We seem to be agreed that the car will run quite happily across the spectrum of generally available fuel grades. What I am still curious about is the question of fuel economy.

As I understand it (which could easily, and not for the first time, be wrong!) the higher the octane, the higher the "energy" content the fuel has, so a gallon of 99 octane has more energy than a gallon of 87 octane.

For a given engine in a given vehicle, the gallon of 99 octane will surely produce more "work" than the gallon of 87 octane? I understand why it might not be able to propel the car down the road any faster, as the ECU adjusts the performance, but surely it must propel the car further down the road? Whether the better MPG is sufficient to warrant the increased expenditure on the higher octane fuel will depend on how much more the higher octane fuel costs. In the UK at the moment it is highly marginal, as indeed it always seems to have been. On the basis it is cost neutral, I tend to opt for the higher octane (Tesco "momentum" is currently advertised at 99 octane), so please , if my man maths is wrong, please say it quietly!

 

Well yes @countyjag everything you said is wrong. The octane number is the fuel's resistance to self detonation. When the fuel is compressed it heats up and it might self detonate without the sparkplug and this is very bad. So that is why some higher compression engines would need a higher octane fuel. The energy content is the same. BTW energy content is measured in calories. Maybe your petrol station might post the TDS for how much energy their fuel has but it's all pretty much regulated and very similar.

 

Thank you JezuzNoChill. For what little it is worth as you consider everything I said was wrong, I agree with what you say about higher octane fuel having a higher resistance to detonation ("knock").
However, the higher octane fuel's greater resistance to knock enables more of the inherent energy within the fuel to be used by the engine, resulting in it containing more USEABLE energy. I accept that my original post would have been better had I included the word useable when discussing the energy content of higher octane fuel.

 

One of the labs I did on university was measuring the heat content of the different grades of gasoline - they are all the same.

From a thermodynamic point of view, the amount of work you can get put of a gasoline engine ( the Otto cycle) is limited by compression. The higher the compression, the larger the area under the curve on the various thermodynamic diagrams, so more work can be extracted. Changing fuels will only benefit if that particular engine can actually utilize the fuel to it's fullest, and it was limited, such as by retarding timing to avoid detonation, by a lower octane fuel. If you put 100 octane in a lawnmower engine with a 6:1 compression ratio there won't be any difference in the power it produced over 87 because it wasn't knock limited by 87 octane.

The engines in the X300 were designed for 90 AKI( 95 RON in Europe) so running a higher octane won't help because it shouldn't be knock limited at 95.

 

The way I understand it, the knock sensor can help adjust the timing to adapt to the lower grade octane, but once the design octane has been reached, there’s no compensation for greater octane….does that make sense?

The engine is tuned to run on a certain level of octane. If you go lower, there’s a sensor to help the engine adapt to that. If you go higher, the engine doesn’t know it’s getting more octane so it can’t make any adjustments to gain any benefit.

 

I've always thought that the octane number in itself is not relevant to power/fuel consumption. However, the higher octane fuels were marketed as providing improved fuel consumption and I do remember reading a product test where this was found to be the case but not by a sufficient margin to justify the extra cost. The benefit came not from the octance rating per se but from other additives in the fuel. As the higher octane fuels are E5 and the 95 octane fuel is, or soon will be, E10 the latter will contain less inherent energy due to its higher bioethanol content. The most important factor for me is whether using E10 will overtime be detrimental to our vehicles? I've just checked the UK government website which says that all Jags produced from model year 1992 are compatable with E10 but I'd like some reassurance from technical experts before using it.

 

I read a lot of theories here but experience is the best way to find out what the consumption and performance are of an AJ16(with Andy bracket). My experience is that on average I drive 70 km more with a full tank of E5(98oct) than with E10(95oct.). Also liters of fuel and kilometers driven confirm this.

 

I'm at expert at nothing.

Anyhow, and FWIW, E10 has been standard issue in my neck of the woods for 20+ years. I've never had any problems/damage using it in my 80s-vintage Jags nor my 1995 Jag. It also works fine in my lawnmower and other yard equipment.

Non-ethanol is available in my town,...at a much higher price. I've tried it in my oldie Jag and certainly didn't feel any difference. But I wasn't really expecting to....and seat-of-the-pants obviously isn't a valid test.

Cheers
DD

 

We are in danger of mostly agreeing here!

There seems to be no suggestion of any harm arising from the use of 95 or 98 octane ( the thread starters question).

For there to be a benefit available on the X300, the ECU has to be "tricked" by the fitment of an Andy bracket.

The resultant benefit is marginal, and may not be economically beneficial depending on the price of the higher octane fuel.