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Unleaded Fuel

Unleaded Fuel

John Horner

George Start wrote:

Is it possible to run a xj6 series 2 on unleaded fuel? if so is there any conversion needed? If not is it advisable to run it on LPG gas? I ask this question due to the cloud hanging over the future availability of leaded fuel here in Australia. I have heard a rumour that lead fuel will be phased out past the year 2000.

The main issue is valve seat recession on those vehicles which don't have hardened valve seats and the appropriate valves installed. I'm not sure when the Jaguar factory converted over to 100% unleaded compatible valves.

If your car doesn't have the hardened valve seats already, it is not a big deal to install them the next time a valve job is needed. If you don't already have hardened valve seats, then the day when a valve job is needed will come a bit sooner.

We went through this same scare in the US quite a few years ago. Frankly, it has not turned out to be the big problem for classic car enthusiasts as many (myself included) thought it would be.

There are practical advantages to the use of unleaded fuel in addition to the environmental ones. Combustion chamber, spark plug and exhaust system deposits are greatly reduced through the use of unleaded fuel. Burning tetraethyl lead releases deposits which typically have a white, gray or brownish coloration. The fact that those deposits tended to form a bit of a wear barrier surface on the exhaust valves was very much an unintended side effect of the use of tetraethyl lead to raise octane ratings. Although exhaust valves benefit from the deposits, the rest of the combustion chamber certain does not. The spark plugs and exhaust system likewise are harmed rather than helped by the deposits.

These days I hardly ever hear evidence of engines pinging (pinking) like was so common in the 60s and 70s when I first started spending a lot of time around cars. I suspect that part of the reason for this is that combustion chamber deposit buildup is much less of a problem now that all cars in this country are running unleaded fuel. (Though I suspect that computerized engine control has a lot to do with this as well).

Editor's note: This seems to be a minor issue. Those wanting further information are referred to Chevron's Technical Bulletin at

Randy Wilson
If Your Still Not Satisfied

Doug Paris wrote:

A previous writer mentioned hardend seats and that is the way to go ultimately, BUT I have a '67 MGB (owned from new) that has been running fine on high octane unleaded with the above mentioned additives and careful tuning. With the relatively "cheap" head and good availability, I will wait for a problem to develop, but as to a Jag, I might think differently.

The idea and practice of hardened valve seats is to machine away the old seat, and press in a steel insert to act as the seat. This is common practice for cars with cast iron heads and the valve seats machined in the iron itself. The need for doing this for use with unleaded fuel is still debatable, as evidenced by the MGB mentioned above. Jaguars do not run the valves on seats man\chined in to the head material. Jaguars have aluminum heads, with steel valve seat inserts. The only reason to put steel valve seats into a head from a Jag is if that head has been technology-downgraded to cast iron.

The axiom that "old cars require hardened seats to run on unleaded" comes from the age old practice of machining the valve seat faces directly in to the cast iron head. Right or wrong (and this is a case of it depends), this does not apply to Jaguars, or any car with an aluminum head. Since you can not run a valve directly on the aluminum, the head has "hardened" (steel) valve seat inserts from the factory. In the US, we have no problems running the cars on unleaded, even the old pre-cat XK's.

As an aside, sometime in the early 70's BL started induction hardening the exhaust seat area of the MGB cast iron heads. No inserts still. These, too, hold up just fine to unleaded.

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