|
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
The best thing to do is try it. There are so many variables that it
really is hard to give a "for sure" answer. Chris Mindy wrote: > 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
The best thing to do is try it. There are so many variables that it
really is hard to give a "for sure" answer. Chris Mindy wrote: > 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
The best thing to do is try it. There are so many variables that it
really is hard to give a "for sure" answer. Chris Mindy wrote: > 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
The best thing to do is try it. There are so many variables that it
really is hard to give a "for sure" answer. Chris Mindy wrote: > 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
c wrote:
> 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 The crux of what you said is that cylinder pressure is the true determining factor for octane reqs. All that stuff you talked about has to do with the air/fuel mixture getting "squeezed" as the piston comes up. (All this commentary is probably way more than Mindy ever wanted, but hey, ask a technical question...) Anyway, I might be wrong, but it was my understanding that the higher the octane, the more the a/f mixture can be compressed before it self ignites (ping). That's why high-compression racing engines use VERY high octanes. If you have higher compression, you can squeeze in more a/f and produce a bigger bang when the spark plug ignites. That's why higher octane is associated with more power. No? I think you gave Mindy the best answer buy telling her to try it and see what happens because this crap can be complicated. But simply put, that's the main purpose of higher octane. Ok SnoMan, tell me where I'm wrong. :) Pete Pete |
Re: 85 vs 87 Octane
c wrote:
> 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 The crux of what you said is that cylinder pressure is the true determining factor for octane reqs. All that stuff you talked about has to do with the air/fuel mixture getting "squeezed" as the piston comes up. (All this commentary is probably way more than Mindy ever wanted, but hey, ask a technical question...) Anyway, I might be wrong, but it was my understanding that the higher the octane, the more the a/f mixture can be compressed before it self ignites (ping). That's why high-compression racing engines use VERY high octanes. If you have higher compression, you can squeeze in more a/f and produce a bigger bang when the spark plug ignites. That's why higher octane is associated with more power. No? I think you gave Mindy the best answer buy telling her to try it and see what happens because this crap can be complicated. But simply put, that's the main purpose of higher octane. Ok SnoMan, tell me where I'm wrong. :) Pete Pete |
Re: 85 vs 87 Octane
c wrote:
> 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 The crux of what you said is that cylinder pressure is the true determining factor for octane reqs. All that stuff you talked about has to do with the air/fuel mixture getting "squeezed" as the piston comes up. (All this commentary is probably way more than Mindy ever wanted, but hey, ask a technical question...) Anyway, I might be wrong, but it was my understanding that the higher the octane, the more the a/f mixture can be compressed before it self ignites (ping). That's why high-compression racing engines use VERY high octanes. If you have higher compression, you can squeeze in more a/f and produce a bigger bang when the spark plug ignites. That's why higher octane is associated with more power. No? I think you gave Mindy the best answer buy telling her to try it and see what happens because this crap can be complicated. But simply put, that's the main purpose of higher octane. Ok SnoMan, tell me where I'm wrong. :) Pete Pete |
Re: 85 vs 87 Octane
c wrote:
> 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 The crux of what you said is that cylinder pressure is the true determining factor for octane reqs. All that stuff you talked about has to do with the air/fuel mixture getting "squeezed" as the piston comes up. (All this commentary is probably way more than Mindy ever wanted, but hey, ask a technical question...) Anyway, I might be wrong, but it was my understanding that the higher the octane, the more the a/f mixture can be compressed before it self ignites (ping). That's why high-compression racing engines use VERY high octanes. If you have higher compression, you can squeeze in more a/f and produce a bigger bang when the spark plug ignites. That's why higher octane is associated with more power. No? I think you gave Mindy the best answer buy telling her to try it and see what happens because this crap can be complicated. But simply put, that's the main purpose of higher octane. Ok SnoMan, tell me where I'm wrong. :) Pete Pete |
Re: 85 vs 87 Octane
Peter Stolz wrote:
> c wrote: >> 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 > > > The crux of what you said is that cylinder pressure is the true > determining factor for octane reqs. All that stuff you talked about has > to do with the air/fuel mixture getting "squeezed" as the piston comes > up. (All this commentary is probably way more than Mindy ever wanted, > but hey, ask a technical question...) Anyway, I might be wrong, but it > was my understanding that the higher the octane, the more the a/f > mixture can be compressed before it self ignites (ping). That's why > high-compression racing engines use VERY high octanes. If you have > higher compression, you can squeeze in more a/f and produce a bigger > bang when the spark plug ignites. That's why higher octane is > associated with more power. No? > > I think you gave Mindy the best answer buy telling her to try it and see > what happens because this crap can be complicated. But simply put, > that's the main purpose of higher octane. > > Ok SnoMan, tell me where I'm wrong. :) > > Pete I know, I get carried away sometimes, but I figure if someone can understand something better if I explain my reasoning, then it is worth typing. Anyway, to answer your question, you are pretty much right. The one thing I would say is that the high compression does not directly allow more A/F to get in to the engine. It does allow the engine to be built with a bigger camshaft and still maintain the proper cylinder pressure. The reason a big cammed engine can use higher compression ratios is that the camshaft, by design, keeps the intake valve open long after the piston has reached the bottom of the intake stroke, and is already on its way up on the compression stroke. This reduces the percentage of the compression stroke that is actually used to compress the mixture. the benefit to this is that the A/F mixture is still actually filling the cylinder, even though the piston is on its way up. Just an FYI for you. The NHRA Pro Stock drag race engines are built with around 18:1 compression, which is the same as some diesel engines. Here is an article explaining pretty much what I explained: http://www.popularhotrodding.com/tec...on_ratio_tech/ Chris |
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