I'm not going to step back into the debate about whether you should or should not be substituting a lower octane fuel on a regular (no pun intended) basis but it does occur to me that since the engine has knock sensors I'd be very surprised if it couldn't properly adjust itself for such an an occasion when someone can't get or decides to substitute a lower octane fuel. Given how often I hear the supercharger whine when I drive mine I think I'd be cheating myself of the very best performance of the car.

 

The points you're raise is exactly why one should consult the manufacturer's opinion on this matter. They did all the development testing of the engine and know how close to the limits it gets under all the conditions the car might encounter. Finding someone who will give you a straight, fully informed answer and not just read it out of the owners manual is what we really want here. But I can't imagine anyone at Jaguar officially suggesting that it's OK to substitute a lower octane fuel even if were feasible.

In any event, I have a very nice topic to completely ruin the jovial atmosphere one should be experiencing at an annual car club picnic!

Wish me luck.

 

Don't ruin it! We've had a good enough answer above from Count Iblis about the AJ27 SC which is near enough the STR. He says it (the PCM) does have knock limits. You're not going to find out exactly what they are. You'd be lucky to find out something as accurate let alone from someone actually in the know.

 

Thank you for the directive ...

None the less, I found portions of his statements confusing. 3 bar? That would be just north of 29lbs of boost above normal atmospheric pressure. I didn't think this SC system boosted that high? Am I wrong about this? Perhaps he could flesh his statements out a bit to clarify things a bit more.

 

It's not boost pressure that he means it is Brake Mean Effective Pressure, don't ask me to explain it though.
Maybe Count Iblis will explain for us?

 

That would be great.

 

Yes.... Octane is the term we should be using. The loose definitions of "Premium, Regular, Mid-Grade" vary from country to country.

When I moved to the Chrysler, and got my hands on a hand held tuner, I got a little lesson in the limits of the PCM myself. See, just like Count Iblis talked about, There are limits that the computer works in. If you take the PCM in the Hemi and reprogram it for 91 or 93 octane fuel (it is designed to run on 89 octane) you get a warning from the tuning company that YOU WILL damage the engine if you run lower octane because the knock sensors will not be able to compensate enough to prevent knock damage. Conversely, they also write "economy" tunes for the car to run at lower power with 87 octane. They also tell you that going up in octane will not hurt anything but will also be a waste of money. So, again, it really becomes a matter of the PCM computer's limitations to handle extreme changes. I really wish for you guys that the Jaguar engines had the support out there to have a company produce a hand held tuner. It is one of the neatest things I've ever gotten for a car. I have played with everything from 87 octane to 93 octane. The power the car produces when tuned to 93 octane is awesome, but it makes it almost to powerful. The drive is more high strung. The best I've found is 91 octane tune, with some of the throttle response dialed back (yes you can adjust that along with a ton of other stuff).

I know that a bunch of that is not relevant to the Jaguar discussion. My point is, yet again, I think people over estimate the actual ability of the computers in their cars to compensate for things out of "specification".

 

Good stuff about bmep in wiki etc - good if you can understand it

 

This post contains interesting info, albeit with all due respect, some of it is anecdotal. It does seem to confirm that yes, the spark will be retarded to avoid detonation but raises the question of what the resulting lower limit for gasoline octane rating would be. Without know more about the petrol used in S.Africa on what engine in which car, it is similar to the chart previously provided for the 4.2L powered Land Rover products that indicate that 87AKI octane gas is acceptable.

And around and around we go..

 

Feed the kitty the best gas you can find (other than the 100LL at the airport, now that's real gas, should sell it as perfume.) I've noticed a difference between 91 and 93 octane in the STR

 

BMEP, Brake Mean Effective Pressure:
For those unfamiliar with the integral calculus used often to describe this, BMEP can best be said to be the combustion pressure averaged out over the cycle. It is the indicated mean effective pressure minus the friction, most of which occurs between piston/rings and cylinder wall during power stroke due to the sideways thrust from the conrod/crankpin assy.
For a plain aspirated petrol engine, something like 10bar would be typical, for a turbo or supercharged engine, say up to 15bar, depending on the boost pressure. Typically, turbo/supercharged engines have to have compression ratio reduced to stop knock-ping due to increased compression pressure, greatest at full throttle of course. One of the reasons pressure-charged petrol engines are less fuel-efficient. The diesel situation is completely different.
Leedsman.

 

A few years ago my wife drove an A3 withthe 2.0 turbo and I had a V8 Touareg. Both require premium. I always put in 91 or 93. She was frugal and always put in 87. So I tested a tank of 87 against a tank of 91. The premium fuel produced 10% better MILEAGE in addition, of course, to better performance. So, back in those days that was about $0.25 a gallon, now it's about $0.38 a gallon difference. It pays for itself.

 

I have been running both in my STR and have seen zero difference in the milage between premium and regular on my test loop 63 miles (mixed highway, full throttle blasts, and city) I was sure I would see some kinda difference if nothing more than a few tenths of a mile but it was the same. As for knocking the has been none, I would expect that as the supercharger spools up that there would be some pinging but unless the knock sensor has psychic abilities i have not even heard a single knock, I of course would run only the best fro the car but i really cannot detect a difference. When I get a chance I am gonna hook up the wds and see if I can get some real time data.