High test does not burn hotter, it just requires more energy to begin burning. This is what prevents knocking in a higher compression engine.

Since the power output from high test and regular are effectively the same (assuming no pre-ignition), the temperature of the burn is defacto the same.

From Wikipedia:

"Octane rating does not relate to the energy content of the fuel (see heating value). It is only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner.", "Higher octane ratings correlate to higher activation energies. Activation energy is the amount of energy necessary to start a chemical reaction. Since higher octane fuels have higher activation energies, it is less likely that a given compression will cause autoignition."

and

"The power output of an engine depends in part on the energy density of its fuel, but similar fuels with different octane ratings have similar density. Since switching to a higher octane fuel does not add any more hydrocarbon content or oxygen, the engine cannot produce more power."

- Nor can the burn temperature be any higher, or due to laws of thermodynamics, more energy (power) would be produced.

I will take this back to my source and see what they say. However you need to read the Jaguar World April 2006 Nikasil – V8 engine issues (link in the body of my subject Nikasil) that supports the fuel issue.

As stated above, this is also false. These are just a few of the dozens of myths regarding gasoline.

I think we have heard that the basic gasoline stock is the same with only the additives changing the octane and therefore the level of initiation of the chemical reaction (burning) being significant. Therefore the chemical energy potential of 89 or 93 octane is the same.

The basic fuel stock being the same there is no difference in burning rate.

Fuels, be they 89 or 93 octane, must meet the minimal specification of the manufacturers regarding fuel injector 'cleaners'.

Additional 'cleaners' are part of the marketing of the gasoline brands and not a magical potion.

The basic fuel stock is the same, therefore there is no difference in the sulfur content between 89 and 93 octane

Pre detonation is damaging to any engine.

Avoiding pre detonation through additives or knock sensors retarding the ignition timing while meeting the power requirements of the throttle setting is the objective.

Because of the timing retardation, not fuel chemical energy potential, the power capability of 89 octane is less than 93 octane.

As a restatement, 93 octane fuel, under aggressive throttle application will produce more power than 89 octane because the knock sensors will retard the timing less.

Please feel free to correct any of the above.

When through we will post on the XK8/R FAQ sticky and then when someone asks about 89 octane we will shout #43 and LOL. (anyone not getting that old joke please send me a PM)

I get the joke! :icon_wink:

Very nice post. Maybe to reword slightly:

An engine designed to run on 93 octane fuel, under aggressive throttle application will produce more power than if run on 89 octane because the knock sensors will retard the timing less.

The idea is that it's the engine producing the additional power due to having the timing advanced to optimum, not the gasoline itself.

In other words, an engine designed run on 89 octane will not produce more power on 93 as the timing is already at it's optimum setting.

Well said!

The lower the octane rating the easier the fuel is to combust under pressure especially in the warmer months (detonate). Think Diesel, I think its octane rating is in the 40's. If your a casual driver you can go with a midgrade gasoline without having any issues.

The rating you gave are european ROM if im not mistaken, I think it falls in this order
ROM
93,95,98,105
US OCT
87,89,91,93/4

dont qoute me there though

And Nikesil engines were designed to run on which fuel?

P.S. Lots of insightful replies.

Gasoline.

Jaguar specified 93 octane to reach the horsepower potential of the engine based on compression and spark advance capabilities designed into the engine. They could have as easily engineered a less advanced spark timing and a correspondingly less advertised horsepower.

That being said, it is obvious that the Jaguar engine fueled with 93 octane will produce more horsepower when called upon to operate at its upper limits. That's just not how I drive.

In the days of even higher compression ratios and RPM driven spark advance control higher octane fuel was absolutely required. Today's computer managed engines equipped with knock sensors have a much greater tolerance for lower octane levels.

Hi,

Sorry, #93 EU or US? As StrateLoss stated, that there is a difference.

Sorry, US.

In that case:

98 RON Octane x 0.95 = 93.1 AKI Octane (US measure)

That means, Jag recommends using 98 RON (EU) octane, thanks a lot!

I found an excellent article written by an organic chemist working for a major oil company on gasoline cleaners. It does not address the pre ignition issue as much as injector clogging and cleaning.

Worth a minute to read.

http://www.vettenet.org/octane.html

+1 to test point and Mikey
Last summer when my ae86 still ran, i did the different fuel for different power thing. factory specs for the car stated the it should run on 89 octane(canada) and the timing advance should be 12degrees btdc. i was able to advance this to about 14 btdc without knocking and slapped her on the dyno for a decent 88whp at 6600rpm peak. after using all the fuel and refilling with sunoco 94, using it and refilling again, i adjusted the timing to about 19degrees btdc according to my timing gun and dropped it off at the dyno shop and came back with an astounding 96whp at 6850rpm. keep in mind the car did not have a knock sensor.

with my other car, an 04 330ci it is reccomended that i use premium unleaded, but i did a test for highway driving only with no aggressive throttle input cand got results back within a very small margin which could ahve easily been due to weather factors. 870km/56.4L premium and 874km/57.6L regular. This small difference would not offset the cost of filling with premium, however, when doing city driving only, i got about 511km/57.3L premium and 443km/56.9L regular making it worthwhile to pay the extra cost of premium fuel.

just bought a 1998 xj8, any idea what kind of fuel consumtion i should be expecting?

Mine 1999 XJR does 20-24 liters / 100 km, bit agressive driving.

Your questions drove me to check into the information given to me and I put me results on my page. http://www.gusglikas.com/AutoRepairNikasilSulfur.htm

Happens to me every day on the way to work. Well, ok, not the semi truck part, but still, if I go to tromp on it and pass someone using regular fruit-punch low octane crap, the valves will clatter in protest and I have to back off. Whenever I use premium, this does not happen and the engine tears right along with nary a peep from the valves. I was never a believer in high octane requirements until this direct experience convinced me otherwise, at least for this car. And when I did the math, it only added just under four bucks per tank full, which is what I might blow on a large coffee these days. Though I could probably get away with cheap gas if I babied the car and did not pass anybody, screw that - I've been using premium ever since.

I believe your engine should have at least one knock sensor. If when using low octane gas there is a 'clattering' present, this infers the sensors are not operating correctly. This should be addressed, and not blame the fuel.

BTW- it's not valves that clatter during detonation, it is single or multiple 'shock waves' created prematurely in the combustion chamber.

Well, call it what you like, but it's the valve train that I hear and it isn't pretty with inferior grade gas versus the premium.

And I will check the engine out again, but as of a few months ago the car was not throwing a code and the situation was the same - crap fuel=crap performance. There might be a reason Jaguar recommends premium fuel after all.

In addition to keeping the octane high, I've found that after installation of a new fuel filter, and a good cleaning of the MAFS (followed by a hard reset) results in virtually no knocking.