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Old 04-21-2013, 10:31 AM
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Mixing gas?

Hey guys,


Just wanted to check with y'all before I do anything. The guy at the shop where I got my oil changed said if I use premium on long road trips that my mileage will be noticeably better. I understand the logic behind this, but isn't it frowned upon to switch between regular and premium?

I always thought of it kind of like feeding a dog. If you're gonna change his food, do it gradually or at least do it on empty and commit to it.
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Old 04-21-2013, 10:52 AM
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Re: Mixing gas?

Quote:
Originally Posted by generals1992 View Post
Hey guys,


Just wanted to check with y'all before I do anything. The guy at the shop where I got my oil changed said if I use premium on long road trips that my mileage will be noticeably better. I understand the logic behind this, but isn't it frowned upon to switch between regular and premium?

I always thought of it kind of like feeding a dog. If you're gonna change his food, do it gradually or at least do it on empty and commit to it.
Octane is probably one of the least-well-understood things about a car that people need to deal with on a daily basis. It's also a good way to start a flame war.

I'll tell you what (I think) I know, and invite you to do your own research.

There is a common perception that high octane fuels have "more energy" or "perform better". But in reality the only thing that octane rates is how likely it is going to spontaneously ignite under pressure. Lower octane fuels will spontaneously ignite (without the spark from the plugs) at lower levels of compression than higher octane fuels. If you have a high compression engine and you put low octane fuel in, the fuel will likely ignite before the spark, which causes "engine knock" and will reduce performance.

However if you don't have a high compression engine, putting in higher octane fuel will do nothing other than drain your wallet faster.

So what you need to do is pair the fuel octane with the compression ratio of your engine. This is usually well-described by the manufacturer when they indicate what octane fuel they recommend using. If the manufacturer recommends 87 then you should use 87. Using 89 or 92 is not going to give you any better performance because your engine was designed so that it should not pre-ignite when running 87 fuel.

The 2014 JGC owner's manual indicates that the V6 should use 87 and the V8 should use 89. The manual also basically reiterates what I stated above by stating:
Quote:
The manufacturer
recommends the use of 89 octane for optimum performance.
The use of premium gasoline is not recommended,
as it will not provide any benefit over regular gasoline in
these engines
(the above quote was specifically for the 5.7V8, not for the V6)
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Old 04-21-2013, 11:01 AM
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Re: Mixing gas?

Oh, and as for mixing fuels, I don't think there really should be an issue as long as you realize that your "actual" octane rating will be based on the weighted average of the fuels and levels you put in. And if you're only putting a little in, it may take a bit for the fuel to fully mix, though if you're only putting a little in, it's not likely to have much of an impact anyway.

Most gas stations actually blend fuels normally. If they provide 3 levels of octane, they'll have one storage tank for the premium, one for regular, and they'll mix they two to get the mid-grade.
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Old 04-21-2013, 11:38 AM
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Re: Mixing gas?

Quote:
Originally Posted by nautiboy View Post
Octane is probably one of the least-well-understood things about a car that people need to deal with on a daily basis. It's also a good way to start a flame war.

I'll tell you what (I think) I know, and invite you to do your own research.

There is a common perception that high octane fuels have "more energy" or "perform better". But in reality the only thing that octane rates is how likely it is going to spontaneously ignite under pressure. Lower octane fuels will spontaneously ignite (without the spark from the plugs) at lower levels of compression than higher octane fuels. If you have a high compression engine and you put low octane fuel in, the fuel will likely ignite before the spark, which causes "engine knock" and will reduce performance.

However if you don't have a high compression engine, putting in higher octane fuel will do nothing other than drain your wallet faster.

So what you need to do is pair the fuel octane with the compression ratio of your engine. This is usually well-described by the manufacturer when they indicate what octane fuel they recommend using. If the manufacturer recommends 87 then you should use 87. Using 89 or 92 is not going to give you any better performance because your engine was designed so that it should not pre-ignite when running 87 fuel.

The 2014 JGC owner's manual indicates that the V6 should use 87 and the V8 should use 89. The manual also basically reiterates what I stated above by stating:


(the above quote was specifically for the 5.7V8, not for the V6)
Well said

Some of Jeeps literature claims the V8 can use 87 but they recommend 89. In this case the 89 will probably give you slightly better mileage. The reason is that the computer can adjust the timing to compensate for the lower octane fuel. The vehicle will run fine, but less efficiently with the retarded timing.

Anything over 89 in the V8 or 87 in the V6 is a win for the gas station and their suppliers.

Mixing octanes a little at a time or all at once has no adverse effects on the engine. Just don't mix any diesel fuel in there.
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Old 04-21-2013, 03:11 PM
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Re: Mixing gas?

Do all (or most) gas stations sell 89 Octane ? (I will have to start checking).

OK....so lets say I was using 89 for the first 3 months in the V8 Engine and then while on a trip (and low on gas) I pull into a station that only has 87. In that instance is it OK to Fill up with 87 for just that one tank full, and then go back to the 89 after that ?

Will that 1 tank of 87 hurt anything ?
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Old 04-21-2013, 03:24 PM
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Re: Mixing gas?

Quote:
Originally Posted by Beski View Post
Do all (or most) gas stations sell 89 Octane ? (I will have to start checking).

OK....so lets say I was using 89 for the first 3 months in the V8 Engine and then while on a trip (and low on gas) I pull into a station that only has 87. In that instance is it OK to Fill up with 87 for just that one tank full, and then go back to the 89 after that ?

Will that 1 tank of 87 hurt anything ?
No problem. You can run it on 87 all the time without hurting anything. You will lose a little power and a little mileage. That's why the owner's manual says it can run on 87, but they recommend 89. Otherwise they would just say use 89.

As to whether or not all stations have it is another question. My MDX said minimum 91, but around here it jumps from 89 to 93, so I was stuck using 93. I know some people were putting half 89 and half 93. I just don't count my pennies that closely.


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Old 04-21-2013, 03:27 PM
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Re: Mixing gas?

Quote:
Originally Posted by generals1992 View Post
Just wanted to check with y'all before I do anything. The guy at the shop where I got my oil changed said if I use premium on long road trips that my mileage will be noticeably better.
He's wrong.

But yes you can mix different octanes. Just run the octane the car is designed for though, higher octane on a vehicle that does not require it is a waste of money.
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Old 04-21-2013, 05:17 PM
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Re: Mixing gas?

Colorado gas stations usually only sell 85, 87, and 91 octane gas. So I will mix 87 and 91 to get the recommended 89.
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Old 04-21-2013, 06:26 PM
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Re: Mixing gas?

Oy und veh.
1) Octane has no relation to energy per gallon.
2) Octane is how fast the gas will burn (flame propagation rate) which relates to spontaneous combustion pressure (where it knocks)
3) Higher octane gas burns slower than low and needs more advance. In the bad old daze you bumped the distrbuter a bit when switiching from regular to premium. If a computer does not have the advance maps for premium, it can't advance far enough.
4) (more complicated). Detonation/knock/spontaneous combustion is a function of "maximun effective chamber pressure" (MECP). In a normally aspirated (no boost) engine in Denver the air pressure is lower than in Orlando and the flame propagation rate drops. In a nutshell at 5,000 feet 85 octane acts like 87 octane at sea level.
5) (Personal Opinion) On a long trip at cruise with a light load & pulling a reasonable manifold vaccuum, you can save some money by running a lower grade gas than around town. This is where I would fill a Hemi with 87.
6 (PO) It also helps to run the engine about 10% cooler than stock (under 200F).
7) E85 is around 102 octane so a flex-fuel engine can run on almost anything unlike an engine designed for gasoline only.

ps I have this nut theory about detonation, temperature, and cyl head problems. Be interesting to see if the incidence kicks up as warm weather returns.

Got that ?
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