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More MPG with 90 AKI fuel vs. 87AKI?

88 Wagonman

88 Wagonman

NAXJA Forum User
Location
Boise, ID
Someone I work with told me that they get better gas mileage from running 90 AKI fuel than the 87. With 15 gallons of fuel at $4.30/gallon (90) that's $64.50. At $4.00/gallon (87) that's $60. That's a difference of $4.50/tank. So at 15MPG you would have to get 1 more MPG to break even. Any opinions? Facts would be nice too, but as these threads go, there is usually a lot more opinion than fact.
 
my stroker gets better mileage on 93. The stock 91 I used to have before giving it to my mother-in-law ran better on 90. I dont have any numbers for you, I just noticed that it seemed to run smoother.
 
Did the friend say what they were driving that got the better mileage? Some cars (usually high performance) are tuned to run on higher octane fuel. They have knock sensors and the computer will pull the timing back when you use lower grade fuel. This makes them run comparatively crappy, and one of the "symptoms" is lower fuel mileage. A friend of mine had a BMW M-6 that would do it. I've never checked with my XJ. 'Always assumed it had low enough compression that it wouldn't matter.

'Good news is, you know it will cost only $4.50 to find out.
 
tbburg said:
Did the friend say what they were driving that got the better mileage?
Click to expand...
1994 Ford explorer 4.0l V6. Not sure about the compression ratio of that engine or if they have a knock sensor, but I'm sure it's not set up for higher octane. On a related note, I work on Yamaha outboards, and they have a very high compression ratio, I don't recall how high right now, but they are designed to run 87 (up to 200 hp) and they say that it you run higher octane you will not gain any power, but you will actually create more carbon by running the higher AKI fuel and that you are wasting your money with it. Don't know if I believe that because they are the same engineers that say to change your oil filter every other oil change because a dirty oil filter filters better than a clean one.:wierd:
 
I've never seen any actual evidence or papers that confirm that higher octane will get you better mileage on lower compression engines. If it were true, there would be swarms of information recommending it.

Octane is quite simply a measure of anti-knock capability and the stock 4.0 needs nothing more than regular fuel (87 in these parts).

As you mention Wagonman, I have seen articles leading you away from higher octane fuel if your engine is not designed for it as it can lead to a slight increase in carbon deposits. There is some actual research that confirms that. Of course, if the engine is DESIGNED for premium, then that's what you should be running.

4.0 is designed for regular, run that and save your money. Gas is expensive enough as it is!
 
birchlakeXJ said:
I've never seen any actual evidence or papers that confirm that higher octane will get you better mileage on lower compression engines. If it were true, there would be swarms of information recommending it.

Octane is quite simply a measure of anti-knock capability and the stock 4.0 needs nothing more than regular fuel (87 in these parts).

As you mention Wagonman, I have seen articles leading you away from higher octane fuel if your engine is not designed for it as it can lead to a slight increase in carbon deposits. There is some actual research that confirms that. Of course, if the engine is DESIGNED for premium, then that's what you should be running.

4.0 is designed for regular, run that and save your money. Gas is expensive enough as it is!
Click to expand...
I am inclined to agree with you, but I am still going to run two tanks of each with the same pump and same temps outside and see where I end up. Of course I don't drive that much so it may take me a couple of months to do so.
 
birchlakeXJ said:
I've never seen any actual evidence or papers that confirm that higher octane will get you better mileage on lower compression engines. If it were true, there would be swarms of information recommending it.
Click to expand...
In 2001 I worked 65 miles from home. During a one (or was it two?) week period, I decided to test the "higher octane" theory. I filled up every morning at the same pump and at the same time. That way, I figured the first click-off should have the same amount of gas in the tank.

I learned that the higher octane did nothing measurable to improve mileage. All it did was empty my wallet faster.

Jim www.yuccaman.com
 
In any case I have seen the increase (if any) in MPG did not compensate in the extra cost of fuel. Not even close.
 
In theory, the low compression of the XJ 4.0 should need nothing more that 87 octane. What I have found though is most fuel brands have a very poor actual actane level i.e most stations 87 octane fuel results in engine knock and low/lower mileage that the 89 from the same station.

Does an average ignition timing of 19* at idle and 38* or more at steady state cruising between 65 MPH and 75 MPH have anything to do with engine knock? My guess would be yes. The timing seem to be high;..... higher than I used to do years ago on Chevy and Ford cars which had a total timing of no more than about 26* and at idle about 12*.

Because of ths, I run almost exclusively 89 octane fuel. My milage and drivability is always better than it is with 87. 91 octane does nothing for the 4.0.

Ignition timing values are derived from the Scan Guage.
 
Here in my town theres a station that does what they call "Whacky Mondays" where you get 89 octane for the price of 87. I've never gotten any better mileage than when I use 87. I do alot of in town driving and almost every tank seems to run 240-250 miles.
 
Running premium fuel can give you better mileage but only if your car is tuned for it. If you installed the jet stage 2 chip you would almost certainly see a rise in gas mileage because that chip advances the timing for more power and efficiency. The gains in gas mileage would probably equal the cost for premium fuel and the cost of the chip over time however you would see a slight increase in power. Maybe 10-15 hp. So if the cost equalls out i would do it if just for that extra 10 hp but thats just me
 
KMK172008 said:
Running premium fuel can give you better mileage but only if your car is tuned for it. If you installed the jet stage 2 chip you would almost certainly see a rise in gas mileage because that chip advances the timing for more power and efficiency. The gains in gas mileage would probably equal the cost for premium fuel and the cost of the chip over time however you would see a slight increase in power. Maybe 10-15 hp. So if the cost equalls out i would do it if just for that extra 10 hp but thats just me
Click to expand...
I thought those chips actually made you use MORE fuel? atleast thats what I've heard. They might give you power...but efficiency too?
 
They advance timing which actually runs the engine leaner. Remember the later you wait to ignite the air-fuel mixture in the cylinder the more power you make and the better burn in the cylinder you get but you risk pre-mature detonation by compression. using less fuel so to avoid detonation requires higher octane premium fuel. So yes chips increase the amount of power produced by your car while decreasing the amount of fuel.
 
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I had a '95 Dakota w/CA emissions that ran the same on regular or premium until I moved to CO. Anything but 91(premium up here) would cause it to knock and generally run like crap. I did notice a substantial drop in mpg(3-4) when not running premium. If nothing else, the performance was a TON better with the higher grade fuel. Don't notice any difference in the XJ, but it's not set for CA.
 
The last guy kinda pointed out one factor others weren't including; altitude.

at sea level 87 might be perfect for the xj, but in denver or something it may need 91 to run properly.

Isn't the stock CR for the 4.0 8.8:1? I know the 2.5 is 9.2:1, maybe I should try 89 or 91 due to such a high CR.

I noticed that if I get 87 from barney's (crap) the engine will knock, backfire, and eventually die, but if I get gas from chevron it'll run great and gain at least 2mpg over barney's fuel.
 
Update:
So, after a month's time here are the results: I get roughly 18-19mpg with steady A\C use with the same driving habits with both grades. The weird thing that I noticed is that with the 87 I get some pinging, but only with the A\C on (more load). I have been told that even with the knock sensor disconnected that you shouldn't get any pinging with 87 on a renix 4.0L. Now, my 91 doesn't do it, but my 88 does. Did the compression ratios change between years? How about the timing? I ask because I have an HO(98) longblock, but it is equipped with the renix (88) electronics. Would this cause my pinging, or is it crap gas?
 
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