Im sure this has been dealt with before, but being new to Jags, I’d like to check.
The 1995 Brochure I have claims that 95 Octane is fine for my Daimler 6, but some say it would benefit greatly from 98. Does anybody have any opinions on that subject, and I take it that it will run on unleaded no problem. I’ve been on diesel for a long time with my other cars, so out of touch.
B

 

The cars were designed for 90 AKI, which is about 94 in Europe. Unleaded is essential, leaded will poison the O2 sensors and catalyst. The ignition system does have knock sensors, so it will retard the ignition if lower quality fuel is used, but if you're using 95 you should be fine, as that is what the car was designed for.

 

Thank you, but would 98 improve performance, smoothness etc?
B

 

No. The ignition will only advance the ignition to what it is programmed to do, it won't go further. Unless there is something wrong with the engine requiring higher octane ( like combustion chamber carbon buildup causing preignition) 98 isn't needed as the computer will not advance the timing further to take advantage of a higher octane fuel.

However, you might wish to look at the "Andy bracket". It does advance the timing and will give you both more performance and better fuel economy on standard fuel. Well recommended by everyone who has bought one, as it's probably the best return of money to performance for a bolt on part. Details here: https://www.jaguarforums.com/forum/x...engines-66536/

 

Here in the Netherlands most agree that the X300(AJ16) performs better with 98 octane. And certainly compared to E10 fuel.

 

Thank you, I have conflicting answers, I’m sure you all have your opinion, and I’m sure I don’t know! Perhaps a mix? Can that be done? all I know is I just want to do the right thing.
I appreciate your opinions, and thank you.
B

 

It shouldn't make a difference, but try both and keep track of your mileage and see if it makes a difference.

 

I have been able to persuade myself that I get about 0.75 mpg more on higher octane fuel, and that she feels marginally sweeter on a light throttle. The difference is pretty marginal though, and I know that I couldnt tell the difference on a "blind" test.

 

Even with andy's bracket I don't think I notice a difference.

 

The engine is tuned to a specific octane rated gas. It is highly unlikely that you would see any real world benefit from exceeding that octane rating. This is true for every car, not just Jaguar.

While many cars can make adjustments for lower rated gas, usually decreasing mpg, there are no engines that can adjust for better gas. None that I’m aware of, anyways.

 

95 RON (approx 50/50 mix of 87 and 93 AKI) max. anything else is a waste unless you have the andy bracket.

i run exclusively on 91 RON, the knock sensors do their job and i don’t care about the mpg hit or retarded timing

 

Right.

The management system has no idea what octane fuel is in the tank. The system only knows, via knock sensors, if there's knocking. If there's knocking it'll retard the timing. If there isn't, it won't.

Cheers
DD

 

Well when it came to collecting the car to bring it home, the garage only had 95 so decision made. It ran fine.
B

 

Hi Winalot,
I was personally responsible for mapping the ignition timing of the XJR6, whilst working for Jaguar. However, I can also confirm the situation regarding naturally aspirated versions of Jaguar AJ16 engine. There is NO performance advantage in using a fuel that has a higher octane rating that 95RON (equivalent to about 90 pump octane / Anti-Knock Index). The only possible exception, is if the engine is fitted with one of my revised crank sensor brackets, or the car is being driven at very high sustained loads in exceptionally high ambient temperatures. Otherwise, save your money and use 95RON fuel. Even if you fit one of my revised brackets, you may not be able to measure the vehicle performance difference between 95 and 98RON fuel.

 

@XJRengineer would running a colder spark plug like the rc8ycc with 95ron on an NA engine with your bracket reduce the likelihood of engine knock?

 

I actually use RC6YCC plugs in my XJR6 that I fitted with an Eaton M112 supercharger, so you can certainly go colder than a Champion grade 9. However, it shouldn't be necessary, unless the engine performance has been increased.

 

Hi thanks
So even with an old and potentially worn engine , there is no advantage in 98 octane?
B

 

Well does your bracket count as increased engine performance warranting colder plugs?

 

I don't recommend changing spark plug grade, if fitting my revised crank sensor bracket. Engine performance may increase if 98RON fuel is used in conjunction with my revised crank sensor bracket, but it is not essential to use 98RON.

 

As all 95 octane fuel in the UK will soon be E10 whereas the 97-99 octane super unleaded will be E5 I'll be using the later as I don't think our cars are designed to run on E10. I also understand that E10 produces a slightly inferior fuel consumption figure.