Just wondering ...

Here in the UK, I ran my old 1980 Series 3 on 95 Octane. I used to use what we called 4 Star that was slightly higher octane rating. In the UK we use the RON system for octane rating, but the US uses another one which gives a lower number for the same stuff.

We can buy a higher rated petrol which is about 98 RON. I don't use it as it costs a fair bit more and we pay enough already with the government robbery tax. My modern Jaguar, an X358, runs on the 95 RON as recommended by Jaguar.

I have been running 89 on mine and apparently im supposed to be using 91, i have no pinging and she runs like a scalded dog. Seems unlikely that higher test gas will make her perform any better, if i bump up the timing to allow higher octane i get a rougher idle... thats why i was asking.

My '87 Series III XJ6 ran fine on 87 octane....except in extreme ambient heat when I got a bit of pinging. So in August I ran 89 and then back to 87 for the next 11 months

I don't recall the recommendation for 89. Is that in the owners manual?

Cheers
DD

Our 98 in all of them.

Whatever it converts to in the rest of the world.

In continental Europe we have a different octane rating than in the USA.

We have RON 95, RON 98, RON 100, RON 105 and RON 110

The USA used a different method of determining the octane number.
In the USA the octane number is an addition of RON + MON octane, divided by 2.
In Europe, only the RON number is stated.

This would result in the following translation:

RON 95 + MON 87 /2 = US Octane 91
RON 98 + MON 90 /2 = US Octane 94
RON 100 + MON 91.5 /2 = US Octane 95.75
RON 105 + MON 95 /2 = US Octane 100
RON 110 + MON 99 /2 = US Octane 104.5



I run her on RON 98 (US 94) and higher only

I use nothing but 87 octane.

Book says 91 which is.genereally reserved for higher .compression motors or thats what i was always led to believe anyways.

Darren MB- what year is your car?

The manual in the pic indicates 91 octane but doesn't say whether it's RON or AKI- not very helpful. North America used the RON system up to the mid '70s. If Jag meant RON, that's about 87 AKI.

Would know the compression ratio of the engine? That would be another clue.

I don't think it is that easy. But, as Grant says, use the highest and all except one's pocket book will be well.


How old are the manuals??? How old are the cars? A lot has changed, including fuels.


And driving conditions? Driver habits? Climate. Elevation.


And, as CA does, others may also do. A different blend for summer than winter.


My 94 Jeep thrives on 87 as does the LT1 in my Jaguar. So, I don't "lighten" my pocket book for 91 or 94.


Way back when, I messed with a pair of Toyota FJ40 engines. I made one from two. Each had issues. Closed chamber head on flat top pistons. Opposite of the way they came... Ooops, bad ping on anything less than 94. But, once satisfied with the latter, it never ran better. Other mods found their way on....


Carl

Actually it is that easy with a little bit of information and research.

Many things have changed with gasoline over the decades but one of the constants is octane rating. It amazes me how the old myths and misconception on the subject won't go away. Not everything is rocket surgery. No need to guess at what the car requires.

My car is an 83, dont know if high compression motor was an option, but car has limited slip diff as well, it is just a 4.2 not the v 12 so i dont know if a previous owner did some mods, or it came that way.

AFAIK, Jag listed their fuel requirements strictly in RON during that era leading to quite a bit of confusion for North American owners where fuels are rated in AKI.

By 1989ish, the manuals were clarified by adding RON next to the octane level required.

I take all of this to mean that your '83 was designed to run on 91 RON, which is 87 AKI.

Reference

What Octane Gas to Use

As far as I can see, the US has petrol (gasoline) grades on sale which are a lower RON rating than fuels sold in the UK and Europe. Of course the equivalent to the European grades is also sold so there is more choice. Certainly in Western Europe, the lowest we get is 95 RON with many garages selling a higher 98 RON fuel that costs significantly more so is not so popular. So only two grades, whereas I can remember four when I was much younger and it was all leaded petrol. We could buy 2, 3, 4, and 5 star petrol although the 2 star disappeared fairly quickly as did the 5 star that was about 101 RON

Usually, with Jaguar XK engines, they are happier on higher octane petrol like our 95 RON. The problem comes when engines get rebuilt and heads skimmed thus raising the compression ratio.

That's correct. Over 90% (possibly 95%) of vehicles sold in North America are designed to run on 91RON/87AKI octane fuel.

As with Grant I run all my cars on 98

I run my saab turbo on 97 octane usually shell v-power, but it is a turbo.
The OH focus I use normal 95 octane but run a tank of the shell V-Power every so often to keep the engine clean. I could use an engine cleaner additive I suppose but it's convenient to just fill up.

It seems to make a difference plus it gives me peace of mind.

When the car was last serviced they offered a "recommended" engine clean, I told them what I did and they said that's fine then.

The 1978 V12 compression ratio is 7.8 : 1 for the US market. I run 87 AKI.

I run 98 in the XJ6, unless I can't get it in some out of the way places. Then 95.
Never 91, it doesn't like it.

mine runs 98, i was told the 86 series3 were a slightly higher compression ratio but i dont know how true that is, goes ok on 95 but pings over 4000revs, terrible on 91 but i guess i could play with the timing if i wanted