New Jaguar owner of XJ6 Sov., 91. Drove ~600 miles using regular gas, before I found out that it needs premium. Should I be worried about engine damage?

 

Firstly welcome to the forum.

When time permits, please do a formal intro in the "New members area".

Your XJ6 is known in Jag world as an XJ40, so bear that in mind when asking questions.

As to the fuel you have used, WORRY NOT, the car is fine. Maybe a tad sluggish, but fine just the same.

Give it a treat of Premium, and enjoy the car.

 

engine will not be damaged. Jaguar engines are designed to run fine on 87 octane fuel.

 

Thank you! No wonder the car been feeling sluggish lately.
I heard from other people about knocking, pinging, high pressure engine, high octane and other things. I understand the car is too old to have a knocker, but what about the mpg efficiency?
If not for performance, will premium improve it's consumption efficiency enough to warrant it's use? Heard mixed things about it, so I'm not.
Right not the tank is nearly full of regular so won't be able to try premium in at least a week or more.
Although I might just get some octane boost from a shop..

 

I understands that this is correct. Jaguar engines of the eras were tuned to achieve full performance on 87 (regular) octane fuel.

Using higher octane fuel will achieve nothing other than lightening your wallet.

 

Anything but that. They're pure snake oil.

 

Your market is way different to mine, as are the cars themselves.

I would concentrate more on the required Jaguar SMILE whilst behind the wheel than the fuel it drinks.

I have run our 98RON in my Jags for as long as we have had it available. That way I dont have to strain the brain as which Jag I am driving and what fuel the thing really needs, simple man that I am. Even the old 3.8 MK10 gets 98.

My S Types require Aust 95 or better, according to our books, so they get 98. The V12's, weeeeell, that was a long time ago, and the handbooks were printed in the early '80's, and we still had TEL fuel, so they got that, and then the 98 when it was phased in, and went like stink.

Octane boosters etc are just a waste. Many of them have TEL in them, and that will cost ya a motza if the cat convertor sees that stuff.

 

yeahh, lots of $$ more octane $$ in their pockets and less in yours!!

Questions:

1) when you open the fuel filler cap, does it have a metal tag that says "USE 87 UNLEADED ONLY" ??

2) do you have the Owner's Handbook? In it you will find the fuel spec required.

If it is a XJ-40, it's probably a 4.0 liter, while the Series 3 XJ-6 has a 4.2 liter engine all the way to 1987 and each fuel filler has a tag that reads: "USE 87 OCTANE UNLEADED ONLY".

Otherwise, how did you "find out" that it needs so-called Premium fuel ??

 

The myths of octane are very deeply ingrained in almost every corner of the interwebs.

 

I might be new to these forums, but it's strange to me how so many here against the supposed difference in fuel being real. Correct me if I'm wrong but isn't the 4 liter sovereign engine a high pressure, short piston engine which requires higher octane concentration to run properly, otherwise it loses power and gets weak gas consumption. As you all know octane is fire retardant, which lowers the combustion of fuel. The reason regular isn't right fuel for such engines is because it has low octane concentration and pre-ignites, thus the engine feels sluggish, loses power and spends more fuel than necessary to keep up. Unlike premium - higher octane fuel. The engine stays cooler with less excessive combustion, keeps up with pressure demands. Of course there wouldn't much of a difference when the engine load is very low, but when it rises, the difference, to me, was very significant.
I understand for other engines, and computers on wheels that might not make a lot of difference, since they have knocking tech which could compensate for the difference.
As for the experience, I've raised octane in my gas tank (regular fuel) by around 2 points and even though I have only been driving 2.5 months, I can already feel the difference. The engine runs quieter, and has significantly more power. Oil pressure stays higher in 80 degree sun. I can feel how more powerful the acceleration has become. And the best thing is the gas consumption:
Just came back after 2 hours of driving and around 150 miles, and my average highway mileage rose from just under 13 to 17.5 mpg! I don't know what kind myths you mean, but some engines specify gas for a reason. And the difference, changing octane concentration, made to my car, speaks volumes about the difference of fuel. As far as I know I haven't changed anything other than fuel.

 

I'm afraid your understanding of fuel octane ratings and their relevance to combustion is way off the mark. All gasolines, irrespective octane rating, burn at the same speed and contain the same amount of energy. One does not 'make more power' than another. That's a common myth. High octane gas is more resistant to detonation (pinging) which has nothing to do with pre-ignition. If an engine does not detonate on a given grade (such as 87) then there is nothing to be gained by using higher octane.

Your engine has a rather mild compression ratio of only 9.5:1 and produces 225 HP from 4 litres displacement. This is well within 87 octane territory.

I'd find out for sure from a reliable source what octane rating your car was built for and go with that, not that I doubt Jose's info above.

 

It seems you misunderstood my comment. I don't believe I said fuel types have different power. I know that all fuels have the same energy. What i was talking about is detonation temperature, which changes with octane concentration. I never said that engine does not detonate with 87 grade, what I did mention is that with higher octane detonation doesn't happen too early, as it could with 87. Which might've been happening to my engine - I don't have proper instruments to diagnose precisely. Also by pre-ignition i was speaking about knocking, which could happen when lower octane fuel is used in place of higher octane as the ignition could happen sooner than needed.
As I said I don't know what exactly was happening to my engine, but what I did intrigue me was a noticeable change in the performance.

 

Detonation has little to do with combustion chamber temperature as it is not heat that provokes it. Detonation is initiated as an abnormal explosion event (hence the name detonation) , vs normal burning (conflagration) of the fuel. Most common causes are excessive ignition advance and/or high compression. Your engine has neither. You're also confusing 'pre-ignition' with 'detonation' which have nothing in common.

No, that's 'pre-ignition'. Did you actually hear any detonation?

Again, the engine by all accounts does not detonate on 87 so any gains you may have noticed by using premium are difficult to explain.

 

Seems to me that this would be an important point to nail down.


Cheers
DD

 

The octane myth again. My cars do just fine on 87. Built that way, a higher octane would do nothing more than reduce the weight of my pocket book.


Around 9 to 1 ratio seem to be the cross over as to the need for higher octane. A very general statement, though. Every engine has it's own needs.


The best place to start is the manufacture's recommendations. Caveat, things, including fuel has changed. The gasolines of today are not those of thirty years ago.


Octane boosters are snake oil.


if one wants to burn off a tank of inadequate 87, just back the ignition timing off a tad and drive gently. Dilute with the highest test at abut 1/2 tank.


Jose:


My gas caps say, actually only one, 'unleaded". No mention of 87 ???


A look n my 83's manual would not help!!! but, perhaps in my 94 Cadillac shop manual...


Carl


Carl

 

Actually, no. Not with respect to octane. 87 octane today is identical in terms of knock suppression to 87 octane of thirty (1985) years ago. Forget the ethanol content, it does more good than harm in this respect.

Where it gets confusing is realizing that there are two different octane rating standards in use, essentially one unique to N. America and the other used everywhere else. When our friends in the UK or downunder speak about using 95 octane, that's expressed in the RON rating system. 95 RON is equivalent to the 91 AKI fuel we get here.

N. America used the RON system up to the mid-'70s at which time we switched to AKI- which typically rates a fuel at 4-5 points lower than RON. When people talk about the 'good old days' of Sunoco 260 and it being 98 octane, it would be rated 93 or 94 in today's system. As an aside there were very few cars back in the day that actually needed or benefitted from those type of fuels but legends die hard as has been mentioned.

Today's high compression (11:1) and supercharged Jags have a factory recommendation to use 91 AKI fuel. The OP's engine being part of the 'low compression' family needs only 87 AKI.

 

Mikey:


I am far from a petroleum engineer. And, you have a far better sense of the methods of octane rating than I.


But, at least in CA, the refineries have a winter and summer blend. Unknown, back 50 years ago. And, way back when, each refinery had its own idea as to what gas ought be. Rough at best as to octane rating.


Around here MTB was used. a benefit as to the fuel. awful as it leaked into the aquifers.


And, E10 or better, a whole other world.


But, as usual, I say to each his own.


Carl

 

Woops, typo. Not 50, only a mere 30!!!



Carl

 

Yes we have summer/winter blends here (nothing new here in fact, possibly new in CA) but this has zero relevance to octane ratings. 87 is 87 year round.

The main difference between summer and winter blends is the RVP (Reid Vapour Pressure) of the fuel. Generally speaking, higher RVP in the winter to make engines easier to start, lower RVP in the summer to reduce emissions.

Again, no connection to an octane discussion.

 

I don't know anything either about octane ratings, all I know is that we are getting ripped off no matter what octane we pay for.

Has the car in question been sitting before you bought it? That could explain many things. I would look in the Owner's Handbook, the little Handbook inside the glovebox that comes with every car. It will tell you what fuel to use.

Carl,
the 3.8 liter carburetted engine in my S type is 9:1 compression, it runs fine with 87 octane. That was my first worry when I bought it, Am I going to have to do work on the valves and such? I was told by several mechanics, "No, it will run fine with regularl unleaded".

But this is the first time I hear of a 4.0 liter XJ-40 requiring higher octane fuel.