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Old 01-05-2008, 01:20 PM   #1
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Octane Question

Hello all,

A buddy that I go crawling and mudding with puts 91 octane in his truck.

I always put 87 in all of my cars. He says it is bad for my injectors but I just smirk and show him my stroking hand gesture.

For the 4 and 6 cylinder, will 87 suffice? I'm good with it. Some say that 91 can even harm the engine.

O well. Tomato...................Toe mah toe.

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Old 01-05-2008, 01:27 PM   #2
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I believe higher octane is required only in higher compression performance oriented motors. The jeep is definatly not that! Dirty tank and or fuel is what gets to the injectors not octane. Most name brand distributors of gasoline put additives in gas to help keep injectors and fuel systems clean. All i run is 87 octane in the jeep. Runs perfect. Occasionally like every oil change i ad sea foam to the tank.

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Old 01-05-2008, 01:29 PM   #3
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i cant give you an answer to that, but i run 87 all the time except for when i wheel. i always fill up with 91 the exit before the park im headed to. im also wondering this...hopefully someone has a firm answer...free bump anyway.
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Old 01-05-2008, 02:00 PM   #4
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91 is useless in a stock (engine) wrangler.. 91 should only be used if modified (advanced timing) or boosted engines. Maybe if it pings in very cold weather you can go with 89 or if you have to 91 but 87 should be perfect in a wrangler with 8:1 Compression ratio. The lower the ratio the less of a chance you will get ping (premature combustion) so you can run lower octane fuel.

my rig sees 87 fuel only and it runs great.
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Old 01-05-2008, 02:02 PM   #5
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if you get anythign higher thatn 87 you're only wasting you're money in these engines
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Old 01-05-2008, 02:09 PM   #6
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Octane has nothing to do with injectors, the injectors have no idea what octane fuel is passing through them.

Octane's sole purpose is to control the gasoline's burn rate and how much compression it can withstand before self-detonating from excessive compression. Octane itself does not add power to gasoline, neither does it help the engine run cleaner. High compression engines require more octane because they compress the fuel more, so more octane is needed to prevent the gasoline from prematurely igniting on its own from the high performance engine's extra-high compression ratio. Premature ignition is what causes the pinging or knocking that is audible when running the engine under a heavy load or if you're running too low of an octane for the engine's design.

In fact, the higher the octane, the more difficult it is to ignite and the slower it burns. In fact, running too high of an octane in an engine can conceivably leave deposits behind.

The only time an engine designed for 87 octane will require a higher octane is when something is wrong with the engine and a higher octane is required to get rid of pinging. That is usually from nothing more than a heavy load of carbon that has built up inside the combustion chambers... which is usually caused by driving the engine too conservatively. Excessive carbon can often be "blown" out of a carboned-up engine by driving extra aggressively for a few days. I.E. accelerating hard to high RPMs which will often burn off/blow out the carbon that has been allowed to accumulate inside the combustion chambers.

If your engine was designed for 87 octane and it isn't pinging, then there is absolutely no... none, zero, nada, benefit to running a higher octane. It won't run any better or faster, nor will it get better mileage.

The only benefit seen by running higher-than-required octane fuel is that the gas station and gasoline manufacturer made a little more $$$ off of you than they deserved.

One more thing... a gallon of 87 octane gasoline has exactly the same energy content as a gallon of 93 octane gasoline. Again, octane is not power, it only helps the gasoline withstand being compressed without igniting prematurely from the compression of the air fuel mixture.

Your buddy is just part of the majority of people that have no real idea what octane is and what its purpose is.
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Old 01-05-2008, 02:30 PM   #7
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Quote:
Originally Posted by Jerry Bransford View Post
One more thing... a gallon of 87 octane gasoline has exactly the same energy content as a gallon of 93 octane gasoline.
yep.. as he said 93 will burn slower so the unburnt fuel will go into the trash decreasing the efficiency of your Jeep and what do we know about less efficient engines?

I like how Jerry said that about carbon build up.. i remember a saying from when i used to own my miata.. A redline a day keeps the carbon away hehe
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Old 01-05-2008, 09:49 PM   #8
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ahh the good ol days when you used to push the same whip as my 3rd grade teacher. do us all a favor and stop bringing up that piece.
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Old 01-06-2008, 12:54 PM   #9
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ahh the good ol days when you used to push the same whip as my 3rd grade teacher. do us all a favor and stop bringing up that piece.
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Old 01-06-2008, 01:01 PM   #10
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Old 01-06-2008, 04:27 PM   #11
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Times 2 what jerryb said, the only difference between cheap 87 and expensive 87 is the mojo they put in it, all 87 comes from the same pipeline to the same terminal where the trucks pick it up. They add their brands mojo juice and it mixes when they dump the gas into the tanker. The cheaper 87 may not have the best additives but all gas is required by federal law to have a minimum cleaning compound. I dump a 20oz bottle of Chevron Techron fuel SYSTEM cleaner in my jeeps every 3,000 or so miles, or Valvoline. If you want cheap, dump 1-2 ounces of Acetone in every couple of tanks, it's a good varnish remover. You can get the stuff for cheap at home depot by the gallon or of you are really cheap sneak some of your moms, wifes or girlfriends fingernail polish remover, thats acetone too Or you might even be able to score some if there are any TV's in your family
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Old 01-06-2008, 05:17 PM   #12
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there are two different definitions of that.

the US OCTANE is = ( RON [research octane number] + MON [motor octane number] ) / 2

the european OKTAN = RON [research octane number]


p.e.
our european cheapest normal fuel has 91 OKTAN = 91 RON
the MON of this fuel is 82

in the US our 91 OKTAN fuel = ( 91 [RON] + 82 [MON] ) / 2 = 86.5 OCTANE ≈ 87 OCTANE.

most people here in europe don´t know that and use low quality fuel in their US cars.
p.e. a granny SRT-8 that needs 94 OCTANE (says the US manual) needs 98 OKTAN here in europe.
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Old 01-06-2008, 05:28 PM   #13
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All Else Fails Look In Your Owners Manual It Tells You To Use 87 Octane .............
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Old 01-06-2008, 06:17 PM   #14
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Jerry Bransford's post is right on the money. I've been a mechanic for 30 years and couldn't have said it better myself. Octane is the measure of a fuels inability to burn. In other words, it won't ignite easily in a high temperature situation like the combustion chamber of a high compression engine. Thats all !
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Old 01-06-2008, 07:02 PM   #15
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One thing to add as far as clogging injectors,, no matter what octane pump gas has junk in it, that is what clogs injectors. That is just how it comes from the refinery, of course some station's storage tanks make things even worse. The only way to get away from that would be to run very expensive race gas. Although I don't think I've ever seen race gas under 90 octane.
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Old 01-06-2008, 07:27 PM   #16
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Or you might even be able to score some if there are any TV's in your family.
We have 4 TV's in our house. I didn't know they ran on acetone!!!

Is that anything like blinker fluid?
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Old 01-06-2008, 07:32 PM   #17
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At the price of a gallon, who would want to run anything higher then 87.

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