Good morning everyone....
Out of the 2 fuels....which one do you all feel is better? I am currently using Vpower 91 contains 0% ethanol......

 

What is your car calibrated (set up) for? Anything higher may well be a complete waste. (Anything lower would potentially damage the engine.)

 

Hello! It says 91 or higher....

 

I'd use 91 then

I might try 94 but first I'd want to know what mpg it does on 91 and then I'd measure mpg on the 94. If not better, I'd go back to 91. (My feeling is that it will NOT be better.)

 

Using a fuel that has an octane rating HIGHER than required is nothing but a complete waste of money. It will NOT give higher mileage, more power, better starting, cleaner engine or any of the marketing BS that's out there.

 

Isn't the point that the software is definitely calibrated for a minimum octane (here, 91) but MAY be able to adapt to higher octane? If so, wouldn't higher mpg result? But if it's not able to adapt then it would definitely be a waste of money.

 

I was using 91 octane as the book says and suffered through 3 episodes of adverse engine codes (can't remember which but I can get them if need be). At the suggestion of the technician I moved up to a higher octane and have had no further incident.

I fill the tank when it gets about half empty and alternate between 91 and 93.5 octane. My wife always fills with 93.5. Our goal is to keep the average octane right about 92. Sounds a bit bizarre but it works.

Eric

 

Nothing bizarre about that strategy at all. It may very well have solved your misfire issues. Along with moving to a higher octane fuel, did your Jag tech suggest that you also have your fuel injectors cleaned?

Two months and more than 4,000 miles beyond our last misfire episode (both times it was code P303 meaning misfires in cylinder 3), I'm beginning to think that the new air filter housing, air filter element, and mass air flow sensor fixed our misfire problems. Time will tell....

 

Engines have an optimum point for maximum spark advance- it's not a situation where more is 'better'. Advancing the spark beyond that point is counter productive- less power and economy is the result.

As I understand the system, the engines are equipped with knock detector(s) which will retard the spark momentarily until the condition clears. This should not be a reason for setting a code.

I doubt very much that any engine that is set for a given octane would have a device that could detect a higher level octane and feed in more spark advance.

 

By way of example, gasoline (petrol) is often 91 in the USA, 95 in UK, 98 in France. Jaguar will have set the software to suit each. But why write and maintain multiple versions, why not have just one? Can't the knock sensors together with all those other sensors provide the data the software needs?

 

This interesting. I have had cars where the manual stated that running anything over the required (89 in this case) octane would damage the EGR system (and it did in a Chevy 3.1 V6). Also, both my Honda 2.4's stated in the manual that higher hp could be achieved by running a higher octane but 89 was the minimum requirement. So, somehow that engine adapted. Again, not really an answer to any question, just curious information.

 

North America and Europe use different rating systems for gas (petrol). Apples and oranges. 95 octane rating in Europe is roughly equal to 90-91 octane rating here in N. Am. Our two systems used to be the same, up until the mid-70s. The change over (which went unnoticed by most) is what makes many people believe that gas used to 'better' in the old days.

 

That still leaves at least calibrations for UK (95) and France (98). There may be others, as I really don't know the typical octane in many countries. (Waits for Jeff to post them LOL)

Honda apparently sorted it out, you'd think with the software. And Jag?

 

I have no idea how or what Honda did, if in fact they did anything. If I was to speculate or second guess, possibly the car was continuously running with retarded timing on 89 octane as sensed by the knock sensor. Substituting higher octane would allow the engine to run at it's optimum setting. I used to occasionally run my old Infiniti on 87 or 89 octane and noticed a considerable power loss under heavy throttle, as compared to the recommended 91.

Without knowing the specifics of the optimum spark advance characteristics of the engines in question, and comparing these to what advance maps are programmed into the computer, the discussion is pretty much futile.

I have far more hard knowledge on Chev small block V8s. A very common mistake of the 'newbs' is the belief that more advance can be cranked in ad infinitum which will produce more power. The truth is far from this.

Another stick in the spokes to the high octane=improved MPG crowd is that any engine will run perfectly on regular 87 grade gas without knocking under light throttle and higher RPM. This means that under steady state highway conditions it makes no difference what grade gas is used (except for cost).

 

Excellent post, Mikey. Our 2005 S-Type 3.0 is my wife's highway cruiser. She drives it accordingly and has probably never pushed it past 80 mph (but don't ask how fast I've taken it up to by myself at dawn one morning when the freeway was empty). As a result, 89 octane runs just fine in our S-Type (and even our Jaguar dealership has acknowledged that the vast majority of these vehicles do quite well running on 89 octane)....

 

Mikey - if the jag's programmed to use (say) 94 properly would it get better mpg than 91 (or 89)? If so, wouldn't using 94 be a way to test whether it IS programmed that way?

 

One other thing 'bout the Honda motors. They were i-vtec motors that had variable valve timing. So, it is possible the motor and software were tuned to deliver optimum power with higher octane but would run fine and possibly cleaner (hence the ULEV rating) with lower octane gas. Don't know... Just know they are built that way... Then again they are also about the only engines I know that actually benefit from a free flowing intake filter as well.... Maybe it's that vtec thing again. All I know is the 2.4 Accords I had contained one of the sweetest 4 pots I've ever driven.. 6800k redline, vtec power burst at around 5k..and smooth as silk...really an engineering marvel.

 

Refer to my post above regarding use of low octane fuels during steady state highway cruise. Since nothing higher than regular (87) octane is of any benefit under these conditions, I'm not sure how you could organize a test so that the anti-knock properties of high(er) octane gas were continuously being taken advantage of (for long enough) to obtain credible data.

I was suprised to learn that one of my Corvette buddies in the US runs all his cars on 87 octane- even his '65 fuelie. His position is that he never runs the engine hard enough to induce pre-ignition/knocking/pinging so why spend the extra money? He's right.

 

Er, I did refer to it but it didn't actually tell me the answer (but in effect you kinda have, or not).

With the sound-deadening I could imagine having serious knock on the jag but not knowing (oh dear).

And I wish to be able to accelerate HARD any time I feel like it so 87 would be unwise. Why run an STR and then use 87?

 

The knock sensors are much more sensitive than the human ear and compensate at levels far below what can be heard or felt. The only way of knowing if the timing has been retarded is by a 'sluggish' feeling under acceleration.

I am sincerely trying to answer your question but think that the ongoing concept of high octane = high MPG is clouding the picture. Perhaps a method of reverse engineering the factory characterisitcs would be to do a series of standing start full throttle tests to top speed on (say) an abandoned airport runway. Do the tests with varying grades of gas. The acceleration time to top speed should decrease with increasing levels of octane as the computer will need to interfere less and less. If the times stabilize at a consistent figure despite using even higher levels of octane, this might indicate the 'factory tuning' level of the car.

If someone has the home phone number for The Stig, and he's available, I'll find a runway and volunteer my car.

You said:

"And I wish to be able to accelerate HARD any time I feel like it so 87 would be unwise. Why run an STR and then use 87?"

Very true, but then again- being a newb here, I'm surprised to see posts from people that buy an expensive V8 powered luxury car and then agonize over fuel consumption.