I was just reading the Owner's Manual (for the first time), and, I noticed the XJS has a compression of 11.5 to 1. Uh, shouldn't I be using premium gas? I've been using 87 octane all this time. There is no pinging. What is recommended? What do the other club members use?

 

All you need to know >> XJS Octane

 

Surprise!!! It wouldnt hurt to try a couple of tankfuls. My xj6s both say "Premiun unleaded only" on the gauges.

 

I run ethanol free gas, and that's only available in premium in my area. For cars that aren't driven all that much, ethanol free is better since it doesn't absorb water, and the XJS fuel system was not really designed with ethanol in mind.

 

You need the highest octane possible in your area. If Marelli car you can select lower octane fuel via a wire plug however that severely hurts performance. Its ugly.

 

We got the real engine, 12.5:1. so Hi Octane is all we use. 98 RON its known as down here.

Differnt markets called for different things, and distributor timing, emissions systems, etc etc were all :market specific" BACK IN THE DAY.

 

The way they do that is by retarding the timing. Earlier Jaguars had 9.0-1 compression in England and the rest of the world except America we got 7.8-1
after 1980 England and the rest of the world got 12.1-1 compression while America got 11.5-1.

The early car 1975-1980 had 40 degrees of timing while the HE only has 17 degrees of timing.

what does that mean to you? If you drive it gently you’ll get by for a while using 87 octane. Eventually you’ll burn valves and maybe put holes in your pistons. And spend a lot of money getting it fixed or selling it cheap to someone who will fix it.
91-92 octane will prevent that.

I know all the talk about premium is a waste of money. Not all the time!! Most cars run fine on 87 octane. Not Jaguars.

 

My 93 4.0 says "premium only" as does my 97 xk8, so that's what I run. I figure if I can't afford the extra for premium, I probably shouldn't have the cars. Lol.