To the poster that said-Just try it because it will "only" cost you $6 to $10 more per tank. To me that could amount to $20-$30/day of driving more which is why I wouldn't unless it is called for in the manual.
14.7:1 AFR. That is Stoichiometrc for gas engines. that is that all the fuel is being burned. (most efficient and cleanest exhaust) Many modern engines will be in the 9.5:1 ratio during high load conditions. the extra fuel is used to cool the pistons and prevent premature combustion of the air fuel mixture. As some others have pointed out higher octane fuels are less flammable. By being less flammable they are, by their nature, less prone to premature combustion (knock, ping, detonation). This allows the computer that controls the engine to run more timing and a AFR closer to 14.7:1 with out pinging.
every engine is different and the load on it is different too. so it logical that some see a great difference in performance (mileage and power) with premium fuel and others not so much. some engine control computers simply can not add timing or subtract fuel enough to compensate for premium fuels.
I am of the mindset that if the manufacturer doesn't ask for it, I'm not wasting my money. I have a 2003 Tahoe which calls for 87 octane so that's what I put in. Not to start another sub-thread but here goes:
If you want to spend a little extra at the pump on something that IMO for-sure will be good for your engines: buy branded fuel (Texaco, Shell etc). Those fuels contain additives that unbranded fuels don't.
Where I live, we get fuel discounts at our local Safeway (Kroger). Every $100 we spend on groceries = $.10 off per gallon. I accumulate my fuel rewards and fill up the Tahoe once a month with Safeway fuel but I try to use Chevron the rest of the time. Chevron is about $.25/gallon more expensive.
We use the same gas as we do when we are NOT towing the trailer (which is NOT premium). Hasn't caused us a problem yet in years, gas is gas. If one spends more for the gas, in my opinion, one is just paying more for the trip!
DiaperDude wrote: If your engine runs high levels of compression it can cause the fuel air mixture to heat up to the ignition point prior to the spark plug firing. This premature detination is what is commonly known as spark knock.
This is the part of the ignition timing equation that I DON'T understand. If spark knock or ping is caused by fuel prematurely igniting, prior to the spark plug firing, how can the solution be to have the computer retard the timing and thus have the spark plug fire later. It would seem that you would want to advance the timing and have the spark plug fire before the fuel prematurely ignites. If the fuel is igniting without the spark plug firing, having the spark plug fire later wouldn't seem like a solution. What aren't I seeing here ?
Ideally, fuel is ignited just before the piston reaches top dead center and begins the downward stroke. That way the fuel reaches full combustion just as the piston is at top dead center and the full force of the explosion is used to force the piston downward. If ignition occurs too soon, then the explosion is forcing the piston downward before top dead center on the upward stroke. In the worst case scenario, something in the engine will break, which in most cases will be the connecting rod. In the best case scenario, fuel efficiency suffers. Over a long period of time, premature detonation can burn a hole in a piston. Premature detonation can generally be heard in the vehicle as pinging and it's generally most prevalent when the engine is under a load, such as going up hill or accelerating hard. Ignition timing is closely monitored by modern electronics and the timing is advanced or retarded automatically as needed. If you have the recommended fuel, there shouldn't be any pinging and all will be well.
If fuel ignites on it's own, then there is little that can be done to solve the problem by engine electronics. Firing the plugs too soon will simply worsen the problems stated above. Higher octane fuel will help in most cases but there could be other causes, such as carbon deposits on the valves or the pistons.
* This post was
edited 05/07/12 07:31pm by tbrowne *
tbrowne wrote: Ideally, fuel is ignited just before the piston reaches top dead center and begins the downward stroke. That way the fuel reaches full combustion just as the piston is at top dead center and the full force of the explosion is used to force the piston downward. If ignition occurs too soon, then the explosion is forcing the piston downward before top dead center on the upward stroke. In the worst case scenario, something in the engine will break, which in most cases will be the connecting rod. In the best case scenario, fuel efficiency suffers. Over a long period of time, premature detonation can burn a hole in a piston. Premature detonation can generally be heard in the vehicle as pinging and it's generally most prevalent when the engine is under a load, such as going up hill or accelerating hard. Ignition timing is closely monitored by modern electronics and the timing is advanced or retarded automatically as needed. If you have the recommended fuel, there shouldn't be any pinging and all will be well.
If fuel ignites on it's own, then there is little that can be done to solve the problem by engine electronics. Firing the plugs too soon will simply worsen the problems stated above. Higher octane fuel will help in most cases but there could be other causes, such as carbon deposits on the valves or the pistons.
I already knew everything that you just stated. My question is: "How does retarding spark plug firing eliminate knock or ping ?? " Everyone acknowledges that is does but how/why ?
My 2005 F150 got slightly better MPG on premium (0.3-0.4 mpg). Not enough to overcome the price differential of premium. I did not notice any difference in power.
My 2008 Tundra gets the same MPG on premium, and no apparent difference in power.
I run the gennies on mid range gas (they seem to like it better, and my local small engine repair shop recommends a higher octane). So I carry mid range gas in the jeep cans.
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Log off and go camping!
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tbrowne wrote: Ideally, fuel is ignited just before the piston reaches top dead center and begins the downward stroke. That way the fuel reaches full combustion just as the piston is at top dead center and the full force of the explosion is used to force the piston downward. If ignition occurs too soon, then the explosion is forcing the piston downward before top dead center on the upward stroke. In the worst case scenario, something in the engine will break, which in most cases will be the connecting rod. In the best case scenario, fuel efficiency suffers. Over a long period of time, premature detonation can burn a hole in a piston. Premature detonation can generally be heard in the vehicle as pinging and it's generally most prevalent when the engine is under a load, such as going up hill or accelerating hard. Ignition timing is closely monitored by modern electronics and the timing is advanced or retarded automatically as needed. If you have the recommended fuel, there shouldn't be any pinging and all will be well.
If fuel ignites on it's own, then there is little that can be done to solve the problem by engine electronics. Firing the plugs too soon will simply worsen the problems stated above. Higher octane fuel will help in most cases but there could be other causes, such as carbon deposits on the valves or the pistons.
I already knew everything that you just stated. My question is: "How does retarding spark plug firing eliminate knock or ping ?? " Everyone acknowledges that is does but how/why ?
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