Petrol

[jwalshe]

Hello all!

Obviously assuming a CX 2.5 happily takes unleaded fuel, do you think there's a benefit to using something like Shell Optimax in a 30 year old car?

James

[CitroJim]

Yes, as petrol was generally of a higher quality in those days.

[Deanxm]

Yes the engine will perform best on the grade of fuel it was set up to use, modern cars can cope with any old rubbish as they have the ability to vary the ignition and injection timing to suit whereas your old car cannot and will perform best running on the grade of fuel it was tuned around.

D

[jwalshe]

Thanks chaps - actually had no idea that was the case. Optimax it is then!

James

[babu]

My XJ6 does not benefit in either performance or economy (according to the trip computer) using premium fuels (it's a 1990 model - but was designed in the 1970s).
The advice I was given on the Jag forums I frequent was that only ricers with very highly tuned Subarus, Mitsubishis etc will get any noticeable benefits from these products. I find it's definitely more economical not to use them.

My CX is dead - looking for another now...

[jwalshe]

And you trusted what a trip computer on a Jaguar told you?

Just kidding. Lovely car, the XJ6. My father had one. Just a shame it broke down at every available opportunity - and always picked the most inconvenient location. Middle of the Severn Bridge was a good one!

[Deanxm]

Higher octane fuel can only benifit performance if the engine has the ability to change alter the ignition timing keeping it running on the verge of detonation so to reap the benifits of higher octane as a general rule if the engine management has a knock sensor you will notice a slight difference in economy and performance, if it does not you may not.
The trouble is these fuels are so much more expensive you will never get better mileage for your pound.
I like to put some high octane fuel in the Xant sometimes as it does make it run smoother with a noticable amount of extra go.

D

[CitroJim]

I like to put some high octane fuel in the Xant sometimes as it does make it run smoother with a noticable amount of extra go.

I find the same both in the V6 and Activa.

I would say the CX is best on Super as I would imagine it was designed originally to run on 4 star leaded which if memory serves was 97 Octane and when 95 Octane unleaded began to be introduced there was much speak of those cars designed for 4-star needing their ignition timing retarded a few degrees to cope with it.

Running super (98 Octane) will allow the ignition timing to be set to manufacturers recommendations.

[Deanxm]

Reading my post again i may be confusing the issue, when i say the Activa runs better on high octane what i mean is it runs worse on lower octane, lower octane gives a higher chance of detonation so with the boost turned up and running 92ron the ignition is being retarded to prevent the detonation, higher octane reduces the risk of detonation and allows the ignition to be more advanced under these conditions allowing the engine to run more efficiently.

Basicaly, run with the octane rating it tells you to use in the original owners handbook, god ive made a meal out of this

D

[CitroJim]

god ive made a meal out of this

No you haven't Dean. It's all good stuff