I've been running 89 octane QuickTrip gas since I first got this truck, thinking the computer would have to do something to compensate for the longer / slower burning fuel, but it seems that most run 87 still. Am I just wasting my money? How is the longer burning fuel handled then? Is 89 of a higher quality / cleaner? Is QuickTrip a good place to get fuel? What about Texaco or Phillips 66? Thanks!!!

-AR

 

Higher octane fuel is not really longer / slower burning... although folks sometimes call it that. It really is terminology. You be the judge.

Gasoline is made of several different hydrocarbon chains. There are 5 carbon chains (pentane), 6 carbon chains (hexane), 7 carbon chains (heptane), 8 carbon chains (octane), plus a smattering of longer chain molecules. By far, the most prevalent molecule is heptane. Attached to the carbon chain are atoms of hydrogen and carbon, plus suplhur, nitrogen, and in the case of reformulated gas, oxygen.

Gasoline burning is a two step process. First, the long chains must be broken into shorter chains, because the long chains cannot combine with oxygen. It is heat that causes the long chains to break down. Next the shorter chains break up into individual atoms and combine with oxygen... this is the actual combustion process.

Now, it so happens that heptane chains (and pentane and hexane) break up at a fairly low temperature, while one specific kind of octane molecule breaks up at a much higher temperature. This is the reason higher octane (larger percentage) prevents knocking... it takes a higher temperature for the molecule to break down into the short chains that actually do the burning. Or, to put it another way, it takes less energy (temperature) to break the bonds on normal heptane than it does for iso-octane. Thus, higher octane gasoline can put up with higher temperatures before it pre detonates or pings than can heptane.

But, as we know, refiners don't actually put more octane into gasoline to boost the octane rating because it costs too much. Instead, they add other stuff like lead, or for unleaded gasoline, they add MTBE, ETBE, or ethanol.

These additives have a higher octane rating than gasoline, but that is not how they work. What they do is retard the breakdown of the long chains into short chains. It takes more time for the long chains to break down into short chains, and thus, the gasoline behaves more like high octane gas.

Note however, that once the chains are broken down, the actual rate of combustion is the same as though no additives were in the gasoline. The flame front, once ignited moves every bit as fast as gasoline with no additives in it.

So, you tell me. If the additives like MTBE, delay the breakdown of the long chain molecules into short chains, but when they do break down, the gasoline burns at exactly the same rate, does premium burn slower?

My answer would be no. It burns at the same rate, but it takes just a bit longer to get around to burning, just enough to prevent pre detonation and knocking.

 

...interesting. Well, are their any advantages to the 89 over 87?

-AR

 

It used to be that only the higher octane brands had detergents and engine cleaners but the EPA has mandated that all grades have to do that now, so no real difference there.

In theory, you magical engine computer's knock sensor should advance the timing more with 89 than 87 because 89 should be more resistant to pre detonation. And, advanced timing should give you slightly better power and gas mileage.

I have not used premium in my truck, but I can tell you that on my motorcycle, advancing the timing 6 degrees and running premium gave me 2 mpg more per gallon, about a 1 HP increase, and about 3 lbs-ft of torque increase.

I'd imagine those folks who have "hopped up" their trucks could tell you more about mileage and power increases.

 

89 octane is 66% 87 octane and 34% 93 octane (some pipe lines
use 65% and 35%) so there's a good chance you may just get 87 octane or 93 octane when you buy 89 octane. as far a better gas, ALL 87 and 93 gas except amaco white 93 octane comes out of the same pipe per octane levels. (one tank 87 % /one 93%
/one 93% amaco white/ect the additives are what make them different. BUT less then 1 gal. of additive per 9000 gal of gas, so can you really tell the diff. ? things that will make you go hmmmmm !!!????

 

??? Huh? When used as an octane booster, there is about 1 cup of MTBE per gallon of gasoline, and about 2 cups per gallon of ethanol. A lot more than 1 gallon in 9000.

 

BeastRider,
i'm not talking about a octane booster, i'm talking about when i load 9000 gals of gas/straight out of the terminal to take to a station , farm ect.../ the additive is less than 1 gal per 9000 gals of gas. (additive- what makes bp branded from unbranded and shell etc...) all i can tell you is what is on the print out when i load. your talking refinery stuff long before i get it, witch my statement is still true, even where some of you run the moonshine/ corn gas /we don't have it in the south. us rednecks would try to drink it . make since now?

i don't cook it up, i just haul it

 

Garebel... gotcha... you're talking about all the other goodies that they put in to keep engines cleaner and to convince us the "high tech techroline" is better than "system 3".

 

OK.
This is all good. Since I have owned the my truck I have used 89 octane. 2 months ago I decided to try 87 after all of the discussion here about it being better etc. I have kept my mileage religiously and know my MPG has dropped since the change. The only real change I have noticed is more water purging from the weep hole in the exhaust while at idle. Bottom line people here often claim better mileage from lower octane rating due to higher energy release from the breaking of the atomic bonds blah,blah,blah. It all seemed reasonable from what I remember from organic chemistry years ago. From what I have seen in practice is when burning No. 2 diesel fuel compared to say No.4 Diesel, you get much more heat per gallon from the less refined grade. So by changing only the fuel do you get better mileage from 89 or 87 octane. My records would indicate 89 is better for my truck. (I have not kept track of miles per dollar to see if the cost is a wash or not)

 

bervinator ,
i would go with 93 octane if i raced, or money wasn't a factor, ect...
i've got better gas milage with 93 octane , but it's cheaper in the long run to stay with 87 octane the very few times i've swapped over, imo . i was just saying 89 was iffy to me because of the likely hood of just getting 87% or 88% . then you might be lucky and get 89% up to 93%. confusing i know, it blows folks mind to find out theres no such thing as 89 octane from the refinery it's blended later.

 

I'm with GArebel... you can get somewhat better mileage out of higher octane gas, if you have a modern late model engine with anti knock sensor, but the percentage increase in the mileage you get is less than the percentage increase of the cost of premium, so you end up paying more.

 

But how does the computer know what octane gas you're running? I was always under the impression that the computer has a "set" figure for the timing, say 12 degrees BTDC (no idea what these motors are set to, just what my wifes 4 banger timing is......). Then, if the knock sensor hears "knocking", it'll just keep retarding the timing until it stops knocking and then advance back to it's "set" timing.

If you are saying the computer just keeps advancing until it pings, then backs off, I can see how the higher octane would help. Just for the record, I just run good ol' Texaco 87 octane.

Like I said, I don't know. Still trying to get used to these "new fangled computer controlled has a sensor for everything motors". I come from the days of carbs, chokes, points and having to set the dwell!!

 

I'll agree with that. The only reason to run higher octane is if you've got a lot of pinging happening. Even then, it's time for a carbon cleaner, plugs, wires, rotor, cap, etc.

Back in the old days of suped up VW Beetles we were always using Chevron94 because maximum timing advance was over 35 degrees and 87 would ping like hell.

Theoretically, yes you do get more power, better gas mileage... but only if your ignition computer takes advantage of it and ups the timing, otherwise no gain.

In Europe most of the smaller cars run 10:1 compression ratio or higher and are setup factory for "super" gasoline. You can run regular unleaded but there's a power and mpg drop only because the computer is setup for premium.

Now if you have a "chip" installed you'll have to run more premium fuel because the chips advance the timing somewhat.

That's one of the reasons I refuse to put a chip in and will look for more power simply from intake/exhaust mods. Not only will the chip cost money but the premium fuel cost will add up over a year.

 

Having tried it and done the math, my truck gets better mileage with higher octane. I've never tried the 89. I normally use the 87 or 93.

As for performance, the claims that higher octance fuel will reduce performance seem out of line to me. I've seen dyno results with a 351 powered truck picking up several HP when switched from regular to premium fuel. My truck idles better, runs a little smoother and gets better mileage with premium.

It's not enough to be cost effective. Right now premium is 20% more in my area. Like many others, I have found my increases other than with a chip. IMO one of the least cost effective mods over time. Figure 20% of your fuel cost and add it over 4 years, then add it to the initial cost. Granted the other improvements are good. But if you're really looking for best "bang for the buck" then get a chip that doesn't require premium and pick up the HP elsewhere.

 

You got it. Somewhat more complicated than that since the engine management computer looks at O2 levels, temperatures, knock sensor, and a bunch more to set both timing and the amount of gas injected into the cylinder.