one more fuel grade inquery
#21
06-01-2011, 03:17 PM
High-octane fuels only become necessary when your engine has a high compression ratio.
All gasoline, regardless of its’ octane rating, have pretty much the same amount of energy per gallon. What!!! "Sacrilege" you say? Well, actually, some higher-octane fuels have a few LESS percent energy per gallon…so as not to argue over this small point, for the sake of this discussion we will all agree that the automotive gasoline that you buy at the pump, regardless of octane rating, has the same amount of potential energy.
Octane is NOT a measure of power but of the fuels’ resistance to ignition from heat. A higher-octane fuel, under identical combustion chamber conditions, will burn slower.
The piston, quickly squeezing the fuel/air mixture into a small space, can generate enough heat of compression to ignite the fuel well before the spark plug fires, with unpleasant results. If the fuel prematurely ignites while the piston is on its way up, the burning of the fuel, in conjunction with the rising piston, creates even more pressure, resulting in a violent explosion. This explosion is equivalent to hitting the top of the piston with a very large hammer. If you want to be able to see through the top of your piston, ignore those sounds that are usually called: "pre-ignition", "ping" or "engine knock". Trust me on this one; in his reckless youth, using this method, the swami turned a few pistons into paper weights.
All gasoline, regardless of its’ octane rating, have pretty much the same amount of energy per gallon. What!!! "Sacrilege" you say? Well, actually, some higher-octane fuels have a few LESS percent energy per gallon…so as not to argue over this small point, for the sake of this discussion we will all agree that the automotive gasoline that you buy at the pump, regardless of octane rating, has the same amount of potential energy.
Octane is NOT a measure of power but of the fuels’ resistance to ignition from heat. A higher-octane fuel, under identical combustion chamber conditions, will burn slower.
The piston, quickly squeezing the fuel/air mixture into a small space, can generate enough heat of compression to ignite the fuel well before the spark plug fires, with unpleasant results. If the fuel prematurely ignites while the piston is on its way up, the burning of the fuel, in conjunction with the rising piston, creates even more pressure, resulting in a violent explosion. This explosion is equivalent to hitting the top of the piston with a very large hammer. If you want to be able to see through the top of your piston, ignore those sounds that are usually called: "pre-ignition", "ping" or "engine knock". Trust me on this one; in his reckless youth, using this method, the swami turned a few pistons into paper weights.
What we really want is a very rapid burn of the fuel, not an explosion. And we want the burning of the fuel to take place while the piston is in a better position to convert this pressure into productive work, like on its way down. Think of this burning as a very fast "push" on the top of the piston. Despite the violent noises you hear from some exhaust systems, it really is a rapid push on the top of the piston making the crankshaft go around, not explosions.
So that we can ignite the fuel at exactly the right time with the spark plug, instead of from the heat of compression, they put stuff into gasoline to keep it from igniting prematurely. The more resistant the fuel is to ignition from the heat of compression, the higher its octane rating.
Higher compression ratios = higher combustion chamber pressures = higher heat… and it is with these higher combustion chamber temperatures that the magic happens.
Higher compression ratios = higher combustion chamber pressures = higher heat… and it is with these higher combustion chamber temperatures that the magic happens.
At higher temperatures the fuel is burned more efficiently. So, while it’s true that the higher-octane fuel does not posses any more energy than low octane fuel, the increased octane allows the extraction of more of the potential energy that has always been there. Conversely, lower compression ration engines utilize a little less of the fuel energy potential (2-4% reduction) but there is also less heat generated in the combustion process.
#22
06-01-2011, 03:22 PM
There is ABSOLUTELY NO BENEFIT to using a higher octane than your engine needs. The only benefit is increased profits to the oil companies that have cleverly convinced some of the public that their new "Super-Duper, Premium-High-Test, Clean-Burning, Used-By-Famous-Racing-Types-All-Around-The-World, Extra-Detergent-Laden-Keep-Your-Pipes-Clean, Extra-High-Octane" fuel is your engines’ best friend.
#24
06-01-2011, 04:17 PM
Overlanding
Join Date: Aug 2010
Location: Morgantown, WV
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I believe using a lower octance than required can damage ur cats, right?
I have a 92 turbo saab and the manual says I can use any octane due to adjustments done from the knock sensors, but to get the max HP out of my engine+turbo I have to use the higher octane.
I try to only buy gas from big name oil companies, like exxon, that have their own oil refineries, they keep the best gas for themselves. would like to use the deep fryer oil from the fastfood places in my town, then get an older diesel car and use it.
I have a 92 turbo saab and the manual says I can use any octane due to adjustments done from the knock sensors, but to get the max HP out of my engine+turbo I have to use the higher octane.
I try to only buy gas from big name oil companies, like exxon, that have their own oil refineries, they keep the best gas for themselves. would like to use the deep fryer oil from the fastfood places in my town, then get an older diesel car and use it.
#26
06-01-2011, 08:04 PM
My wife's Volvo's owners manual says the exact same thing, "for maximum MPG, performance and engine life fuel with a octane rating of 91 or higher is recommended"
#27
06-01-2011, 08:05 PM
#29
06-02-2011, 02:29 PM
It is a little more than the compression ratio... it's really the whole design of the combustion chamber and it's ingnition characteristics, but otherwise the reasoning is sound.
The Land Rover V8 is a bit of an oddity. In the UK, a 4.0 V8 is a luxury engine for rich bastards. Premium petrol costs an average of 143p per liter or about $8.80 per gallon, and you know what your mpg is, so you can very well imagine the average Jack doesn't tolerate a V8's expense. They buy the little diesels. Because of this, there's no reason to design a V8 to sip cheap fuel. The compression ratio and the combustion chamber design can be made to require premium petrol. It makes no difference. The bloke paying for it's loaded anyway.
But then they started selling them to stupid Yanks. The Americans already had the impression the Range Rover was a luxury vehicle alternative to the standard yuppy b'mer, and had pretty well legislated any kind of diesel out of the picture, so it was easy and really the only option to sell them the V8. But to an American, the 4.0 is a wussy little engine and we all think gasoline should be $1.99 a gallon and as long as we can keep it there it doesn't matter how much we use. As a result, the idea of paying $4.30 a gallon for premium just to fill a wussy little 4 liter seems ridiculous. I mean, 91 is supposed to be for 7.0 liter (high compression) 500hp 'vette's, right? Why should I have to put it in my lowly foreign station wagon?
The only question I have is, does the Rover V8 really need 91 because of pinging and detonation due to the compression/combustion chamber design, or does Land Rover specify this simply because the exhaust valves stick and the higher detergent levels in premium gas prevents that?
The Land Rover V8 is a bit of an oddity. In the UK, a 4.0 V8 is a luxury engine for rich bastards. Premium petrol costs an average of 143p per liter or about $8.80 per gallon, and you know what your mpg is, so you can very well imagine the average Jack doesn't tolerate a V8's expense. They buy the little diesels. Because of this, there's no reason to design a V8 to sip cheap fuel. The compression ratio and the combustion chamber design can be made to require premium petrol. It makes no difference. The bloke paying for it's loaded anyway.
But then they started selling them to stupid Yanks. The Americans already had the impression the Range Rover was a luxury vehicle alternative to the standard yuppy b'mer, and had pretty well legislated any kind of diesel out of the picture, so it was easy and really the only option to sell them the V8. But to an American, the 4.0 is a wussy little engine and we all think gasoline should be $1.99 a gallon and as long as we can keep it there it doesn't matter how much we use. As a result, the idea of paying $4.30 a gallon for premium just to fill a wussy little 4 liter seems ridiculous. I mean, 91 is supposed to be for 7.0 liter (high compression) 500hp 'vette's, right? Why should I have to put it in my lowly foreign station wagon?
The only question I have is, does the Rover V8 really need 91 because of pinging and detonation due to the compression/combustion chamber design, or does Land Rover specify this simply because the exhaust valves stick and the higher detergent levels in premium gas prevents that?
