the harder the fuel is to ignite. > > No it will light just as easy, the differnece is that it has a higher > auto ignite tempature and a slightly slower flame speed but it is no > harder to light its fuse with a spark > >> And higher octane fuel has no more power than lower octane fuel. > > Actually it has a few less BTU in it but in a higher compression > engine it allows for a more favorable ignition and expansion cycle > than low octane fuel does (which has to use a retarded curve to > prevent to rapid of a pressure build up and auto ignition or > detenation) > >> What your experiencing is lower air pressure at altitude. The same >> thing that makes you tire more easily at higher elevations also means >> your engine is not getting the same amount of oxygen with each intake. >> Less oxygen means less fuel can be burned. Less fuel of course means >> less power unless you can push more air into the engine with a >> supercharger or turbocharger. > > Mostly true. In the old days you would advance base line timing about > 2 degrees for each 1000 feet above 1000 MSL up to a max of 12 degrees > or so. It did not restore all power but it does help some. Some modern > cars can advance timing a bit more automatically as well as lean > mixture out. At start up it reads MAP sensor just before engine cranks > to set elevation data. The problem with this is if you start engine at > 4000 feet and drive to 9000 feet the computer does not know it is at > 9000 feet until you shut it out and restart it. > >> You want to use the lowest octane fuel that does not cause ping. > > This is not cut and dried as it seems. This works with old fixed > timing engine but not with some modern engine with knock sensor > because many of them (especailly GM's ) will never knock no matter > what, performance will just suffer. BTW, octane requirements do > decrease generally with elevation and this is why above 4000 MSL or so > you start seeing 85 or 85.5 octane fuel for regular, 88 or so for plus > and 91 for premium. You do not want a tank of 85 when you are headed > to lower elevations especailly in hot weather (ambent temp effects > octane needs too) Generally if your engine has a CR of 9 to 1 or > higher you cannot burn 87 at sea level without some spark timing > trickery as 9 to 1 and above really needs 89 or more (especailly in > warm weather) for optimal timing curving. At 10 to one and above you > really need 91 or better unless you run a compromised timing curve > which also reduces efficency and power too. > > ----------------- > TheSnoMan.com Compression ratio is not necessarily the only factor involved. Engines with aluminum heads can run higher compression ratios than engines with cast iron heads due to thermal conductivity of the metal. Also, the valve timing influences octane requirements. More overlap on the valve timing reduces cylinder pressure, which is the true determining factor for octane requirements. Other factors that affect octane requirements are the efficiency of the combustion chamber design and the quench area around the outer diameter of the combustion chamber. I have a 350 in my S10 truck that runs on 89 octane with a true blueprinted 10.7 compression ratio and cast iron heads, but it also has a cam with quite a bit of overlap and the combustion chambers are highly polished. The quench distance between the top of the piston and the head is .038" which is the tightest recommended distance, but also the best for reducing detonation. I do not have to compromise my ignition timing with this setup. I am running 12 degrees initial timing and 24 degrees mechanical. Chris |
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