Given that the computer adjusts the timing to the fuel you typically use, I would think it wise to choose one octane and stick with it.

Running 89 most of the time and an occassional tank of 91 probably doesn't help anything, since the computer will still set timing based on an average of 89 octane gas. By the time a random tank of 91 is used up, the computer probably advanced the timing some (since it could get away with it without pinging), then suddenly it's back to 89 and pinging until it compensates back down to 89.

Predictability is a good thing when it comes to fuel maps.

 

Could you cite a reference for this statement please?

 

Here is an interesting response taken from another forum.

Might need a bit of tweaking for today's prices but the math show the question of premium vs regular not to be that big a problem.

>>>>>Pasted<<<<<

Where I live 93 octane Premium is generally 30 cents a gallon more than 87 octane regular.

Considering that premium gets 2 mpg better than regular and it does at a minimum, this means it cost .87 cents more for premium every 400 miles.

400 miles divided by 27 mpg = 14.81 gallons used X $3.20 per gallon = $47.41 (Regular gas)
400 miles divided by 29 mpg = 13.80 gallons used X $3.50 per gallon = $48.28 (premium gas)

$48.21 - $47.41 = $.87 cents more for premium.

At 15000 mile usage per year, It would cost $32.63 extra to use 93 Octane premium using 30 cents more a gallon.

>>>>>>END PASTE<<<<<

An interesting argument over all, perhaps worthy of discussion...

Vince

 

As a complete newbie to the Jag world (I'm loving it, and these forums), but a longtime owner of various high performance cars, I feel compelled to add to this thread.

First, a re-cap.
#1, the price per annum is nearly negligible for a car that is tuned to require premium fuel (the $32.63 per 15k miles argument).
#2, the "engine knock" is just one of several things detrimental to the cars optimal performance.
#3, there is no known benefit for fueling with a "treat" of octane higher than the 91 recommended by the manufacturer.

The only thing I will add is this bit of anecdotal evidence for doing as told: my '03 x-type (2.5L, Manual), had the "check engine" light come on after a week of ownership. I had put in one tank of premium, one of mid-grade, but not let the whole original tank burn off (I suspect it was regular). Upon taking it in for inspection, the mechanic said the O2 sensors had failed, and both catalytic converters needed to be replaced--$4k!! I freaked, called around until I found a jag mechanic, and he re-assessed the issue. Turns out the sensors were fine, and everything was good to go, but I needed to make sure to put in premium fuel so the catalytic converters would "bet hot enough--near 500 degrees." His explanation was the premium fuel was required for proper function, and to not skimp. Made sense to me, and happy to report a steady diet of premium has kept the problem at bay.

I had known about the compression ratio, etc. being the impetus for the "good stuff" but never the exhaust temp. Good to know.

Cheers!

 

I'm afraid there's some misunderstanding somewhere- premium gas burns at the same temp and speed as any other grade. Operating with retarded timing (knock sensors activated due to presence of detonation) would tend to raise the exhaust temp, not reduce it. Even then any variation up or down would be well within normal operating range for any cat. converter.

 

FYI here in the states, most all gas staions in a region get their gas from the same refinery, so it's not so much brand as condition of their tanks !!

 

I've had my 00 S Type since late last year. I started putting in Shell gas in my other cars on a recommendation from a friend. I was not using premium in those (88 Cougar LS/95 Taurus). I did notice a jump in MPG in both cars. However, when I put Shell premium(91 octane) in the Jag it ran a little rough. Someone actually asked if it was a deisel. I then switched to BP and it ran like a champ. I didn't notice at first but BP gas in my area is 93 octane for the most part. More power, better idle, decent mpg(up to 30 mpg hwy @55-60mph). I'll stick with BP for now.

 

93 octane for me. Chevron or BP, no off brands. MPG is better than first couple of tankfulls with 89 octane. I'll stick with 93.

 

I am picking up my first Jaguar ( 1990 XJ40 - 170,000 mi ) tomorow in Denver Colorado ( 5,300 ft elevation ) and taking it back home up in the mountains ( 7,600 ft ). My 91 octane cars ( CARB. AND 9.2:1 CR ) run wonderful on regular because of the altitude at my place and good on mid-range fuel in Denver. Can I expect the same in the Jag????

 

This might help, aside from the coffee discussions.

https://www.jaguarforums.com/forum/x...-lately-70132/