On a side note, looking at the owners manual, it states that "the PREFERRED fuel is 95 RON unleaded". The "RON" rating from what I can gather is more of a British rating. In the US, we use the AKI rating (similar to what the RON rating is, but using different measuring techniques). 91 AKI is the same as 95 RON. If you drop to say 87 AKI, that is roughly 91 RON fuel. Different regions of the US have different fuels. Premium in say Maryland (where I live which is at sea level) is 93 AKI. But, if you go to say Colorado (where the elevation is in the 5000 foot/1800M range), the "same" premium is only 91 AKI. This may not be true everywhere, but higher elevations have a varying "definition" of what premium fuel is.

This is all based off of measuring the concentrations of certain chemicals inside the gasoline that either allow it to burn really easy, or retard its ability to burn.

If you want to get more into this, here is a good summation page of the ratings and how they are determined. Yes, it is a Wiki, but I think for the average joe, this is good enough to make them an informed buyer/user.

Octane rating - Wikipedia, the free encyclopedia

 

Thermo-

Your explanations were quite good except for some minor but essential points.

1) The dieseling phenomena is more correctly called 'detonation' when applied to four stroke Otto cycle engines. In general 'dieseling' does not occur in gas powered four stroke engines except in cases of engines apparently running by themselves after the ignition is shut off. Even then, this phenomena is properly called 'pre-ignition' as the fuel is being ignited by hot spots in the combustion chamber and not through the act of compression.

The high octane gas recommended by Jaguar is intended to prevent or lessen detonation, not dieseling. There are distinct differences between the two phenomena- true dieseling does not occur in our engines as the compression ratio is nowhere high enough (usually in the range of 16:1 or much higher) for this to occur

2) The RON rating system for gasoline is used just about everywhere in the world EXCEPT North America. We adopted the AKI system in the 1970s for obscure reasons- and here we are almost 40 years later and it still causes confusion.

3) As far as anyone has documented, the knock (detonations) sensors and associated software are capable of dealing with any sort of octane rating gasoline that can be thrown at them and simply reduce timing advance as required to avoid damage, at the expense of peak HP and fuel economy. No actual engine damage occurs- that's why the 95 RON fuel is only recommended' and not 'mandatory'.

Mike

 

Dang...super brains of the world unite!

I personally decided that I don't want to chance any sort of damage to either my 2002 Jag or my 2003 GSX-R (motorcycle) so I'm sticking to 91/93 for now. However, I assure you that most people out there that lease new luxury vehicles which require high octane only put in the lowest cheapest gas. Well they basically know that they are getting raped by the car financiers and they have to turn it back in anyways so who cares about longevity. So today's cars definitely run fine on low octane. But I don't feel like testing it with my 10 year old car and bike.

 

Great thread!

Dissertations from both "The College of Advanced Timing" and the "School of Hard Knocks"!

(as for myself, I had an English Major and a French Minor. When they found them in my dorm room they kicked me out)

 

priceless!!!!

 

Yuck... ethanol in that cheap stuff;-).

 

What specific problems have you had with ethanol? I see you're from Ottawa. You guys have had E10 gas longer than most of Canada, well over 20 years or so.

 

knocking while under heavy foot.
Anyhow, even my ever high output Brigs and Stratton V-twin (yes, on the lawn mower) hates ethanol ;-)

 

What specific problems have you had with ethanol? I see you're from Ottawa. You guys have had E10 gas longer than most of Canada, well over 20 years or so.

 

We bought our 02 X-type in 05 with 19,000 miles. It now has, 7 years later, 67,000 miles and has only had, at the most, 4 tanks of premium in it. Not knowing any better I went from the dealer to the gas station and filled it with regular from Arco. Never had a problem with regular or noticed any decrease in performace. Gas mileage is still up around 31 mpg on the highway at 65 mph with cruise control. It has only known Arco regular its entire life.

 

Ethanol has no negative effect on knock/detonation, in fact pure ethanol has a much high octane rating than pure gas which is why E85 is being used in the latest generation of ultra high performance engines.

If you're buying '91', it's '91' irrespective of the % of ethanol.

 

>why E85 is being used in the latest generation of ultra high performance engines.

Ya, that's a trick with the turbo guys to keep their engines from blowing up with detonation. Some add an additional fuel injector into the intake and
blow ethanol straight into the engine when the boost is high.

================================================
Jaguar: Grace, Pace, and Space - Sir William Lyons

 

I've tried regular, and my father, who owned my car before me tried regular too. We noticed uneven acceleration in "our" 2003 2.5L manual. We also noticed a decrease in overall power and throttle response with regular. This is due to the knock sensor pulling ignition timing. We never heard any pinging (yes, I know exactly what it sounds like) (and yes, I am qualified)

Truth be told, the gain in MPG by using premium is quite minimal, if any at all, and won't offset the price difference.

It's not likely to fry your engine. A properly functioning knock sensor will probably prevent any detonation.

 

What a great explanation!

I moved to Denver about three years ago from Chicago and will say that my X is definitely more lenient about gas here than it was there. In Chicago I couldn't even use "cheap" premium gas (there was one place specifically) or the car would knock and smoke almost immediately. Out (up?) here I still typically use the good stuff, but occasionally have used mid-grade or regular and have never noticed an issue.

(of course, the vacuum leaks are much worse at altitude, but whatever...)

 

Thermo, i'm glad you answered this question the way you did because i came here tonight because i had been running premium gasoline in my 2002 x type 3.0 until 3/25/2013. On 3/24, i filled up on 90/non ethanol and went outside the next morning to start my car and it wouldn't start. It was 33 degrees outside, every 1/2 hour i tried to start it, when it finally warmed up to 45 degrees, she started right up and purred like new. Did the same thing the following morining, wouldn't start, so i removed some of the non ethanol and added 93 octane and now all is well. So, IMHO, even non ethanol gas can make a difference in certain conditions.

So, Thermo, keep up the GREAT work and please continue to answer another "umpteen" questions if you have the time to do so. It's because of people like you that some of us are here, because we don't know it all and don't mind asking questions to get great advice from good people like you. I know that i speak for alot of people when i say that:

"YOU ARE GREATLY APPRECIATED FOR YOUR WISDOM AND PATIENCE"

 

angel, thanks for the vote of confidence. I am just a person that loves to learn something each and every day. Sometimes the questions you guys ask is something that I don't know and then that sends me off on a wild goose chase to see what I can find. Then I take what I learn and apply it to what I already know.

So, if there is something you want to know about, just ask. I will do what I can to keep it on a level that most people can understand.

 

FYI in europe we only have 95 octane (named super)
98 octane (named super +) and shell sells 99 octane used in my bike (+10.000 rpm)
The higher the octane, the higher the price...
If you want to go fast and/or luxurious you pay the price

 

I think the manual said 95 octane, to me we drive premium vehicles. Respect the Motorcar.

 

Mine went into limp mode the time my wife filled it up with 87. Had to pour in some octane booster to get it running right again. I use 93.

 

Thermo,

You know that i believe in you. I'm still bringing my baby to you to get work done to her, HIDs and other things that we've already discussed. Hope you're not still working so hard. See you soon