If you can afford it, in this case go with 91 or 93, the Hemi knock sensors are pretty sensitive and will yank timing at the slightest instance of knock or heavy load.
Nope. Higher octane gas does not contain any more energy than lower octane gas. The reason you might see high performance cars require high octane is because they are running turbos or very high compression, both of which raise the temperatures of the gas/air mix which contributes to pre-ignition - IE: detonation. The octane number is just a representation of how resistant the blend is to pre-ignition. It is also sometimes advisable to run a higher octane when towing or during hot weather. Towing can raise engine temps especially at low speeds and if the intake temps get too hot the vehicle will start pulling timing and you lose power. Bad for towing. Moral of the story - just follow the manual and don't think you are 'treating your engine' or gaining anything by running higher octane fuel.
Nope. Higher octane gas does not contain any more energy than lower octane gas. The reason you might see high performance cars require high octane is because they are running turbos or very high compression, both of which raise the temperatures of the gas/air mix which contributes to pre-ignition - IE: detonation. The octane number is just a representation of how resistant the blend is to pre-ignition. It is also sometimes advisable to run a higher octane when towing or during hot weather. Towing can raise engine temps especially at low speeds and if the intake temps get too hot the vehicle will start pulling timing and you lose power. Bad for towing. Moral of the story - just follow the manual and don't think you are 'treating your engine' or gaining anything by running higher octane fuel.
89 octane allows the spark to be advanced more than 87. The higher octane fuels are more resistant to early ignition or detonation.
The PCM controls the spark advance and it is anything but static. I have a reader than can display engine parameters and the spark advance goes from -3 to +27 degrees, depending on engine speed and load. Have tried 87, 89, and 91 octane gasolines. Do not see a great deal of difference among them. Having said that, most of my driving is rural, driver-only, flat ground, no load.
If I drive on the interstate or carry a big load, I will be sure to use 89 octane to avoid the knock sensors from taking over and retarding the spark (as mentioned earlier in the thread).
Remember that the EGR valve will open if the combustion temperature gets too high and reduce it. That also affects power and timing. On and on and on.......
I think a better choice is to use gasolines that do not have alcohol in them.
As a general statement, this is true. My point in the previous post was that the timing is constantly varying as conditions change. The PCM determines where it wants to be. The knock sensors will take over if they get excited (I believe they are piezoelectric crystals). I watch spark advance as one parameter on my DashDAQ XL screens. Not only does it vary by load and temperature, it also varies by MDS activation. No longer a centrifugal advance mechanism that is set statically, then final adjusted by timing light and dwell angle.
While there probably is some difference in the PCM-set timing between 87 and 89 octane fuels, I cannot set the conditions exactly the same to determine if there is a significant change or not. I ran 87 for 2 weeks just to see if a pattern developed. No joy.
BTW: I generally run 89 octane in mine. I bought the HEMI for a reason and do not want to limit its operation unnecessarily..... Whatever octane you run, it's better than a Prius.
FWIW, I've never measured timing on my Jeep with a scan tool. On another car I own that I have measured timing (a Honda), I've only seen gains in timing with higher octane gas at WOT. Even went as high at 96 octane and the timing kept advancing.
Sorry haters, the Prius is a terrific 2nd or 3rd car, especially when your primary car gets 15 MPG! I drove a well loaded model...slow, but full of techno-fun.