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  #1  
Old 01-24-2014, 12:42 PM
Jkanebell Jkanebell is offline
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Default 91 oct

I recently made the jump from 89 to 91. Ive noticed a MPG decrease. Is that normal? I may add that is is 10 degrees here in Kansas so that may also contribute. Just wondering if anyone else has noticed the MPG differnce. On the other hand - the truck hauls serious ace now.. even with 35x12.5 MT's and a 3 inch lift.

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Old 01-26-2014, 12:06 AM
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You're getting a hotter burn in your fuel, so you're gonna have more HP, which doesn't necessarily mean better mpg. Could be your driving habits, too. You're noticing the fun factor going up when the skinny pedal goes down. And when the fun factor goes up, the mpg usually goes down.
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Old 01-26-2014, 08:33 AM
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[QUOTE=RogerRamJet;1308500]You're getting a hotter burn in your fuel, so you're gonna have more HP,


Curious where you got that info from.
My understanding is there's less energy the higher the octane,as in less heat output or BTU's from a gallon of gas.
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Old 01-27-2014, 12:06 AM
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[QUOTE=Hornet;1308655]
Quote:
Originally Posted by RogerRamJet View Post
You're getting a hotter burn in your fuel, so you're gonna have more HP,


Curious where you got that info from.
My understanding is there's less energy the higher the octane,as in less heat output or BTU's from a gallon of gas.
Less energy in higher octane, eh? So, when the NASCAR Cup series cars put out 850 horses, it's not because their running 110 octane in their tanks? Yeah, yeah, I know. For all of you motor heads out there, it's the compression ratio. But the higher the octane, the better the fuel will compress, giving you a hotter burn and a bigger boom.
Study up on what octane is, Hornet. In this case, bigger is better.
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Old 01-27-2014, 07:07 AM
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I have,i think you better do some reading lad.
Um how does an 850 horse Nascar engine compare to a normal street engine,two totally differant engines,with differant fuel requirements by a long shot.
Don't be dumb enough to try using those 2 engine classes as a comparision,not gonna work
The lower the octane,the more complete the burn,hence more power output from a gallon.
There is no "Boom" as you say,it's a "Burn".
If you have a "boom" you've got pre-ignition,that'll wipe out ring lands pretty fast.
Higher octane fuel has been proven to cause "carbon" build-ups too.
The lower the octane you can get away with the better

Maybe dig out some articles backing up your claims,i'm always up for some reading,and not adverse to changing my mindset,so prove to me your claims.

Last edited by Hornet; 01-27-2014 at 07:15 AM.
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Old 01-28-2014, 12:07 AM
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Congratulations, you just answered the OP's original question. Well done, sir.

Too time consuming to argue this via a keyboard. Let's just say the OP got his answer as to why his MPG went down with higher octane and leave our discussion for another day, another place.

Reps to you Hornet, for a great discussion and answer.
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Old 01-28-2014, 09:52 AM
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I've had it happen before also. Usually happens in the winter it seems like, near as I can tell. Normally I get better though, atleast when it's above freezing. I'm not positive, but I've been told before that part of it is the truck ECM "learning" how you drive again when you've changed a factor. Give it a couple tanks before you do your math and see how it's going then.

For octane, again this is the way I understand it. The higher the octane, the higher the potential in each molecule. It also burns more consistently because it requires more heat to combust but it actually burns cooler once combusted, thus giving a more complete and controlled burn. The more input required from the electrical spark is why the higher octanes are more resistant to a pre-ignition.
Hope that helps some.
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Old 01-28-2014, 11:09 PM
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Quote:
Originally Posted by THasse View Post
I've had it happen before also. Usually happens in the winter it seems like, near as I can tell. Normally I get better though, atleast when it's above freezing. I'm not positive, but I've been told before that part of it is the truck ECM "learning" how you drive again when you've changed a factor. Give it a couple tanks before you do your math and see how it's going then.

For octane, again this is the way I understand it. The higher the octane, the higher the potential in each molecule. It also burns more consistently because it requires more heat to combust but it actually burns cooler once combusted, thus giving a more complete and controlled burn. The more input required from the electrical spark is why the higher octanes are more resistant to a pre-ignition.
Hope that helps some.
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Great post, T!
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Old 03-03-2014, 10:29 AM
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Octane deals with how much compression/heat the fuel will take prior to pre-ignition. The higher the octane rating the more you can compress it. The reason that when people use tuners they have to go to higher octane is because they lean out the air fuel mixture and advance timing to increase power, a lean tune is a powerful tune. In stock form the 5.7 hemi is set to run 87-89 octane. Combustion temp are not hot enough to reliably burn all the fuel in the engine when higher octane fuel is used. That is why the owners manual states that 91 octane is not recommended.
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Old 03-03-2014, 07:47 PM
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I will second this ^^^, Unless you are running a Tuner with a 91 or 93 Octane tune I would not use 91 Octane, I would also try to stay away from fuels that have ethanol in them if at all possible, if you kind find a plain old school unleaded 87 that would be best for the truck, however if you get a tuner and a few bolt on Mods I would run a 91 or 93 Octane fuel with a 91 Octane tune, I know a lot of people swear by the 93 octane tunes, however I have seen pretty high KR's with the 93 tunes on 93 Octane especially the winter blends. And if you are not running a catch can I would not even dare try to do a 93 tune, you might be able to get away with a 91 Tune but still better if you have a catch can to reduce the contaminates introduced into the fuel.

If you have any other questions or would like to get a tuner so that you can effectively run your higher octane fuel feel free to reach out to us via PM or posting in our Vendor Section, you can also email us directly at sales@bullysperformance.com or call us toll free at 855-432-FAST we are always more than willing to help out forum Members, just remember if you contact us off of the boards to mention you are a Z member to insure you get the best discounts

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