This is for U.S. Jag owners only. In California and much of the U.S. only 91 octane is available for premium fuel. I hear all the time that 93/95 octane gas won't make a X308 XJR perform better than 91 octane. So what was the purpose of 93/95 octane then? I know in some states it is still available. Why do they still make it if cars won't perform any better with it?

 

Because its not directly for performance, its for compression (which is tied to performance)...

 

In almost every gas station in my area (DC/MD/VA) the typical octane ratings available are 87/89/93, almost never see 91 as an option unless it's a really remote location, or one of the few i've seen that offer 4 choices (87/89/91/93). Maybe it's a marketing thing? Especially if everyone else in the area offers 93 it looks bad if you're "only" offering 91 as your top choice.

Especially since most people don't know a thing about how their engines work, knock, predetonation etc, they just assume "oh the higher number is a better/cleaner/more efficient gasoline" and waste money putting 93 in their Civic.

EDIT: Highhorse beat me to the punch on the knock video. TLDR, higher compression engines (and forced induction) are more likely to predetonate gas due to high pressures, and the higher octane rating gas is more resistant to predetonation.

 

91 is a California thing (or Rocky Mtn States). If you could buy 93, you would see the difference. The ECU compensates, but milage and performance will be slightly lowered. Most won't even notice.

 

Jag supercharged engines have variable pressure (dynamic compression) in the cylinders as the supercharger boost varies according to the demand for power, i.e according to how much the accelerator pedal is pressed.

If you drive with moderate acceleration and at moderate cruising speeds, the boost will be low and the engine will operate at the compression not much higher than the basic 9:1. In this case, whether you use 91, 93 or 95 octane fuel will make no difference. However, 91 octane may not be sufficient to prevent detonation in the cylinders at high boost (hard acceleration) and, if the detonation (knocking) is detected by the Knock Sensors, the ECU will adjust the ignition timing and probably reduce the boost to eliminate the knocking which will result in somewhat reduced engine performance (power). In this case, the 93 octane fuel will give you better performance as it will withstand higher boosts better.

On the other hand, the Jaguar "Vehicle Care Handbook" states that 91 octane (AKI, the US measure) is the minimum to be used. The handbook provides separate data for NA and SC but makes no difference between the two when recommending the fuel octane.