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Old 02-05-2012, 12:41 AM
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89 or >91 octane gas?

The manual says 89 and doesn't recommend premium, but why? I would always thinking higher octane = better performance etc.?
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Old 02-05-2012, 01:06 AM
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Re: 89 or >91 octane gas?

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.
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Old 02-05-2012, 08:04 AM
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Re: 89 or >91 octane gas?

Quote:
Originally Posted by armoredsaint View Post
I would always thinking higher octane = better performance etc.?
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.
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Old 02-05-2012, 08:46 AM
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Re: 89 or >91 octane gas?

Quote:
Originally Posted by brandont View Post
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.
+1 So completely right.
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Old 02-05-2012, 09:07 AM
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Re: 89 or >91 octane gas?

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Originally Posted by armoredsaint View Post
The manual says 89 and doesn't recommend premium, but why? I would always thinking higher octane = better performance etc.?
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.
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Old 02-05-2012, 09:08 AM
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Re: 89 or >91 octane gas?

Everybody seems to get this wrong, but you nailed it. I often run 87 octane gas and no problems at all.
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Old 02-05-2012, 09:21 AM
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Re: 89 or >91 octane gas?

Quote:
Originally Posted by brandont View Post
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.

good point, i currently have a BMW w/ twin turbos and it always say 91 or higher only, past was Audi's with turbos also. i guess i will be saving a little bit of money
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Old 02-05-2012, 09:48 AM
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Re: 89 or >91 octane gas?

More octane, more timing.

More timing, more power.
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Old 02-05-2012, 11:05 AM
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Re: 89 or >91 octane gas?

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Originally Posted by andyh View Post
More octane, more timing....More timing, more power.
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.
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Old 02-05-2012, 11:23 AM
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Re: 89 or >91 octane gas?

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.
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Old 02-05-2012, 11:31 AM
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Re: 89 or >91 octane gas?

Quote:
Originally Posted by Walt View Post
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.


Who was the member here that quoted "I rather get herpes than drive a Prius?"
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Old 02-06-2012, 02:51 AM
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Re: 89 or >91 octane gas?

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Who was the member here that quoted "I rather get herpes than drive a Prius?"
[/COLOR]
I had to ride in the back of a Prius taxi in California. BOY it made me miss my Grand! I was eating my knees the entire time. I swore then that I would never buy one, no matter what!
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