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Who says 91 octane is not needed?

9

96XJ_4.0

NAXJA Forum User
Location
ca
I recently purchased a 1996 XJ with 120k miles. The thing looks, runs, and drives like new.

When I purchased it I filled it with 16.3 gal of 91. I drove it 328 miles then filled it with 16.2 gallons of 87 after reading here that 91 is not needed. I got 278 miles out of the tank?
I filled it with 15.9 gallons of 91 again. And today my odometer reads 306 and my gas light is still off.

So...
I get on average 20.1 MPG with 91 octane, and 17.1 with 87 octane. It's cheaper to pay 2.00$ more per tank! No matter the price of gas...At least for my rig.
 
Get us 6 accurate/calculated pump-to-pump samples before you decide to fork out for 91 octane. Your sample size is too small.

I stand by my recommendation and use of 87 octane for a stock 4.0 engine. Octane is simply an anti-knock index.

Don't believe me? GOOGLE it.
 
Last edited:
Are you driving the same way and the same routs/distances?
 
Johnnie Walker said:
Are you driving the same way and the same routs/distances?
Click to expand...

I note he's in San Bernardino. One thing that springs to mind from living near that part of the world: headwinds. I've seen the instant fuel economy readout on the overhead drop (and stay) as low as 11mpg for sustained periods due to headwinds in that area. And good luck getting over 65mph if you're driving into one of the really strong ones...
 
Exactly the same driving routine. About a tank a week on the dot. And it really does not matter the price of gas. It's always 20 cents more for 91, @ 15+ or - gallons it equates to 3 dollars every fillup. Bleh it's 3$ when it over 40 to fill it 3$ doesn't matter. The knocking does.

I always keep track of my mileage and I can post weekly as I drive.
 
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96XJ_4.0 said:
3$ doesn't matter. The knocking does.
Click to expand...

Do you have a knocking problem that is reduced with the use of premium fuel? That wasn't mentioned in your original post.

If so, that could be a buildup of carbon on the top end. Different octane fuels detonate differently and use of premium can result in less knock if its a carbon issue.
 
Last edited:
Could it be a, more or less, Alcohol or reformulated difference?

I think the owners manual recommended 91 octane if towing or hot weather, as I recall faintly from way back there.
 
Should try some MCCC to clean out the engine and then see how the knocking is.

That said, my Wife's old 1989 Toyota Tercel Owner's Manual recommended 91 octane for knocking.
 
orange said:
Could it be a, more or less, Alcohol or reformulated difference?
Click to expand...

x2. Do they both have ethanol? Mine definitely gets worse mileage with that crap. At least in oklahoma I could chose the pure gas, but here you don't have a choice.
 
srimes said:
x2. Do they both have ethanol? Mine definitely gets worse mileage with that crap. At least in oklahoma I could chose the pure gas, but here you don't have a choice.
Click to expand...

Good point. Oregon has just this January started allowing the sale of premium non-ethanol fuel.
 
i've heard many people claimed that they gained better MPG with higher octane. I might give that a shot for a month to see if I get more miles out of a tank. I'll decide if the extra $$ will pay for itself by more miles per tank.
 
It doesn't knock at all. I just don't want to deal with it. It just runs better. And I filled it with 87 this morning. I'll report back on mileage soon.
 
I have tried both the 87 and 91 in my jeep and my jeep runs so much better and gets better mileage on the 91. I have been running 91 for about 6 months now and its proven on my end that the mileage is way better with 91 than 87. The extra couple bucks for the 91 is worth it to me, it pays off in the long run. I never had a nocking issue i just thought one day to try it and it works better for me now
 
i also experiment with this concept out of my own curiosity. while i averaged about 35 miles more per tank, my jeep had delays while starting up...??? not sure why to be honest but issues with starting only occurred while running 91 octane. had a brand new starter a couple weeks before this experiment.
 
winkosmosis-
dug out the 96 OM pg 123:

"when pulling a heavy load or driving a fully loaded vehicle when the humidity is low and the temperature is high, use a premium unleaded fuel to help prevent spark knock. If spark knock persists, lighten load, or engine piston damage may result."

(Also mentioned to avoid fuel additives being sold as octane enhancers as most have high concentrations of Methanol (=BAD for fuel system))

(Ethanol 10% ok)

Of course that may have just been the factory worrying about warranty costs, plus fuel has probably changed a lot since 96. (Smells worse! lol)

It still seems to me that fuel from different stations, brands and grades perform differently!

Regards
Orange
 
orange said:
winkosmosis-
dug out the 96 OM pg 123:

"when pulling a heavy load or driving a fully loaded vehicle when the humidity is low and the temperature is high, use a premium unleaded fuel to help prevent spark knock. If spark knock persists, lighten load, or engine piston damage may result."

(Also mentioned to avoid fuel additives being sold as octane enhancers as most have high concentrations of Methanol (=BAD for fuel system))

(Ethanol 10% ok)

Of course that may have just been the factory worrying about warranty costs, plus fuel has probably changed a lot since 96. (Smells worse! lol)

It still seems to me that fuel from different stations, brands and grades perform differently!

Regards
Orange
Click to expand...

Wow.. Maybe my 1999 doesn't say that because the more advanced OBDII system retards timing etc. Even in that case, higher octane would probably give better gas mileage.

I'll try to do some back to back testing with my Scangauge.
 
96 is the OBD2 switch.

95- are OBD1


Good point also! I didn't mention that I only use Mobil because it's local.( like 100 feet LoL)
 
Only way to tell would be a back to back DYNO with AFRs mapped out for us.

Right? Closed experiment in same conditions. Only change being fuel grade.
 
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