I got gas

Taking a little survey as to which type of gas everyone uses in their xj. I go with 100 octane ethanol free gas.

 

87

 

Getting ethanol free gas is hard enough. Where in the world do you get 100 octane?

 

There is a great place here in Titusville Florida that sells it. They always serve up 93 and I believe 86 all ethanol free. The 100 octane ethanol free is about 4.20 a gallon. But well worth it in my opinion

 

These smaller engines should be using 89 shouldn't they?

 

91 RON ~ 86 PON(what we use in North America)

 

Oh no another subject I've spent too much time obsessing over. I used to think higher octane (isooctane to n-heptane) meant more power because performance tuned engines required it, but that's not the way it works. The right octane for your motor at your altitude will create more power. To confuse everyone even more: modern engines adjust ignition timing and can compensate-but not an XJ.

Quoting from the fuel bible: "The higher the rating, the slower and more controlled the fuel burns," and "the octane number is actually an imprecise measure of the maximum compression ratio at which a particular fuel can be burned in an engine without detonation," and "Power is limited by the maximum amount of fuel-air mixture that can be jammed into the combustion chamber."

Helpful chart needed here: (note this guide is presented as a rough estimate and +/- 8% is possible)

Compression ratio Octane (at Sea Level)
5:1..................... 72
6:1..................... 81
7:1..................... 87
8:1..................... 92
9:1..................... 96
10:1................... 100
11:1................... 104
12:1................... 108

Based on this information it is clear that you don't want to go over your optimum octane because you will slow combustion and it will be harder to start when it's cold. Going too low would also be bad because it will drop your fuel economy.

For altitude considerations I will quote the bible once again: "The higher the altitude above sea level, the lower the octane requirement. As a general rule of thumb, for every 300m or 1000ft above sea level, the RON value can go down by about 0.5. For example an 85 octane fuel in Denver will have about the same characteristics as an 87 octane fuel on the coast in Los Angeles." Humidity and air temp are considerations as well, but I think they are minor at best.

Of course for all you XJ'ers o'er the pond your numbers are different than ours-your 95 is like our 89.

I always put super-unleaded in mine which is usually 91, but I am mostly doing this to avoid ethanol (the curse of all carbs). Ethanol is often added to the lower grades and they don't always tell you. My advice would be to use the fuel that works best for you. Check your fuel economy, your power, the smoothness of your engine, and how it starts when cold-then base your petrol on those characteristics. Unless of course your racing up Pike's Peak-then your gonna need to add lower octane as you climb!