Octane

[jeepzj]

I drive a '98 Jeep Grand Cherokee Laredo (ZJ) 6 cylnder (I6) and have been using 89 octane which is only .08 cents less than 93 octane. That's only a $1.60 savings on 20 gallons of gas....not much at all. Can I use 93 octane for my Jeep? Any ill effects, disadvantages, good bad? Will a higher octane gas give better fuel economy or worse?

Thanks!

 

[labman]

I don't think it could hurt mileage. I think most modern engines have knock sensors and are programed to give better performance including mileage with higher octanes. I see trying it as very low risk. After a few tankfuls, decide is you see any difference and decide if it is worth it. If the difference in price stay the same as the price climbs, it may be worth it is you do get better mileage.

 

[SteveD2]

You should be using 87 octane in an I6. That's what it was designed to use. There is no knock sensor in your engine, so using premium will have absolutely no effect. If anything it will cause more carbon buildup and less power.

 

[dkcase]

Unless you have a high-compression engine that requires a higher octane fuel to prevent detonation, you are wasting your money.
If it runs well on the 87, that's all you need.

A friend of a friend visited him with a Boss 429 Mustang. I remember at the time we stopped for gas. He said he could only burn Sunoco 260 or Super Shell. this is circa 1969 or so.

[ April 05, 2004, 10:44 PM: Message edited by: dkcase ]

 

[Ken2]

An older engine that originally used 87 octane gasoline may need higher octane now because of combustion chamber deposits causing ping. The right thing to do is to use an effective combustion chamber cleaner to remove the deposits.


Ken

 

[The_Jeff]

Pump ~60/40 with 87/93 and save a few cents. The station mixes it on the spot from two tanks anyway.

Buy the 93 first, so you get the hose-full of the hi-test when you switch to 87.

This works best when you pay at the pump, where the cashier won't give you a hard time.

 

[Ryan00TJ]

Why would you want to use a higher octane in a 4.0L? Are you experiencing any pinging on 87 octane? If so you need to clean your combustion chambers to reduce the carbon deposits so it won't ping on 87. Running a higher octane in a engine that doesn't require it only hurts your wallet and could cause problems down the road. A higher octane burns cooler and when used in a engine that doesn't require a higher octane, it can lead to combustion chamber deposits. Your 4.0 has an 8.8-1 compression ratio! Stick with 87.

 

[cryptokid]

im sure there are other cars, but the only car i have ever seen get better gas mileage with premium was an 89 jetta dohc 16v 1.8L engine with 10.5:1 compression which enabled it to more than offset the cost of the higher octane fuel. this car got just enough better gas mileage with premium to just barely offset the cost of the extra price for premium compared to regular. it beat the margin by about 1-2 cents a gallon.
the owners manual says that 93 was recomended although anyting from 87 on up could be used with the exception of leadded gas.

there is alot of confusion concerning compression ratios and the octane requirement. the design of the engine plays more of an important role in octane requirement than the actual compression ratio. case in point : toyota prius with 13.5:1 compression requires 87 octane gas, and using anything higher does not increase fuel mileage. how does toyota get away with 13.5:1 compression and only require 87? the answer would be efficent combustion chamber design, which is somthing most cars and trucks on the road today do not have.

so in a nutshell, you really need to test youre engine using different octane rated fuels to find the one which gives the cheapest price per mileage. in my car i get better mpg using a higher octane fuel than 87, but it is not enough of a gain to offset the additional price of a premium fuel, so therefore it is still cheaper to run 87, which is what i do.

 

[rob-the-oil-nut]

I went through the same pro & con. I was told it was a waste and possibly detrimental (or just plain mental)

I bought a steady diet of gasohol and tried a few consecutive tanks of 'real' gasoline and kept rigid records. I ended up getting the best gas mileage after 4-5 consecutive tankfuls of premium gasohol, that was with my now-gone '93 Elantra (1.6 DOHC) although I shrug as to why exactly that should make mileage go up. It was significantly better than the previous year, I kept the records plus I had a mental note that if I ever got that many clicks from a tank of gas, maybe play it safe by not trusting the gas guage

BTW this peak in fuel economy happened in the heat of summer (all jokes aside, we get days / weeks when we are right up there with Texas) and the oil was at least 6 months old. As memory serves, it was RedLine with the by-pass filter.
Hope my input helps someone out there

Rob-the-oil-nut

 

[rob-the-oil-nut]

Oops, forgot to mention, my #1 mechanic explained to me (not that I'm sold on his words) that "premium" gas is a joke and a bit of a hoax. He explained how gas was made and that to make 1000 gallons of "regular" you pretty much just have 1000 galons of regular, but with premium, you need 950 gallons or real gas, and 50 gallons of, ummm, well, his point was it was essentially BSIAB

Namely that the more additives you put in to resist detonation, the less "real gasoline" you are putting in your tank.
If I haven't made my point, think of this: You are armed with doses or double-doses of FP, Redline Fuel System Cleaner, 104+, and a few others. If you are going to add them all to a 20 gallon tank, you sure as heck won't be adding them all to 20 gallons of gas, will you?
I'm not saying this is my point of view, just that he said it, and no one else did on BITOG. Could be 110% true, or misguided (just like many posts... Just cuz it's on a forum, even BITOG, does not mean it's true)
Cheers!
Rob

 

[drm7]

I used to have a Jeep Cherokee 4.0 and it would ping ever so slightly on 87 and only under the right condition. I took it in and they flashed my ECU and it felt like I picked up 10hp but the slight ping was still there. Not severe enought to warrant higher octane and the power was great. I don't think very many cars have the kind of programming that allows them to advance timing for higher octane to increase power. This includes the Jeep 4.0. Most knock sensors are just to retard timing to prevent knock. I think an exception may be the Toyota V8. Don't know about any others.

 

[MNgopher]

On the 4.0 there should be zero need for anything but 87 octane. I know my '88 XJ has a knock sensor, but it does not allow the timing to advance - it only retards it if knocking is detected.

 

[Audi Junkie]

My car instantly runs like crap on 87. I allow it's use on highway trips that comsume more than one tank and if a good price on super is not available. It takes only 10hp-15hp to go steady down the road. I read in a trade publication at my mechanic's that a test was done on random samples of regular gas and (almost) none had detergent levels that met established standards. Premium gas has more and better cleaning addititves than regs. So, it is not all about octane...it's about detergents. Also, to respond to the above statment, just because 93RON burns cooler, does not necessarily mean it leave deposits. Toluene burns slow and cool, but actually offers better performance over regular gasoline, especially in turbos -it allows faster spooling. Where do you think F1 cars get 1000bhp? Toluene, diluted down to meet mandated octane limits, ASTM defination of gasoline is like 10%-20% toluene, add your own and see how it works in your car. Excellent detergent action. BTW- my mechanic testified as an expert against Mobil for his customer in a skunk-gas case and won!

 

[MNgopher]

I think the basic point we are trying to make here is that the 4.0l is a ridiculously tolerant engine on many fronts. One is the type of gasoline. Every other vehicle I have owned has shown some tendencies to knock or ping on a few brands but not on others (and before anyone comments its all the same, note that FOUR different refineries supply this market, and in testing, all show pretty different formulations that meet current regulations). My Jeep Cherokee flat out doesn't care.

The cheapest 87 octane availible should be more than adequate for the 4.0l.

 

[SteveD2]

Yeah, the 4.0 would probably run fine on straight naptha.

Definately a big waste of money to use anything but 87.

 

[GT Mike]

Run some GM Top Engine Cleaner thru your 4.0L if it's pinging on 87. Like mentioned above, there's no need for more octane than 87 in this engine. However, since they are a fairly low compression engine, and don't biuld up a lot of heat in their combustion process, carbon buildup is inevitable. Even if you drive the thing like a maniac they still need to be de-carboned about every 10,000 miles for best results.
My '01 Cherokee Classic actually ran worse on 93 than it did on 87, especially when it was cold outside.
My '96 Grand Cherokee would ping a little bit on 87 when it was time for a top engine cleaning, and then it was fine for about another 6 months. Amoco or Mobil seemed to work the best in these engines, though they will run on almost anything with no noticeable decrease in performance.

(Looks like there's some JU members here...I recognize a couple names. I posted as "nolimits" there when I had my Jeeps.)

 

[Gary Allan]

Some diclaimers clarified


"Use 87 octane gasoline because premium will do nothing for your engine"

Reason: Initially only premium fuel was used by FI vehicles (BMW, MB, etc.). The "myth" that was somewhat supported by the refiners was that premium fuel was "better" for your engine.


This cause a federal pannel to compell the auto manufacturers and the refiners to dispell this 'myth"

Now the myth in the myth. There is no data that says that the use of premium fuel WILL NOT give you increased "drivability" under certain conditions. I've had 87 octane rated vehicles that, in the summer heat with the A/C on, can't get out of their own way without downshifting ..while the same car under the same conditions ran much better with a higher grade fuel. It didn't give me anymore gas mileage.

It's primarily an energy and consumer policy ..NOT a performance LAW.

If the assertion is totally true (and not just true "most of the time") then the following MUST also be true:

For a vehicle with a knock sensor ...knock and the subsequent timing retardation has NO EFFECT on vehilcle performance or economy ...since "less retardation" (a by-product of higher octane and therefore less knocking) during high load situations has no positive impact on performance or economy (according to the dogma).

See the "hole" there?

Now I've got to admit that I've had some engines that could care less what I threw in them. On the other hand, I've had some that responded well, in terms of "drivability", with higher grade fuel.

Until someone has a drag strip shoot out (or hill climb) with the identical 87 octane car, using different fuel grades, ...I'll stand by my opinion.

Do I use premium or "plus"? Not usually, the problems with "drivablilty" are usually only present in the extreme heat of the summer (that's 90+ here in southeastern PA).


All gas is generic in the pipeline. The local terminals do all the blending for the "brand". How else do you have Sunoco, Exxon, Hess all in the same neighborhood? This was, btw, on the Discovery Channel not too long ago. The stuff is piped all over the place as a generic comodity. East Coast refiners supply a region ..the Louisiana pipelines supply some regions ..the west coast others ..but "brands" are everywhere.

 

[MNgopher]

A few years ago, Car and Driver did the exact thing you are looking for. They took several cars that required varying grades of gasoline and ran performance tests and dynoed them using 87 octane and premium gasolines.

The results were that cars that "required" premium gasoline did indeed perform better running the premium octane gasolines. The cars that only required 87 octane showed little enough reponse that any response was lost in the margin of error.

 

[labman]

I think over the years, Car and Driver has done some great technical stuff. In something like that, likely they are honest.

 

[MNgopher]

One other item - as you point out, gasoline is often shipped and traded as a commodity.

What it really boils down to is you need to know exactly how gasoline is distributed in your own market.

In my case, there are exactly four refineries that supply well over 90% of the gasoline consumed in this market. One maintains an entirely different distribution system entirely separate from the other systems. Two other barbds are exclusively refined at one of the two refineries, and proven by testing. Thus, it is easy to pinpoint three of the four refiners gasoline in this market through testing - all have very different formulations.

Thus, once you figure that mess out, you CAN draw differences based on brand, but only once you figure out who supplies who.