I'm a newbie with a 2007 XK. I know the manual says to use premium gas but most modern engines will automatically adjust for the octane. Can I use regular when I'm just driving around town at low speeds?

 

The XK requires 95 octane gas for driving. It also can be filled with E10 95 octane gas.
No need for higher octane, costs more brings no advantage as long as you haven't upgraded the engine by mods like ECU mapping and higher Superchargerpressure.

 

The highest octane in North America is generally 91, although some is around that is higher.
Here in Canada the 91 at Shell, Esso and Ultramar does not have any ethanol, and the gas at Petro Canada does. I prefer the non-ethanol as its more efficient.
I believe the measurement of octane in Europe and N.A. is a bit different, or at least thats what the tech's at the Jaguar academy told me a few years ago, so the 91 is likely about the same as the Euro 95..not totally sure.

I would not use anything less than 91 as its the heat build up from detonation that the computer registers and retards the engine timing which produces less power and less heat.
My guess is if you 'grenade' your engine for whatever reason, Jaguar would test the fuel and realize the low octane and/or the computer would record the detonation condition... note that the manual specifies 91.
Only my opinion, but I doubt its worth it.
Lawrence

 

Mulmur is correct.

In most filling stations here in the USA 91 Octane is usually the highest you can find. Some stations offer 93 octane. Anything higher than 93 can usually be found at "speciality fuel stations" only.

I have a 2009 XKR 4.2L SC and burn 91 Octane without any known problems. I live in southern New Mexico.

 

Gas in Europe and many other places is rated differently than in North America. Your gas is rated by Research Octane Number (RON), ours is rated by Anti Knock Index (AKI)

Roughly speaking, 95 RON = 91 AKI.

Using anything higher is a waste of money. Using anything lower may result in reduced engine power and fuel economy if (emphasize IF) the car is driven under conditions that induce detonation and the knock sensors kick in to retard the timing.

Using E10 fuel (10% ethanol) will reduce power and fuel economy by approx. 3% as compared to pure gas.

 

The general rule most should follow is the manufacturers wording.
"Recommended" indicates the system has tolerance for lesser grades.
"Required" indicates the system has been designed to function with a specific grade.

The Jag lists Premium as Required.....

my 2 cents
Vince

 

93 octane is found all along the east coast. Lower octane is needed at higher elevations I'm told. In New England, only Maine has gone to cheap 91 octane so my 93 tunes are no good up there.
FYI most important time of year to run highest available is in heat of summer when AC running, loaded car on vacation etc. In cold of winter, predetonation less of an issue anyway as fuel is much colder and more resistant to this. I never cut corners in summer but in winter experimented for years with several cars and effects were minimal (I'm talking temps below 20 degrees F at least).

Kind of a counter phenomenon to turbos running better with cold air than becoming heatsoaked in summer and losing boost.

Also realize a stock XK 5.0 is detuned out of the box to handle lousy fuel around the world (this comes directly from my ECU tuner). If tune done, fuel requirement much more important as vehicle is now running at 100% instead of factory ~65% tune.

 

Just to muddy the waters, here's what the 2007 XK150 owner's manual says:

The preferred fuel should have and octane number of at least 95 RON (Research Octane number)Super Green Plus 98 RON unleaded fuel (where available) may be used as an alternative to the standard 95 RON unleaded fuel


Here's what the 2012 owner's manual says:

The recommended unleaded fuel should have an octane rating of 95 RON (Research Octane Number) or higher. However, you may also use unleaded fuel with a lower RON, but performance will be reduced. The minimum octane rating for fuel to be used is 91 RON


91 RON is equivalent to about 87 AKI or what we call 'regular' in most of North America..

 

93 everywhere here.... 92 or 91 and I don't touch it.

 

Burning a lower octane gasoline is likely a false economy and not particularly good for your vehicle. If you insist on trying the lower octane, keep track of your mpg for a tank or two of lower octane gas and then compare it to the mpg you get with 91 octane.

 

93 is pretty much a east coast thing... Very hard to find here on the west coast where even 92 is a rarity. You don't miss what you have never had access to.....

V

 

We can buy 94 up here. No idea why- can't think of one car that requires it.

 

Octane ratings, simply put, are the gasoline's burn point, the higher the octane, the lower the burn point.

High compression, turbo charged & supercharged motors require higher octane due to higher cylinder pressure increasing the likely hood of compressed gasoline burning before it is ignited hence the term "pre-ignition" or "knock". The more pressure that is applied to the air-fuel mix, the hotter it gets.

Some years ago, motors did not have the sophisticated electronics to sense this anomaly and compensate for it which meant there was a greater risk for engine damage. Modern cars can sense this and make adjustments as needed to compensate. This compensation will reduce ignition advance which translates to reduced performance especially during hard acceleration.

Use of lower octane fuel in our cars has 2 other negative consequences, reduced MPG & increased exhaust temperatures which can damage related components over time.

 

Not really the correct or full story. The condition is called detonation (not pre-detonation) and it is not premature burning of the fuel but actual detonation or 'explosion'. It is this undesirable and uncontrolled act that results in the pinging or knocking noise which in itself can destroy an engine in relatively short order.

The octane rating of a fuel is it's resistance to detonation, not a lower burning point.

The controls on our engines are sophisticated enough to detect the onset of detonation long before the driver could notice it and avoid the condition by temporarily retarding the ignition advance as required. It is important to understand that these engines would not suffer detonation continuously if fed low octane fuel- it is a transient condition affected mainly by RPM, load and throttle position. A gentle driver may never experience detonation at all.

As such, most drivers would not see a reduction in mileage or notice a decrease in power.

 

I beg to differ… And its resistance IS due to lower burning point.

Obviously meant pre-ignition.

I also doubt the majority of high performance supercharged cars are driven gently so performance reductions will be noticeable. Studies have proven MPG is reduced and increased exhaust gas temps CAN damage related components.

There is a reason engineers require specified octane or RON minimums.

 

This might help explain:

Engine knocking - Wikipedia, the free encyclopedia

Pre-ignition is a very different fault, it is not another name for detonation.

The OP has a normally aspirated engine, no supercharger. As quoted from the 2012 owner's manual, the car can be operated on 87 AKI fuel.

 

I thought the NA 5.0L engine was at a higher compression then the SC motors. How much of a difference would that make.

.....USA (AKI/PON) = Europe (RON)

Regular........... 87 = 91
Plus................ 89 = 93
Premium......... 91 = 95
Premium Plus... 93 = 98

 

The engine compression quoted is without any boost added by the supercharger. This also means that a supercharged engine is less likely to experience detonation when off the boost.

 

Ditto.Cheers!
G.

 

Mine's supercharged, so I use 95 RON (standard in UK) = 91 US. I only use 98 RON for autobahns and track, when I'm pushing the engine closer to its limits.