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Jaguar V12 Octane Requirements

Jaguar V12

Fuel Requirements

Just determining what fuel is recommended for use in the Jaguar V12 engine is a bit of a challenge.  For example, in the XJ-S H.E. Handbook for the US-spec 1983 model year, on page 111 it merely lists a "Recommended minimum octane rating" of 91 (RON) Lead Free.  Many people have read this and concluded that the car calls for Premium, since the typical octane ratings in the U.S. are 87/89/91+ -- but that's because they fail to notice the reference to RON.  US gasolines are not rated in RON; they are rated in the average of RON and MON, commonly marked on the pump as (R+M)/2.  Unless you know how to convert RON to (R+M)/2, the spec in that handbook is meaningless.

Apparently, Jaguar improved their handbooks later.  Douglas Dahl says, "It states for my '89 H.E. with Lucas ignition 'In the United States, Federal Law requires that gasoline octane ratings be posted on the pumps.  The octane rating shown is an average of Research Octane Number (RON) and Motor Octane Number (MON).  This number is also referred to as the anti-knock index (AKI) or Average Octane Number (AON).

"'Unleaded gasoline is available in two grades; 'Regular' labelled at the pumps as 87 AKI (91 RON) and 'Premium' labelled at the pumps as 91 AKI (95 RON).  Use unleaded gasoline with an octane rating of 91 RON, i.e. Regular fuel.'"  Apparently Jaguar was unaware of the availability of mid-grade 89 octane fuels.

Now, in 1989 Jaguar replaced the mechanically-advanced Lucas CEI ignition system with an electronically-controlled Marelli system.  Apparently, at the same time, they decided that regular unleaded was no longer good enough -- even though the mechanicals of the 5.3 liter V12 itself did not change, including the 11.5:1 compression ratio.  Kirk Mather provided the following scans from the handbook for his '89 with Marelli ignition.  Apologies for the size, but when reduced significantly they become unreadable.

First, page 98c:

And page 209:

Mather also explains that it clearly states "Premium Unleaded Only" both on his dashboard and at the fuel filler cap.

Note that the Marelli ignition system features a jumper under the hood for retarding the timing for using lower octane fuels.  It's entirely possible that this jumper is intended to be used only when Premium is unavailable, or when someone accidentally fills the car with the incorrect fuel, or when the owner gets a tank of fuel that is supposed to be Premium but clearly isn't up to par and the engine is knocking.  Or, it's possible that the jumper was provided to permit the same ignition ECU to be used in the US and in other parts of the world where fuels with higher octane than US Premium are available.

Any and all Jaguar V12's have a tendency to knock if they've been driven too gently for too long.  The solution is the "Italian Tune-Up" -- drive them anything but gently for about 20 minutes.  A proper Italian tune-up involves being within about 1000 RPM of redline for a few minutes, but more than that it requires having the throttle wide open.  Achieving both of those conditions simultaneously results in about 140 mph in this car, so alternative methods are recommended.  One idea is to hold the shifter in 2nd gear and accelerate at full throttle from 5000 RPM to redline, slow back down, then accelerate again.  This is still sporty, so yet another possibility is to do it in first gear -- but the ride is not likely to be very comfortable.

Most owners who do this know when they're done; the first couple of throttle stomps demonstrate really lousy throttle response and result in huge clouds of black smoke out the back of the car, but as it clears up the car gets much snappier and cleaner-running.

Obviously, it is highly advisable to make sure the car is in satisfactory operating condition before attempting an Italian tune-up.  A seized centrifugal advance mechanism on the Lucas ignition system would be most disastrous, as the retarded timing at such elevated power conditions can cause piston damage.  An obstructed fuel pickup in the surge tank can cause the engine to run lean, which will also cause piston damage.  And if the car has any tendency to overheat whatsoever, this will put it over the top.  All three of these faults are so common that, if you don't know for a fact that they have been addressed and corrected on your car, you should avoid the Italian tune-up until they have been taken care of.

So far, this page has only addressed the concerns of the US-spec XJ-S owner.  The UK-spec owner has his own problems, since the earlier handbooks clearly require "Four Star Leaded", and leaded fuel is being phased out in the UK.  The "leaded" part of that requirement wasn't critical; the components of the engine that would normally be the areas of concern for converting to unleaded fuel (valve seats, valve guides) are in fact the exact same parts that have been used on unleaded fuel in the US for decades.  The reason the spec spelled out Four Star Leaded was that was the only fuel available with adequate octane -- 98 RON.  And when the phasing out of leaded fuel was first announced, it was expected that there would be no unleaded offered with a higher RON than 95, so the owners of the UK-spec Jaguar V12's with 12.5:1 compression ratio might have been up a creek -- but reportedly 98 RON unleaded became available after all.

Pre-H.E. engines are a separate issue, although they seem to parallel the concerns of the H.E.  The pre-H.E. also came with two different compression ratios, and the US-spec called for 91 RON while the UK-spec called for 98 RON.

Production of the V12 was suspended in 1993 and reintroduced in 1994 as a 6.0 litre with 11:1 compression ratio -- worldwide, apparently.  The Marelli ignition system remained, although a distributorless system was introduced just before the V12 ceased production permanently in 1996.


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