[modern] Unleaded Fuel and the AJ6 Engine

I bought an XJ-S 3.6 recently and was told by the dealer that it needed
to run
on Leaded Fuel 97 Octane or better. This was confirmed by the green
owners manual
that comes with the car, “Use ONLY leaded Fuel”.
I rang Jaguar Ireland (I’m in Dublin, Ireland) and asked about it. The
Service Manager said
that Jaguar recommend a Conversion Kit to enable this engine to run on
Unleaded. This kit

  • a bracket - retards the timing 3 degrees. Downside of this is that it
    saps power from the engine.
    Quoted �100 for the job. Something in his manner made me ask if the car
    would be ok on unleaded
    without the conversion. He just said that Jaguar recommended the
    Not fully convinced, I got onto AJ6 Engineering in the UK. Spoke to a
    very helpful guy who said
    the car would run fine on unleaded at normal revs. The problem, he said,
    was that at full or near-full
    revs the engine could “ping” or pre ignite. This could damage the piston
    crowns. This could be rectified
    by fitting their “U-Box”. This is a device that retards the timing only
    when the revs get near the red-line.
    Price �130 Sterling. Fitting involves 2 wires and a vacuum pipe.
    Good value I thought.
    So I went to the petrol station and put some Unleaded Petrol in (95
    Octane) and guess what? The car
    runs absolutely fine. Granted, I rarely, if at all, rev the engine to
    the red-line. This sort of thing on a 12 year
    old car is asking for trouble. Anyway, the car has plenty of power at 3
  • 5000 revs - well short of red-line.
    I may get the U-Box if I am feeling flush one day - but for now,
    unleaded will suit me just fine.
    My advice - for what it’s worth - is to try unleaded first, and see what
    Hope this helps,