Something interesting just occurred at the gas station.

Saw another XJS...93 pull in just after I finished pumping 93 octane into my 96 XJS.

Wanted to look at it so I walked over and he was just pressing the 87 octane on the pump.

Obviously I asked what are you doing and he stated that the XJS has an electronic sensor in it that detects the lower Octane and adjusts timing etc.


IS THIS TRUE???

Please Comment!!!!!!

Softball Paul

1996 XJS, 1957 XK140 Drop Head and every other British Car known to mankind.

 

I wouldn't even dream of using that stuff in my Car, not even 95 Octane unless I was really desperate.

I've always used Tesco Momentum which is 99 Octane and while I don't know if it's true, is supposed to give you a few more miles per gallon.

It costs about 5p per litre more, which is pretty insignificant on the over all cost of running an XJS.

The pay off is on 99 Octane my Car runs like a dream and so I cannot see the point in trying to save a few pennies by using anything else.

Being a Performance Car, I don't feel like feeding it 'Junk Food' any more than I would fill the engine with unbranded Oil.

 

Many cars do indeed have a knock sensor that detects pinging/knocking. The sensor notifies the ECU which dutifully retards the timing.

However....

I don't think a '93 is one of them

AFAIK Jaguar started using knock sensors on the AJ16 engine for 1995 model year

Cheers
DD

 

My understanding is V12 doesn't have knock sensor or anything to detect octane rating.

What they do have on some models is a map for low octane fuel for use in countries that only have low octane fuel. You have to enable by removing a link wire

 

There are different octane rating systems used in North America and Europe, so the numbers are not directly comparable. 95 in the UK is roughly equal to 91 in North America.

 

That was my understanding as well. My 1995 Technical Guide shows knock sensors on the XJ16 in the X300 range, but not on the V12 sedans. Given the system was more advanced for the sedans than the XJS, I doubt the XJS V12 had knock sensors.

 

I still wouldn't use it, if I could get a higher Octane.

Do they do a 99 Octane equivalent in the US?

 

Depends upon where you are. Some places, yes.

Octane requirements vary with elevation and temperature, so there would be no point in offering it in Denver for example (elevation 5300') as the engine simply doesn't need all that octane.

There is no point in buying a higher octane than the engine needs, it can't take advantage of the higher rating.

 

Here in Venice, Fl. the price differential between regular (87 octane) and Premium (93 Octance) is almost $.50. This is significant....it's more than $10.00 on a full tank cost differential.

Would my 96 XJS (6 Cyl.) run on regular octane? Not sure I would want to try, just curious.

Softball Paul

96 XJS, 57 XK140 Drop Head and every other British Sports car known to mankind.

 

The 96 6cyl car sould be the AJ16 engine, and Nipon Denso engine management.

That being the case, it will have knock sensors, so "SHOULD" be OK.

My X300 down here has a label on the fuel cap "95 RON Minimum". It gets our 98 as do all the others, including the V12's.

$10 is really no biggy, an engine failure from detonation will cost $5000 at least, and pistons for the AJ16 are near impossible to locate.

 

I assume that's per US gallon? Here the difference is 20 cents/litre, or 75 cents/US gallon.

 

Another plus for the higher octane 98/99 stuff (which I use 100%) is that it is much less prone to turning into jelly in the tank over the winter or long periods of not being used than the 95. Ditto in the jerrycans. That is the reason I put it into all the mowers, etc that I have. In the car, I feel for high rev blasts it is better protection against any detonation, as Grant said.
Greg

 

The AJ16 engine was designed for 90 AKI fuel, which is about 93-94 RON (European scale), so 95 should be in excess of what it needs.

I don't believe the ECU will advance timing beyond the map to take advantage of higher octane fuel, just retard if it senses knock. A question for XJREngineer on the X300 forum, as he's the one who designed the system.

 

So now I am confused.

Are you saying that running 87 octane (USA) should be adequate for my 96 XJS?

Inside my gas cap cover it clearly states PREMIUM ONLY

Thanks

SoftballPaul

 

No. The engine was designed for 90, so running premium ( 91) is what you should do. However, if you put in 87, the system will detect knock and retard the timing to reduce knock. That will reduce power and typically fuel economy.

 

Thanks for the clarification!!

I will stick with Premium and play it safe. Car runs Great and fuel economy if very good...17,18,19 city driving....Like to drop it into Sport mode and give it a go...UP to 26 highway.

Thanks to all for the response

Softball Paul

1996 XJS, 1957 XK140 Drop Head and everyother British Sports Car known to Mankind

 

The 6.0 V12 has Nippon-Denso. The AJ16 has Sagem/Lucas. In any case, the AJ16 does have knock sensors, so it is able to adjust for lower octane.

 

6.0 V12 in sedan (saloon) has Nippon Denso engine management, but same engine in XJS has Lucas 36cu (fuel ecu) and Marelli MED 123E (ignition ecu)

 

You should also try and get ethanol free fuel. It rots the rubber components.

 

I have had about a dozen V12's, both the 5.3 and the later 6.0, and also had several each of both the AJ6 and AJ16 versions of the 4.0 and racked up probably 500k-600k all combined, plus another 100k+ on my wife's X-Type 3.0L AWD, and I have noticed more difference in brand of fuel rather than in the "grade" of the fuel when using the top-tier brands.

It seems to be much more about the brand than the octane rating ~ I have had my cars run fantastic on Shell, Exxon, BP, Phillips 66, etc. "Regular" grade, yet seem to loose some MPG and a little power when running on some of the off-brands "Premium". One thing I do buy when I can get it is the "Ethanol Free" fuels, again sticking with the top-tier brands. Ethanol free DOES make a difference, I can feel a small amount of performance difference but where I really seem to see a difference is that on ethanol free my 6.0 and AJ16 4.0 both seem to gain 2-3 MPG.

But as long as I am using good brands I actually have not noticed a difference between the grades of fuel.

I have been in situations in the past where in some parts of the country premium was just outrageously priced compared to regular and I ran my cars on regular in those places (but still from from top-tier brands) during those high gas prices and have never seen any noticeable difference beyond that premium will sometimes give you an extra 1-2.5 mpg on all of the later 6.0 and 4.0 engines.

I have never had any of the tip-tier brands make any of my cars knock, hesitate, or do anything to indicate they were in any way unhappy with regular compared to premium.

Now, that being said... Since in my area premium isn't that badly priced compared to regular (about .18 a gallon extra) and since gas prices have dropped so much, I use premium, but it's really just because it says to on the tag by the fuel fill, not because I have ever noticed ANY difference in performance or the way the engines act when using premium vs regular when using the top-tier brands, beyond a small increase in fuel mileage with some types of driving.


As I said, what I have noticed IS more important seems to be the brand. When I stick with "Top-Tier" brands I really can't tell any difference in performance, and have never had any knocks from the top-tier brands. If you haven't heard of top-tier fuels, just do a web-search for "Top Tier Gasoline" and do some research.