There have been many threads about whether premium or high octane gas must be run in a Jaguar. For the past 14 months and 24K miles I've been running my 04 VDP on 87 octane. Since I have a very light foot I haven't noticed performance problems. I did notice a hesitation sometimes when first stepping on the gas from a complete stop, but I thought that had more to do with the transmission.


Then about two weeks ago I threw a catalytic converter bank 1 fault code. It cleared on its own and then tripped again after a hesitation when starting up from a complete stop. Then the engine light cleared itself again after a couple days. My theory is that the lower octane gas may be affecting the emissions system because of its long term effect on engine performance.


I changed to 93 octane gas and ran some Cataclean and later some Lucas cleaner fluid through. I don't have the hesitation upon startup anymore.


Any thoughts about this idea of the effect of 87 octane gas?

 

I don't know about catalytic converter problems but I have seen at least one difference between 87 and 93 octane. When I got my '05 XJ8L I, too, tried 87 octane and didn't seem to notice any performance difference.

But then, over the course of a month, I made two identical road trips: from home to son #2, then to son #1, then son #2, then home. Total of 668 miles. Each time I did all the driving, took exactly the same route, had the same passenger/luggage load in the car. I did one trip with 87 octane and the other with 93. (there was enough time and mileage between the two trips to purge the 87 octane and make sure I was running entirely on 93)

What I found was that the 93 octane gave me 7% better gas mileage, for a 4% increase in price. So it's cheaper for me to buy the more expensive gas! With more careful observation I think I noticed that she took some of the hills without downshifting when I ran 93, but did downshift when on 87.

As they say, your mileage may vary.

Ray

 

It's unlikely that there's any connection. The ONLY difference between low and high octane gas is the resistance to detonation (pinging) which has no effect on the anti-pollution equipment.

Part of the increase could have been the % of ethanol in the low octane fuel. Many high octane fuels are ethanol-free which will give a 3-4% boost over E10.

 

My vette manual recommends high octane but says it's no problem to use 87 but to expect a very slight decrease in power. Jags are probably the same. The computer adjusts the timing back a smidge. (Smidge is technical jargon for a pinch). My seat of the pants dyno can't tell any difference, though I'm sure there is a smidge.

Mikey, good point about the ethanol content.

 

The decrease in mileage is due to running the engine with automatically retarded timing to prevent pinging.
There is a difference in regular and high octane fuels. The lower octane fuel actually has more BTUs per ounce and burns hotter. I doubt the temp difference would fry a converter, but there is a difference.
An engine running retarded also runs hotter, so maybe if you are combining both heat increases, you might affect something.
When not following manufacturers recommendations, you should expect some difficulties.

 

I see no difference when switching from 93 octane to 89. I see a noticeable reduction in performance, and an occasional rattle, if I put 87 octane in it.

 

The BTU variation is immeasurably small and can vary more from one batch of high octane to another more than low octane vs. high octane.

There is no variation in burning temp associated with octane level. That's an old myth and a common misunderstanding of what detonation is and how higher octane level fuels avoid it.

 

It all depends on how clever the Engine Management System is.

On old mechanical (bob-weight) and vacuum advance ignitions (distributors, remember them ?) then using a higher octane makes no diffrerence at all ... unless ... you also retime (advance) the ignition when using a higher octane.

The down side is you must retime the ignition the other way when going to a lower octane, otherwise you'll get a lot of detonation (colloquially called 'pinking' in the UK)

Modern ignitions with detonation sensors in the cylinder block(s) will automatically keep adjusting to be just short of the detonation range. If you fill up with a higher octane then the system will find, when it tries out advancing the ignition, that it can run with a more advanced setting ... which will give a slight power increase.

 

I use premium in both my jaguars the XJR6 is necessary as it is highly modded

The XJ8 I don't use a lot its a distance car no round town usage as I hate traffic also the premium fuels have a lot of cleaning agents in them having had to buy injectors is costly and have had to do that in the XJR6 because premium fuels only got popular here in Australia in the late 90,s so for its first few years it would have been run on base unleaded

As for the XJ8 I am the third owner the second was a enthusiast like me but the first was a doctor who probably just used what ever he could get at the time

When I replaced the catalytic converters the originals where actually crumbling inside car ran fine no codes and quite economical I thought

I haven't done a decent trip with the new cats but I have noticed it feels stronger

I cant see the rationality of using low octane fuel in such an advanced engine you might not need the ultra high octane but I wouldn't use less than the recommended octane

If you are worried about fuel prices and trying to save money by using lower than recommended octane cut back elsewhere to save the money

I would hate to have to price what a new engine for one of these cars would cost

 

The supercharged engine has a slightly lower compression engine. Usually that would mean it does not need such a high octane. Do we know if that still applies with SC?

 

oh dear no. You need high octave for any forced induction engine (as a general rule; there are exceptions). Lower compression yes, but you are shoving the air into those cylinders which causes lots of heat (and a not very efficient SC IIRC), which causes pre ignition.


I'd never not use the highest grade available with an XJR.

 

Using octane lower than recommended has the potential drawback of reduced engine power and increased fuel consumption. Using fuel with octane higher than recommended is of no benefit, just a waste of money.

 

So it sounds as if no one sees a connection between the octane rating and the catalytic converter fault. And there are lots of good reasons to use 93 and not use 87. Is it possible for an engine misfire or some anomaly type of engine action to cause the P0420 code? It has tripped twice now but has once again cleared on its own. I notice nothing unusual about engine performance.

 

I couldn't say that the octane had something to do with the cats either maybe maybe not my car has 165,000k,s on the clock that why I replaced the cats on mine

They did look crumbly inside still got the old ones in the back yard am going to take them to the scrapers as they are still woth a bit of money

 

Some years ago, we had Caltex 100 octane pump gas and I was using it in my Mitsubishi Airtrek (Outlander) Turbo and it ran great. Then they stopped selling it as very few people were willing to pay the 5% difference in price between the 100 octane and the 98 octane. When I was forced to switch to the 98 octane, I too realised that my fuel consumption suffered, by about 10%. Ever since then I've given up the idea of trying to save money by using lower octane fuel as my fuel consumption increase negates the cost of savings at the time of purchase.

 

Again I can see no connection. Your car was built to achieve full performance on 91AKI, not 93.