I have a friend that has a 2000 XKR. Says that except for pings and internal detonations, that the grade of fuel does not matter and the cars that say they require unleaded premium will run just fine (maybe not quite as hot) using regular octane rated fuel.

I really do not think that is correct, but not an expert like many of you on this forum and am curious as to what effect reg. octane will have on the Jag engines (and frankly other brands) that indicated they require the highest octane.

Thanks

Tom in Dallas/Plano

 

There's some truth and some myth in what your friend has stated. The octane rating of gasoline is strictly a measure of it's resistance to detonation (knocking/pinging) and nothing else. It is not a measure of a fuel's 'quality', purity, type of additives, quantity of additive or anything else.

Contrary to myth, high octane gas does not burn faster/slower/hotter/cooler than low octane gas, not does it contain more/less energy.

Some engines, usually those with higher compression ratios are prone to detonation, which is an undesirable 'explosion' of fuel, rather than the normal burning (deflagration) through combustion.

To avoid detonation, the fuel is blended with various compounds and additives to reduce this tendency to detonate which in turn allows the engine to produce it's full rated output.

Old school engines had no built-in controls to detect detonation and it was up to the driver to listen for the telltale bearings in a coffee can pinging sound in order to avoid engine damage.

Today's engines have knock sensors which are far more sensitive than human ears and retard ignition timing which is usually all that's required to eliminate detonation. Retarded timing will have the effects of reduced engine power, as well as increased fuel consumption.

Your friend is correct that aside from detonation (pinging) there are no other side effects.

As to whether you could/should operate your Jag (or any car designed for high octane fuel) on low octane, you'll get 12 different answers for every 10 people you ask. I've operated my S-type on 87 octane and notice no difference in power or economy. YMMV.

 

Since is there and I looked.... I hardly ever use premium. 87 or 89 is the standard here and I use that. What I have noticed is the cars preference for a particular brand. I don't know why.

 

A small worry for me is that the 2000 XKR uses an older, simpler, cruder PCM and may not be able to compensate properly for lower octane fuel. With the extreme cost of an engine vs the slight extra cost of fuel I personally would avoid using low octane fuel.

I can't imagine having such a nice car and taking such a risk.

 

That makes sense to me. Further why would anyone want a supercharged car that runs at less than optima performance?

Tpm

 

Being as I am considered "ancient" and been around Jags all my life, and am biased a small amount.

We used to have "Standard" and "Super" as our only 2 grades of fuel.

I used our Standard back in the MK7 days. It was heaps cheaper than the Super, and the car really did not care too much.

Compared to what my mates were driving, the Jag went like stink, and was what is was, an awesome car.

My later models got the Super.

Then came our adventure into Unleaded.

3 fuels on the chart. 91, 95, 98.

Being as how the 98 was about the same price as the Super was, and my understanding that Super was around the 98 mark anyway, I simply went with 98 in ALL the Jags and have never given it any more thought.

I tried our 95 in the X300 out of need (country outlet only had 95), and to me the car felt sluggish.

The S Types only get 98, no discussion.

I know different markets have different "oomph" fuels, and then the Ethanol blend (quite rare here), has me understanding (right or wrong) the engine management systems are "tweaked" to suit that particular market.

Like our V12 HE engines got 12.5:1 comp ratio, the USA got 11.5:1. The truth as to why, who knows.

 

There's no evidence of any technical difference, at least on the S-types, between Euro, N.Am and down under specs that I have ever seen when it comes to with engine ratings, compression, octane etc.

Keep in mind that manufacturers are required to follow certification rules, primarily for emissions purposes, when it comes to saying how much HP on what fuel with so many grams of CO2, NOx etc. Yes, I know- 'Volkswagen'. ;(

There's no evidence beyond 'butt dyno' that using higher octane than OEM recommendation does anything one way or the other. We have on fuel brand here that sells 94 AKI/98RON but I'm not sure why. No cars sold here require that high of an octane rating.

Ethanol is not part of the discussion when it come to octane rating. If the fuel is sold as 98 RON, it is irrelevant whether it contains 0% or 100% ethanol. It's still 98 RON.

 

Possibly he's found through practical experimentation that the engine does not experience detonation with his usual driving style or possibly Jag's factory advance curve is conservative enough that it's difficult to induce detonation. The latter is true with the previous generation of engine that could use the 'Andy's bracket' mod to correct.

I suspect the same combination with the modern S-types.

 

Possibly. I really can't see the point of modest savings (if any - mpg may be reduced) when severe engine damage liable to write off the entire car may be the alternative.

The 2000 car has a much reduced ability PCM as compared to the one in your or my S-Type.

 

Relatively speaking the differential cost of the various grades of petrol are so small, compared to the overall cost of running a car, that I can rationalise using lower grades of fuel.

Benefits for me are the cleaning additives (they are present and do work) and its a higher spec product that gets less messed with at oil terminals to get it in spec. I have zero cars that need more than 95 for combustion, but use the 98 products. (even in the Morris which I think would run on kerosene).

Some creative blending is often done in terminals to get product in spec either because it is just a poor batch, or gets contaminated (few thousand litres of the wrong product goes into the wrong tank)

 

Again- octane rating has nothing to do with the volume or type of additives, nor is high octane gas any better quality than other octane levels. There's no technical reason to explain why a lower compression, less efficient engine would need less additives or could deal with inferior quality fuel as compared to a high performance version.

These are both old myths and misunderstandings prompted by exaggerated advertising claims from the fuel manufacturers.
.

 

Ahem .. the official stance of Jaguar according to the 2000 XJ Vehicle Care Booklet is:

(North American Version, hence the reference to AKI)

at Specifications 6-9

It is highly doubtful that the XKR would have a different spec
than a XJR.

Those who are in the habit of telling others to look in the
manual should now feel free to argue why in this instance
they feel the the manual should be ignored.

++

 

Who suggested that the manual be ignored?

 

For our supercharged engines I wouldn't even consider lower octane, unless the power of the engine isn't used.

Over time safety margins can become smaller, being it fuel injectors causing leaner mixtures in certain cylinders, or carbon deposits causing hotspots, a higher octane fuel will provide some small extra insurance there. Even more so for those under us that tune the engines as there you already start tampering with safety margins for the full engine.

 

Not sure who said anything to the contrary, you appear to be arguing with yourself.

The Shell and BP 98 products as sold here have cleaning additives that the the other petrol grades do not (you can call them by their pump names V-Power and Ultimate if the RON number is a problem). They do work (I know the Shell one does I worked for them for 8 years, I'm guessing the BP product is basically the same), its just a matter of the value you place on that. May be good, may have no measurable effect over the life of the car. We all make value judgement on these religious items like oil, tyres, spark plugs and of course fuel.

You used the plural about myths , dont know what else you refer to.

 

Ah- you're now referring to 'brand names' and not 'octane ratings', followed by broad statements that some brands are better than others.

This was not the OPs question at all, nor the topic of discussion by anyone else.

Here's an interesting discussion on the power of advertising done by one of your fellow countrymen, and directly attacking one of the two brands you mention. His observations are spot on. Caution: NSFW

 

Well its on the Internet it must be true :-)

Motoring journo's are much more reliable sources than petroleum chemists.

I do share the feeling though that much marketing is BS or at best gilding the lily in one form or another. We shouldnt confuse the horrors the marketing people perpetrate with the real products.

 

Here's the disclaimer we get for the BP fuel

"Ordinary Fuel refers to minimum detergency gasoline. Dirt refers to deposits on critical engine parts. Requires continuous use over 5000 miles. Restores an average of 3 - 5 miles per tank that had been lost due to deposits. Based on fleet testing representative of the U.S. car population. Fuel economy can be affected by many factors. Benefits may be more significant in older model vehicles."

The same journo published this the other day. Also NSFW


worth a listen.

 

I have read this same discussion many times on the forum.

What it comes down to is that some folks are just very frugal (AKA cheap) in all aspects of their lives.

They will put a 40 watt bulb in a 60 watt fixture to save a few pennies on the electric bill. They buy bargain brand batteries, toilet paper, tires, oil and fuel. Keep the heat on low and the A/C on high and suffer the cold and heat along with their families. They will drive all the way across town to save .02 per gallon on gas, burning $4 worth to get there. They clip coupons, shop sales and resale shops, and wear the same shoes, belt and jeans for 20 years.

They love to talk about how much they "saved" on anything they bought and why you got taken on your purchase of a similar item.

The last jewelry they bought their wife was her engagement ring 25 years ago, which may be actually be a cubic zirconia. Who is stupid enough to pay that much for a silly "rock" anyway?

They have convinced themselves that we are the foolish ones who "spend more that we have to" to get the same results. And usually, you cannot convince them otherwise.

They also buy luxury, high performance cars used because they let someone else take the depreciation hit because they are too "smart" to buy new, then run them into the ground for lack of proper maintenance, fluid change requirements and such. They are smarter than the automotive engineers who make up the specs and refuse to be taken in by the higher costs of the maintenance recommendations suggested by the manufacturer.

They are smarter than you too.

They are universally loved and appreciated by local merchants, neighbors and their in-laws, who run when they see them coming. They are a lot of fun to be around too.

You will know when their funeral motorcade finally goes by. It will be the one with the hearse towing a trailer full of all the money they "saved" over their lifetime.
Vector

 

I've only had my Jag 2 months and I've been burning regular because my understanding has always been that Premium is a waste of money unless you need it. And if you engine doesn't ping on regular, than you don't need Premium.

However, now that I've read many of the responses on here I think I'm going to start using premium to see if performance or gas mileage improve.

I live in Missouri and I know some years back they passed a law concerning the octane ratings purported at the pumps because they felt they weren't truthful. I haven't been out of state for awhile so I don't know if it's still the same but regular used to be 89 and premium was 93 octane. Here it's 87 and 91 and the mid-grade is 89. That may mean I can use mid-grade and satisfy Jaguar's recommendation. For now I going to go with premium for a while and see how it goes.