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Old 03-19-2012, 09:10 AM
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What grade of gas are you usin(g and do you feel the diff?

Just wondering what is recommended for our hemi overland out there? What do you guys use and recommend and do you feel a difference with the grades 87 89 93? Thanks
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Old 03-19-2012, 09:26 AM
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Re: What grade of gas are you usin(g and do you feel the diff?

89 is recommended in the manual and specs for the HEMI
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Old 03-19-2012, 09:29 AM
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Re: What grade of gas are you usin(g and do you feel the diff?

I have a Hemi in my Magnum and e manual says use 89, but it says NOT to use 93. There have been time I have used 87 and I feel the difference. The car seemed to not have that push. It's still fast, but the car drags and doesn't have that speedy acceleration.
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Old 03-19-2012, 11:50 AM
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Re: What grade of gas are you usin(g and do you feel the diff?

Now only 87 octane.
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Old 03-19-2012, 12:35 PM
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Re: What grade of gas are you usin(g and do you feel the diff?

OP, did you not get a owners manual with your Jeep? The Hemi runs on 89 octane mid grade, and the V6 runs on 87 octane regular. Running premium fuel in either engine is NOT recommended. Use the fuel that the manual says to use!
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Old 03-19-2012, 12:39 PM
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Re: What grade of gas are you usin(g and do you feel the diff?

Suspect if you monitor the advance on the hemi you will see more total with 89 than 87. Maybe an IMAP difference also. . BTW my OM (5th) says that you may run 87 in a Hemi but 89 is recommended.

Gas is gas. All grades have about the same BTU content. The difference is that a higher octane (PON is R+M/2) gasoline is harder to autoignight (detonate) and burns slower.

This means that you can squeze it tighter before it knocks but also need to dial in more advance to compensate for the slower burning. If the computer maps are optomised for 89 PON you may lose power on 93 because the fire is retarded from optimum.

Back in the day a change to premium was accompnied by advancing the timing about 4 degrees on a SBC. That is difficult to do here without flashing.

Am interesting experiment might be to tell a flex fuel vehicle that 93 octane is really high percentage ethanol which also burns slower.
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Old 03-19-2012, 02:53 PM
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Re: What grade of gas are you usin(g and do you feel the diff?

What happens if 93 is in the tank? Besides wasting money? Any issues with the MDS or Hemi engine?
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Old 03-19-2012, 03:10 PM
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Re: What grade of gas are you usin(g and do you feel the diff?

I always buy the cheap stuff for my hemi. I get mostly 9 mpg on all city driving so I figure paying more for 89 wont make much difference in the mpg. It's a hemi, I don't see why it can't handle 87.
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Old 03-19-2012, 07:41 PM
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Re: What grade of gas are you usin(g and do you feel the diff?

I run premium (91/93) but regular is totally fine, I can't see any ill effects regardless of what gas you run. That said a lot of people will tell you otherwise and have a lengthy explanation of why. If you can afford it I feel that higher octane could help with MPGs and helps the engine make power intern not working as hard. This is just my $.02
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Old 03-19-2012, 10:36 PM
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Re: What grade of gas are you usin(g and do you feel the diff?

Could spend a few hours on octane from Tom Midgley to the Lead company employees who used to wave their ties over knocking engines. Won't.

Just believe it or don't gas is gas and octane has little to do with british thermal units per pound. Too low octane and it is not a good. Too high and are just thowing money out the door.

That said each engine is different. Bore (more than stroke), chamber shape, plug location, MCP (max compression pressure not master control program), all make a difference. With DBW the brane has more contrlol over things than when the nut behind the wheel is connected directly to the butterflies.

Part of what makes high compression on trash gas (87 PON) possible is control of MCP aided by VVT i&e. Makes retard on knock (kludge) mostly unnecessary.

Is one of the reasons why the 3.6 gets such extraordinary MPG in a GALB. (WK2 is getting bettr MPG than my Reattae under the same conditions).

If you really want to know why, read Ricardo, capabilities have changed a lot but the physics have not.

Will say that for my Pentastar 87 is fine. However in the Hemi with the same compression, a quarter inch larger bore, and less efficient chamber (sorry but four of a kind beat even a big pair) design needs a little more flame retardent i.e. 89 PON is advisable around town but on a long road trip on Interstates I'd try 87 PON (or local equivalent, for a non-boosted engine altitude lowers the MCP and why you find 85 PON regular in Colorado.) Your pocketbook will be happier and at cruise the engine will not notice the difference.
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Old 03-19-2012, 10:46 PM
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Re: What grade of gas are you usin(g and do you feel the diff?

It's fact. If your engine give full timing advance with a certain octane fuel, you will see zero difference with a higher octane fuel. It is not a more is better type thing. Octane only helps until there is full timing and no knock. Anything else is wasting money.

In gross simplification, the more advanced the engine timing is, the greater the likelihood of knock (or pre-ignition), but also higher temp combustion chamber temps. Higher temps lead usually to more efficient burning of the air/fuel charge. However this is only up to a point and then you see power reduction and other problems. With high temps and overly advanced timing you also get knock, necessitating the computer to pull the timing back and reducing combustion temps. Like what Padgett said, higher octane is the ability of the fuel to resist the temperature of the combustion chamber in lighting the charge off early, ahead of the (timing) spark plug. You are more likely to have knock when under heavy load and when the ambient outside temperature is high. High air temps lead to slight reductions in radiator cooling and higher gross air intake temperatures that can lead to overall higher combustion temps. Small, but combined with advanced timing, they add up.

So, in the height of summer in Pheonix towing you might find a need for premium 91/93. In a Buffalo blizzard, you can probably get away with 85. Both would probably show the same amount of timing and thus same combustion temps/efficiencies. That means the same power.
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Old 03-20-2012, 08:35 AM
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Re: What grade of gas are you usin(g and do you feel the diff?

Thank you Ashton, suspect sometimes my teaching/engineering background shows. Looks like we disagree only on minor points such as the most efficient peak chamber temperature.

Did leave out the effects of carbon deposits in high milage engines raising octane requirements but then haven't seen a carboned up head for some time. Heck, they don't even include head removal and cleaning instructions in the Owner's Manual any more.
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