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running premium fuel= Bad MPG

mitsumotors

NAXJA Forum User
Location
Winnipeg
I read a couple post on here saying that people running premium fuel in their xj's seemed to get better gas mileage. I ran a couple tanks and found that i was getting noticably worse mileage when using premium. A mechanic i talked to told me that if my vehicle is not set up to run premium it will not get good mileage if you run it because it runns cooler and will mess up your exhaust temp readings and stuff. can anyone else confirm if this is true?
 
I think this is pretty accurate.

Zuki-Ron said:
You will likely just pay more for the same or less performance.

Generally speaking, putting high octane fuel in a low compression engine is not all that productive.

Low Octane fuel, burns more rapidly, High Octane fuel burns more slowly. Counter intuitive, eh.

High performance, high compression engines require a slower burning fuel to prevent pre-ignition during the compression stroke and to burn more evenly in the quench and power parts of the stroke.

Adding a high Octane rated fuel to an engine that is not designed to take advantage of it's slower burning characteristics, is not that productive.
 
Back inthe day when we weren't paying $4/gallon, I was doing a 70-mile drive each way. I ran regular and premium through it, topping the tank off at the same pump. The only difference I had with the 89 octane was that it emptied my wallet faster. There was less than a 1mpg improvement over the lowest 85 octane.

Jim www.yuccaman.com 96XJ 4.0L
 
i was running regular and getting like 15mpg an then i did spark plugs, cap rotor, wires and O2 sensor and started running premium and my mileage dropped to 13mpg. makes no sense to me and i'm about ready to blow my brains out!!!
 
i haven't run any mid grade in my xj, but i know that my 88 fury likes the 89 over the 87 but it has 9 to 1 comp. factory and says to run it in the mid grade in the owners manual. as to the mileage issue, i dont keep my foot out of it enough to get any mileage.
 
Most people think that a higher octane rating gives more MPG. It is simply not true and is just a waste of money unless your vehicle is tuned for it.
 
I have moved my CPS as far over to the passenger side as possible on my Renix. I always pump premium. I get 17 mixed city/highway with 4.10's and 31's and a 3" lift. The performance to me is worth the extra couple of bucks.
 
the_weirdo said:
Most people think that a higher octane rating gives more MPG. It is simply not true and is just a waste of money unless your vehicle is tuned for it.

Exactly. I am getting 19-22 currently, with 2/3 highway, and regular gasoline. Lucky I guess, the previous owner took care of it.
 
I run higher octane in the summer when we go wheeling in the desert. The xj still runs about 205, but it keeps it from getting hotter than that. That the only time I run higher octane.
 
Hallo Jeepers. I run Shell Super power. (97 oct). It is a special high octane fuel with cleaning additives. It keeps the valves and the injectors very clean. That is very important! 200 K without problems of dirt.
Greetings from Holland:laugh:
 
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mitsumotors said:
i was running regular and getting like 15mpg an then i did spark plugs, cap rotor, wires and O2 sensor and started running premium and my mileage dropped to 13mpg. makes no sense to me and i'm about ready to blow my brains out!!!

premium gas has a higher flash point. if you were able to adjust timing on an xj and if it were a high compression engine, than you might see better results.

i believe whats happening is the gas is igniting too late in the compression stroke.
 
How much play do those adjustable renix CPSs give you. Is it a matter of moving it ~1/2" closer to the passenger side? I've built a high comp(10:1) 4.6 and it doesn't ping on 93(haven't tried lower yet), but am tempted to slot my cps and see what it does.
 
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