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post #1 of (permalink) Old 12-27-2000, 07:10 PM
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When do you need to use premium gasoline?

Happy Holidays to all Jeep folks around the world!

This forum is running a little slow, so I'll ask a question that has been puzzling me for many years, and no one has ever given me an answer that I believe or can understand. Maybe it was correctly explained to me, but I simply failed to understand. Or, maybe the person explaining didn't really know the answer either, and tried to BS through it. I'm certain the members of this forum can help me out with some real information..

At the gas pumps, there is "Regular", "Medium Grade", and "Premium" choices. Occasionally, there are even other choices. I have never understood when you need to use premium vs. regular gas, much less intermediate grades. In the past, I have tried premium vs. regular in several cars, but have never noticed a difference.

I've heard that the octane is different, as if that is supposed to answer everything. I have even noticed octane numbers posted on the pumps. I am an electrical engineer, and being told "the octane is different" doesn't really fill in the blanks for me. I need some real basic help on this one, so go easy on me with the answers.

So what is the answer? When is premium appropriate? What about mid-grade gas? Is it all a marketing scam (I don't really believe it's a scam...just kind of a rhetorical question)? Does the grade really make a difference, and if so, what is it? Is is a matter of what engine you are running, or is it a climate/altitude thing?

This is not a bogus question. I would really appreciate any help understanding what this is all about.

Thanks for the help.


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post #2 of (permalink) Old 12-27-2000, 07:34 PM
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Re: When do you need to use premium gasoline?

Howdy !
IMO , for me it`s been engine specific . I had a 4.3 Chev that was very fussy about fuel type and my last 4x4 was a 92 Z71 w/350 . I could just about pour anything closely related to a deceased dinasaur in the tank and the engine never cared. My back & forth to work ride is a Prelude and it doesn`t seem to know the diff between grades either.
My friend bought a Motorhome last fall and the PREVIOUS owner LOUDLY proclaimed that premium was the way to go , he said there was more power going up hills etc.. On a couple of the fishing trips we used it for , we put premium in one tank and reg in the other , switched back and forth up the long hills and never noticed any difference at all . It is a 460 Ford engine . As a kid I owned a 4 cyl. dual carbed little white EPIC GT . That thing wouldn`t even give me the satisfaction of running without premium fuel . So , there is my experience with fuel . Personally I think it`s a waste of money for the added cost . Maybe others have a different opinion .


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post #3 of (permalink) Old 12-27-2000, 07:41 PM
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Re: When do you need to use premium gasoline?

As far as pump gas goes the explanation is very simple. The higher the octane rating the more reactive the fuel. These higher octane ratings also allow for better performance from the same motor by producing slightly more power and allowing it to run more effiecently by combusting at a faster rate. The primary reason for the higher grade fuels usually has to do with the type of engine. Higher performance motors generally run higher octane fuels to achieve both great performance and power. generally sports cars and the like will run 93 octane because the fuels are cleaner burning and those engines are generally designed to be run with the fuels. Ole jeeps like ours arean't really going to get huge advantages from higher octane fuels because most are designed to run on cheap 87 or 89 octane gas and the benefits from higher octane gas don't outweigh the small benefits.

Generally speaking though, higher octane will equal greater power, more effiecntly and cleanly burned fuel, and overall better performance in a motor.

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post #4 of (permalink) Old 12-27-2000, 07:53 PM
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Re: When do you need to use premium gasoline?

Most of my knowledge is based on practical application, with a little from my skoolin.

In skool, we had Chevy 350 engines we could vary the compression, stroke, fuel/air, and basically every other parameter you could imagine (it was a VERY good school). What we found was a stock Chevy engine, simulating under 100K on it, ran best and gave its peak performance with 87 octane. Economy and performance actually dropped with higher octane fuels. However, as compression ratio increased, the 87 octane no longer performed well and the higher grades were required for performance. The same with an engine indicative of a lot of wear. The lower grades created a "knock" and had very little "performance" (hp and torque). The higher grades regained some of the lost power, but not very much. It did stop the "knock", however.

I'm looking to TR to get his input on this. My "knowledge" is about 15 years old now, and growing older every minute.

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post #5 of (permalink) Old 12-27-2000, 08:32 PM
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Re: When do you need to use premium gasoline?

from my understanding.. and that is about what it is.. is the higher the octane.. the slower the burn.. (not sure if that is the right word) therefore giving you more oomph (my technical term)... and like said.. in an old jeep engine.. you wont notice the difference.. but in todays "sports" cars.. if you will.. little rice burners that run at higher rpms.. benefit from a higher octane..
i did notice a difference in my toyota.. had the v6.. and for long highway runs.. i could get 2-4 mpg more out of it using the higher octane.. but around town.. it didnt do anything..
and im sure TR or cjdave can give us the correct technical termology and better clearfy it for us.. backyard boys..


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post #6 of (permalink) Old 12-27-2000, 09:01 PM
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Re: When do you need to use premium gasoline?

Keep in mind that when I say higher octane I am mainly talking about the cam2 fuels which are around 112 octane but it will make a good exagerated look at the differences. One thing about higher octane is the fact it is slightly more refined and that gets rid of the stuff that causes the carbon buildup. The main thing about high octane is it has a higher flash point or it actually takes a more powerful spark to ignite. For instance When you raise the compression the low octane fuels will want to ignite and that causes ping. The higher heat will also ignite the lower octane fuels easier. This is why when you have carbon buildup and it is hot and glowing in the combustion chamber it will preignite and causes a ping. When you put high octane in that engine it might be just enough to hold the fuel igniting until the spark from the spark plug like it is supposed to. In race engines it is obvious when you need to step up and in the two stroke bikes it is even more obvious. If you really up the compression in a 250 two stroke you will have to run high octane or it will ping and jump all over the place. As far as our every day cars go I beleive a stock low compression engine will not benefit much other than cleaner burning fuel. The smaller 4 cylinders that are usually higher compression might be the reason for a slight improvement in those. I do know that a stock small engine like a go cart will actually loose a little power on higher octane fuels because the higher flash point makes it more dificult to ignite. This is just my experience with higher octane by being around drag racing and the dirt bike circuit so I might be corrected here.

post #7 of (permalink) Old 12-27-2000, 09:19 PM
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Re: When do you need to use premium gasoline?

The higher octane fuels are design for higher compression motor. The motor that have been design from the early 80's will not benefit from higher octane fuels. In fact worst mpg will occur due to unburn fuel being pass. This would affect the washing of the clyinder walls and failure of EPA emission.Today motors are perfect happy with the lower octane fuels.

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post #8 of (permalink) Old 12-30-2000, 08:05 PM
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Re: When do you need to use premium gasoline?

C'mon TeamRush and CJDave, I'd really like to hear your input on this one.



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