California switched to 91 octane from 92 octane gas in 2001. As we all know the X300's were made from 95-97. I am curious what octane gas the X300's were originally designed to run on. It is 100% legal to buy 100 octane unleaded gas (pun intended) in California to mix with 91 octane gas. Too bad 100 octane gas is $8.99 a gallon. I am guessing that is an 'environmental tax' so people don't buy too much of it.

 

I remember reading some handbook material saying that the X300s for Hong Kong, Japan and possibly other RHD regions were designed to run on "95U" fuel.

 

The US version is designed for 91. I use 89 in my XJ6 with no problems. There is a slight reduction in power, but I'm not drag racing, and I do not see a difference in mileage. The knock sensors will retard the timing , if necessary.

 

That's the spec of fuel used in the UK , Europe, Oz, NZ and elsewhere. Their rating system is expressed in RON, ours is expressed in AKI. Their 95 is our 91, it's same-same.

 

The cars were designed for 90 AKI, which is the system used in North America, or 95 in Europe.

 

Pages 93 - 96 of the X300 "Car Care Document” carry a description of the fuel in terms of Octane ratings on X300s in different regions. It is predominantly "95U" for most regions (regardless of LHD and RHD), and fewer "91U" or "91L" for regions where X300s are not fitted with catalyst converter. But strangely, the document shows rare places even like Malta, China, Indonesia, Panama and Paraguay, but is silent on North America. These were preceded by a general description of “Fuel Requirements” on Pages 91 and 92 where “RON” and even “MTBE” were mentioned, but not “AKI”. I also believe that XJREngineer’s timing bracket is also relevant here with regard to belief that the knock sensors will correct the timing – the Andy bracket is to advance factory timing by 5 degrees, I recall.

 

If only RON was mentioned then the fuel 95U would 95 RON unleaded. That's 91 AKI in North America.

91U and 91L are 91 RON unleaded and 91 RON leaded respectively which would be known as 87 AKI in North America. Leaded fuels have not been available here for road use since the 1990s I think.

 

Oops!
I think you mean "leaded?"

 

Right you are. Thanks.

 

AKI is Anti Knock Index. It's the average of the RON and MON values.

 

Try a bottle of octane booster in the tank and then fill with regular, if you don't get any pinking, carry on using regular.

I've been using this system in my AJ6 for the last 3 months with no downside so far.

fun fact: Higher the octane rating, lower the volatility

Larry

 

Typical octane boosters in a bottle raise the rating by only .1, ie to 91.1 from 91. Very poor return on investment.

 

Members on the Dutch Jaguar forum use almost all RON98 petrol and are very positive about.

 

What octane were the Dutch spec cars built for, (98 or 95) please?

 

Built for RON95 (the lowest number what can)

 

Thanks. Why do they use 98 then? No advantage.

 

Experience most members, better throttle response, more power and lower fuel consumption. Better burning, cleaner engine.

 

Interesting. Similar claims made on later cars (like the 1999-2008 S-type) were found to be unsubstantiated and were more from people believing in old myths and misconceptions. Placebo effect.

For a car to deliver the benefits mentioned in your first sentence, the engine must suffer sufficient detonation to cause the control system to retard the ignition timing over a prolonged period. It's very unusual for an OEM to release a car that's factory calibrated to operate in this manner.

As for better burning and cleaner engine, these are most definitely old myths.

 

To measure is to know. I drive eight years with RON98. Always liters and kilometers noted, also, when I used RON95. clearly have a saving on fuel found when using RON98. Financial benefit has not because RON98 is more expensive.

No big differences and the best results with BP Ultimat98. We have also RON102 in Netherlands but too expensive.

 

BP Ultimate98 is the ethanol poorest which can be purchased with an ethanol content below 2% but usually under 1%.. ...

Most fuels in Europe have an ethanol content between 5 and 10%

Fuel with ethanol results in a worse combustion and higher consumption.