I have heard many, including RACQ, say that there is no advantage in putting high octane fuel in your engine and yet others will say there is. I previously owned a 1989 420 SEL Mercedes and my mechanic said that those who said it made no difference were wrong. He said I should maintain using 98 Octane although the cost was higher and I was happy to do so. I used to get quite reasonable MPG / KPL out of the Merc and I believe part of that came from using the higher octane.

I now have a 2005 XJ8 { 216,000 Kilometres } and plan to continue using the 98 Octane. from BP Mercedes always stated not to use Ethanol fuels and I won't use them in the Jaguar either.

 

I an confident it's harmless in that car but pointless and costs more money than it's worth.

Your car, do as you wish, of course.

Some cars can make good use of higher octane but I'm fairly sure your one doesn't (and it's happy with even E10).

 

You would get better responses of how your specific car handles fuel control related to the year/model Jag you have if you were to post in the correct forum.
You posted this in the forum a later design of XJs. It would expectedly handle combustion control differently from yours.
It would also help if you made the topic of your post relevant to your question, otherwise many of us will pass it by without interest.

 

The SV8 would seemingly dynamically adjust timing based on the fuel fed to it though I never read anything out of the ECU to confirm one way or another. I always avoided the higher ethanol fuels simply because as a classic owner I detest ethanol and the trouble it causes.

 

 

Plantenagista, being someone who has worked on cars for more years than I care to admit and a self taught person, I think I can provide some insight into the reality of things. When it comes to the octane that a motor should have run through it, yes, it is possible to run a lower octane than what is recommended. With this being said, I would only do it in more modern cars, not on a classic car. I say that because a modern day car has the ability to shift the timing on the fly. The computer monitors the engine via what is called a knock sensor (knock is a "banging" sound that the engine makes when the cylinder tries to fire too early - which can be caused by either the actual time of spark or compressing the fuel too fast, leading to spontaneous combustion). With high compression engines like what Jaguar tends to use, the possibility of the spontaneous combustion increases. This is where using a fuel with ethanol in it can help as the ethanol is a high octane fuel and the rating is based on that of the gasoline, not of the ethanol. So, you have some margin when running 10% ethanol fuel as compared to straight gasoline with 0% ethanol.

As for the timing of the engine, advancing the timing is desired as this allows the engine to get more power out of the engine. Obviously, you can't advance it too far as you will actually spark the fuel to burn as it is still compressing, leading to the knock sound. This is where the knock sensor comes in to play. The computer monitors the knock sensor and tries to time the spark so just a very small amount of knock occurs, essentially putting the engine at the point that it can get maximum power out of the fuel. A lot of the timing is already programmed into the computer from the factory with a given situation (ie, running high octane fuel for example). Running lower octane fuel will force the computer to back off on the timing as it may start to see some spontaneous combustion. This retarding of the timing reduces the power output of the engine and can result in a drop of mileage. So, now, the question comes, are you saving money in the end because you are using cheaper fuel with lower mileage or spending more to get higher mileage. Most people that have really looked into this say it is a wash at best. So, why risk damage to the engine for very minimal benefit.

 

My $0.02 worth......Our cars have "premium" fuel specified as the preferred fuel. In my geographic area, my choice of fuels centers around 90 octane, 0% ethanol fuel or 93 octane, 10% ethanol fuel. I generally run the 0% ethanol fuel, and both of our current Jags seem to like it.

 

He's talking about 98 octane for a 2005 XJ8. That's higher than it wants.

Will do no harm but no extra good, yet costs more.