4 pages of speculation, has anyone actually got their O.M. out and actually read it?
2011 Jeep Grand Cherokee Owner's Manual said:
This engine [5.7L V8] is designed to meet all emissions regulations and provide satisfactory fuel economy and performance when using high-quality unleaded gasoline having an octane range of 87 to 89. The manufacturer recommends the use of 89 octane for optimum performance. The use of premium gasoline is not recommended, as it will not provide any benefit over regular gasoline in these engines.
This is the 2011 O.M., so later years might be different.
This is not the only engine from FCA with this recommendation, and likely not the only Manufacturer either. The PCM recognizes what octane you're using and detunes the engine to run that octane (within a certain range, in this case 87-89). So you can safely run 87 Octane, with slightly less performance, but 89 Octane will produce the best performance.
And that recognizing and detuning is not simply the Knock Sensor retarding ignition timing a few seconds. Its an algorithm that does use the knock sensor, but more a testing of the indicators for which octane, and switching to alternate set of tables for the tune. Testing again after a few minutes and either staying with or switching to the other tables.
I think the BTU content, or Energy Density is negligible. Higher Octane fuels usually have a higher pressure vapor, making it harder to atomize and mix with air. That definitely can have an effect on cold weather starting and stalling while cold. Both Ford and FCA have TSB's out, for customers complaining of hard starting and stalling in cold weather, first ask if they are using higher than recommended Octane fuel.
Lower Octane fuel's better atomization and mixing, plus the slightly higher energy density, could provide a slight mileage increase.
Just as E10, 10% ethanol, since alcohol only has half the energy density of gasoline, using E10 results in a slight decrease in mileage.
So what are you saying? fill our Jeeps up with Jet A? Yeah that will work haha :lol:
Jet A is actually Diesel fuel, which your still right, if the engine runs at all, it will run horribly and need some expensive repair to get all the diesel out. Different additive packages. But Military Trucks now have filter and fuel control kit, that they can switch them over to running JP-5/JP-8 in an hour.
I saw a Brit Car show that actually tried this. Surprisingly, both Diesel and Gasoline engines ran, although poorly, on the wrong fuel. Eventually the engines quit after a while, the gas engine ran for hours on diesel. The Diesels died in a few minutes to nearly an hour, and the diesels suffered very expensive damage. Diesels need extreme high pressure fuel pumps, and those pumps are lubricated by the thicker more oily diesel. Gasoline can't lube a pump nearly as well, and thus the pumps burn up right away and need to be replaced, and they are expensive.
I fill mine up with the mid-grade.. whatever octane that may be. Here where I live, it's 87.. up in the mountain towns, it could be 85.
People have been talking about Compression Ratios determining the Octane needed. Actually it's the peak pressure and temperatures in the cylinder, which the Compression Ratio has an enormous effect on that. But so does cam timing, duration, lift and overlap. So does intake, exhaust tuning and ignition timing.
So if you live in the mountains, where ambient air pressure is lower, you're starting with a lower pressure and thus less of a peak pressure and temperature. So you do not need as high an octane. That is why they sell lower octane fuel in the mountain communities.
I like to keep things simple and it looks like the conclusions we can draw for the 5.7L engine are:
1. The manual recommends 89 octane.
2. Many people here run 87 octane without any apparent problems.
3. If you live at higher altitude, like Denver, the 87 octane may be the essential equivalent of 89 octane at sea level.
4. Running 91 or 93 octane doesn't add any value.
5. Ethanol is bad because it corrosive and Benzene is bad because it's carcinogenic, so pick your poison.
Bottomline for me is I will run 89 octane from a reputable gas company as long as it's available, if not I'll run 87 octane. I'll run premium if nothing else is available. Tell me if I've gotten something wrong.
That's not what the manual recommends, see above. Sure you're not far off, but why pass on something kinda of right, instead of actually reading what the O.M. really recommends?
And someone was talking about the knock sensor is there to let you use lower octane? Ummm, no. Sure in the Hemi you can, but it's designed to use the lower 87, and its not simply the knock sensor retarding timing.
To get the most efficiency out of modern engines they're pushing the tune closer to the limits, so knock is more likely. In the past, they just detuned the motor to make sure knock would not happen under any circumstance, then dealt with the lower mileage or if you wanted more power, just make a bigger and more fuel hungry engine. There are hundreds of variable to determine the best fuel and ignition timing, and they can change in an instant, so even with a computer processing all the calculations, you're still going to get knock once in a while. The engine is designed and tested to use the recommended octane, and it will still get knock in some circumstances, that is what the knock sensor is for.
In some cases yea, you could use lower than recommended octane and the knock sensor retarding timing would still protect the motor.
In some cases no, you use lower octane, get knock and the knock sensor retards ignition and that still won't be enough to stop the knock and you'll get engine damage. They didn't test and design that motor to use a lower octane and have the knock sensor protect it for knock from lower octanes.
In some cases yes/no, you use lower than recommended octane, the knock sensor retarding timing is enough to protect the motor from damage, but the constant retarding of the ignition reduces mileage so much, you would have saved more money if you just bought the correct octane.
And in the case of living in the mountains, you could use lower than recommended octane and never get knock and thus not have the knock sensor retard ignition.