What are the consequences of using 89 or 87 octane fuel in an engine rated by the mfg at 91? ie '04 Lincoln LS 6.
The engine’s computer will adjust the timing and other things, you will have less power, and likely shorter engine life. This is OK once in a while when you are stuck in Rattlesnake Gulch in West Texas, and it is the only gas available.
Yeah, less power, and it will not shorten engine life - because computer will retard the ignition timing to avoid detonation. You will get worse mileage, however - if you care about it 
You’re between a rock and a hard place, sad to say. If you go with 89 or especially 87, timing will retard to prevent pinging. Pinging can ruin engine if prolonged. Problem w/retarded timing is cooling system will run hotter, even though combustion chamber will be cooler- and there’s the problem- you won’t get the real good complete combustion designed into the engine and your gas mileage will suffer; probably
get carbon buildup in combustion chamber after a while; which; if severe; will make timing retard even more… snowball effect. It might be possible that the poor gas mileage you get will offset the $ you save by getting cheaper gas. maybe you could get away w/89 octane… kinda doubt it. I think the cheapest overall scenario w/fewest headaches is to stick w/91 octane. You’re talking here about a relative trickle of $.
Click on: http://en.wikipedia.org/wiki/Engine_knocking. Learn the difference between pre-ignition and detonation. One is an early normal rate combustion and the other is an explosive rate burn. The effects of one are vastly different than the effects of the other.
If you care about this car, and want to keep it running correctly, DO NOT put anything but 91-octane gasoline in its tank.
If you can’t afford 91-octane, or are unwilling to spend the money, please consider selling this car and buying something designed to run on regular. A Honda Accord, perhaps?
I would use 91 octane fuel. You won’t save much. Let’s say you drive 20,000 miles each year, you average 25 MPG, and the difference between grades is 10 cents per gallon. You would save $80 each year by using 89 octane and $176 per year by using 87 octane. But your mileage will suffer - what is the break even point? It’s just 3% per grade drop. That is, if you go to 89 octane you only need for your mileage to drop 3% before the 91 octane and 89 octane gasoline cost is the same. It’s a little less than 7% for 87 octane vs. 91 octane. So the $80 to $176 is the most you would save; probably much less. It hardly seems worth it to me.
I might trade my 1992 Honda Accord, with only 320,000 miles, for that Lincoln. I won’t ask for much boot, and the Honda uses 87 octane gasoline.
Davendiana, Does your owner’s manual reccomend premium or require it? Please post back and tell us; it could change the nature of this discussion. For more about this subject click here:http://community.cartalk.com/posts/list/214813.page
You said it better than me, jt.
You may be getting 100% gasoline with the higher octane. If you have a V-8 engine with any power at all, you probably should be using 91 even without it being recommended. Do the right thing.
The owner's manual will say that it must have high octane or that it may use regular but should use high octane. The parts the leave out are:
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If it needs high octane, using regular can damage the engine.
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If it should use high octane then it has a system to sense the low octane and to make adjustments to the car that will protect the engine. However the adjustments will result in lower mileage (reducing or wiping out any economic advantage) and will reduce the power, defeating the reason most people buy cars that require high octane.
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Using high octane in a car that does not recommend it does nothing but cost you more.