I can only get 91 in San Diego but 100 unleaded is available at $9 a gallon. I have mixed 91 and 100 to make 93 for my Nissan 350Z and it makes a difference.

 

The fuel available in the UK is around 95 for normal, and 98 for premium. I do notice a difference with premium, but it is only 1 or 2 mpg (not to be sneezed at when you are hovering around 20-21 on a good day!) . The difference which makes me buy it is that the car feels as if it wants to run on smaller throttle openings, and just feels smoother.

 

North America and Europe use different octane rating systems for petrol/gasoline. 91 octane in North America is the same as 95 octane in Europe.

Jag certified the cars as delivering full rated performance on 91 octane (North America). Using even higher octane fuel would only make a difference if the engine were to experience detonation and resulting reduction of ignition timing advance while on 91. This is not likely.

Higher octane fuel does not necessarily contain more or better additives contrary to common myth and in most cases doesn't.

 

My 95 runs fine on 89, and I don't see a difference in mileage.

 

Uh-oh! (ducking for cover)

 

On that topic, I've thought the following was an interesting article:

Fuel Rating - Octane Comparison - Dyno Tests Graphs - Hot Rod Magazine

.

 

There's very little info in that article that applies to a modern EFI computer controlled engine.

Some parts of it:

"But most of all, we discovered that our presumption that higher-octane fuels burn slower than lower-octane fuels (and therefore require more ignition lead) is largely incorrect. There are too many other fuel-formulation issues at work to assign a general rule about octane. Race fuel tends to have a more powerful formulation than pump gas, regardless of octane rating, because it is denser and can release more power and heat. (Note that we made the best power with 114 octane with the least ignition lead, indicating it had the fastest burn time.) California pump gas is blended with methyl tertiary-butyl ether (MTBE), alcohol, and other ingredients damaging to performance. "

are just down right bad science. Octane rating has no relevance to how fast a fuel burns. No fuels in California or elsewhere are blended with MTBE.

 

Yes it is

especially:

Not worth the expense to buy racing fuel, but it points out that the additives
do have an effect on burn rate and energy density.

They also contribute no or less energy by volume than the equivalent amount
of pure gasoline that they displace. In other words, anything that displaces
gasoline in the mix is likely to make the fuel less energy dense.

No, Shell V-Power 91 octane without ethanol is not available
in CA as reported by other members repeatedly.

But, it is available to me, and I have repeatedly measured
fuel consumption to be lower when running it on the same
long distance route at the same speeds as compared to
Esso Regular 87 octane with 10 percent ethanol.

Due to pricing practices, the actual dollar cost is about the
same. I pay more per unit, but get more energy per unit.
So cost is a wash, but I get to avoid the ethanol and more
power on tap. At least the throttle is crisper. The choice
becomes very easy then.

 

Ditto. I use regular (89) in both my XJ6 and XK8 with no problem what so ever. Tried testing the performance with the highest available and cannot see any difference.

 

I can't see how the energy content and knock resistance of a given fuel mix knows or cares if the engine is fed by carburetor, mechanical FI or EFI. EFI may have more complex ways of detecting knock and modifying timing, but the propensity for a given fuel to pre-detonate (all other things being equal) is primarily due its own chemistry.

Also MTBE was used extensively for years in California, including around the time this test and article was published in 2001. I remember the MTBE issue well when I lived there:

https://en.wikipedia.org/wiki/MTBE_controversy


I'm not arguing or making a case for using higher or lower octane rated fuels for a given application by referencing the "Hot Rod" article above. The discussion was whether a given octane rating improves the running of an X300 and it reminded me of an interesting article I read recently on the topic of octane and engine performance.... so I shared


.

 

Agreed (other than there's no such thing as 'pre'-detonation sorry, personal OCD thing) but it was the type of engine and it's controls I was referring to. Old school engines as used in their test had crude fuel supply and ignition advance systems. Their observations of trying different fuels with different ignition advance settings doesn't really apply to modern engines other than noting that higher octane ratings can sometimes support greater ignition advance, resulting in the potential for greater power output.

The above discussion really does not apply to modern Jags as there is no method of advancing the timing to take advantage of the additional detonation resistance of the fuel.

It appears from other posters here that the cars run without penalty on 87 or 89 fuel, inferring that there's no detonation even with those low octane ratings.

 

Well........ there's the AndyBracket for the AJ6/AJ16. You may argue that our 20 yr old X300's are not "modern" jags (yet we are a sub-forum in the modern section)......OR you may argue that the AndyBracket does not advance the timing "to take advantage of the additional detonation resistance of the fuel."
I'm not sure about that one. IIRC, he stated that he developed the bracket based on engine development test data which set the timing based on very hot, high power settings. He reckoned it could stand a few degrees more advance given where most of us operate our engines. I would assume development testing was carried out using the "premium" fuel recommended in whatever octane rating system you choose. So maybe if some members who run the 89 have fitted one, they could weigh-in on whether they've experienced detonation? I suspect not. I've fit the brackets on 2 of mine... but always top them up with the 93....


A long way round.... but my point is that there is "kinda-sorta" a method, though you have ample avenues of argument to make the claim stand...

 

Octane ratings, almost as bad as an oil forum..!

Andy's bracket advances the ignition curve about 3' from memory (perhaps 5' - can't remember)
this is because when he did the original ignition programming at Jaguar, they ran the engines under full load on the test bench at any given rev point until knock (detonation) occurred, then dialed back the curve about 3' to protect the engines from knock/pinking/detonation.

Andy's rationale, if I may presume to interpret his original post about the bracket, is that in real-world motoring, except perhaps storming the Autobahn at full noise for mile after mile, the engines never run at full constant load - we always have our feet on and off the gas. So the state of steady full load never exists and therefore spark can be advanced.

I have read about octane quite a lot. It is of relevance to me because I run my father's 1928 Alvis. It is a typical pre-war English design with small bore and long stroke (to keep the hp tax-rating low), although relatively sophisticated as it is OHC not SV. It was designed when petrol in UK had an octane rating of about 65-68, although the Octane system hadn't been invented yet, and tetra-ethyl lead was only just entering the market around 1928.
Anyway, the Alvis doesn't like 91 RON petrol very much, and if I could buy 70 I would. It helps slightly that the compression ratio was raised a bit in 1961 when he had the engine rebuilt (originally about 5:1, now about 6.5:1).
An English guy with the same model Alvis thought his car had run better in 'the old days' so he bought some Heptane and made his own 70 octane fuel by diluting petrol with heptane (pure 100% heptane has a zero octane rating). It ran faster, cooler, and used less petrol, so it wasn't just the rose-tinted spectacles - he was right. (I bought a 5 litre of heptane to try for myself, but have yet to do this)

My point is, engines run best on the fuel they are designed for, and higher octane fuel isn't necessarily better in our Jags.
Having said this, I put 98 RON in my XJR and it seems faster!
I also mix 1 litre 100 Octane (about 104 RON) leaded AvGas to 10 litres 91 RON in the Alvis, just to protect the valve seats, and it runs quieter with a dash of the ol'AvGas and lead.

So I am very contrary, go figure..

 

I do have the AndyBracket fitted and definitely no detonation. I was using 93 octane when I fitted the bracket, but switched to 89 two years ago.

 

Ah- didn't realize that the Andy bracket could be used on your engines- sorry.

If one was fitted and the engine detonated on 91, then using higher octane fuel would be warranted. If there's no detonation while on 91, no point in using anything higher.

 

AL NZ has put his finger on the key point; engines run best on the fuel they were designed for. In the case of modern engines, which for this purpose the AJ 16 most certainly is, things aren't quite so simple however, as the engine is designed to optimise its performance in a variety of different circumstances. This is achieved through the various "maps" which are stored in the ECU. Sensors on the engine and the vehicle allow the ECU to understand what conditions the engine is operating in, and cause it to select the optimum parameters from the most appropriate map in its memory. For sure, the range of maps loaded into the X300 at the factory allow for the range of fuels commonly available globally, and have appropriate parameters for each. Also for sure, the car will deliver the specified power outputs on the specified fuel grade ( no Volkswagen story here!)
However, the use of premium fuels allows a different selection of maps to be used, which translate into more power or more economy depending on how the car is driven, marginally in both instances. Of course if the fuel is of such a high octane that it exceeds the mapping range used by the factory, no benefit would result, as the ECU would not have an appropriate map to use. Hence those wishing to go racing and use special fuels would need to have their ECU remapped.
As has been discussed elsewhere in the forum, there is scope to extract more performance from the AJ16 by remapping ( or "chipping") the car, but the lack of volume demand makes it uneconomic. Andy's bracket is a very clever way circumvent the lack of a map in the ECU, and create an optimum set of parameters from hardware rather than software.
So in short, higher octane pump fuel does make the X300 run better!

 

YES.

 

Again, this would only happen IF the engine was detonating on a given fuel. If an engine does not detonate on (say) 91AKI, then there would be no reason and no method for the controls to select a different map that might allow more ignition lead. From the reports above, the engine does not appear to detonate on 91, or even 89.

As you've said, the creation of the Andy's bracket was to compensate for the fact that the most adventurous factory map maxes out on 91AKI fuel.

The key is determining whether the engine detonates on 91 with an Andy bracket. Does anyone know?

This subject gets regularly beaten to death on the modern S-type board. It appears that the factory timing also maxes on those cars out with 91AKI fuel and the cars are quite happy with 89 or even 87 fuel. We don't have an Andy's bracket to play around with though!