The 'E' Type Forum

Just wondered how many people out there are running leaded AvGas LL100 as an additive in their fuel - at around a 10% mix with their regular unleaded petrol ?

With the withdrawal of leaded fuel for cars in the UK many years ago, as a form of insurance and peace of mind I have used a lead replacement petrol additive, but these are becoming harder to find and increasingly more expensive - especially the Castrol Valvemaster with the extra octane. AvGas LL100 is a 100 octane fuel - but rated on the Motor Octane Number (MON) method, which would make it on the car-orientated Research Octane Method (RON) around 108 to 110 octane points - or Nine Star to us land dwelling car users. The "LL" means "low-lead" - but only "low" in the sense of other aviation fuels which could contain up to 1g/litre - whereas in practice LL100 contains lead at around 0.4g to 0.5/litre.

Our local airfield sells it at ?1.69/litre, which bearing in mind you are adding 10% to your fuel rather than a small additive, does not seem so bad. With the other advantages of a long life and not going "off" in storage and not gumming up needle valves, many people use it in lawnmowers and generators. Also if you spill it on yourself, you don't reek of petrol for ages.

I understand an issue around lack of tax and therefore it should not be used exclusively as a fuel or on public roads, but with other users out there, do they have views on it being used increasingly as an additive by the classic car fraternity ?

regards,

Dave

The XK engine, having a aluminium head, has hardened valve seats as standard so not sure why you would need any 'leaded super fuels'. Great for the track but on the road it severely limits your range. I have tried leaded fuel and various additives with excellent results but with today's road conditions and general availability it does not seem worthwhile. We drove some 240 miles today and I doubt I could have gone any quicker with a special brew - and we were quick!

LL100 is aircraft fuel, not strictly speaking an additive. The extra octane might help a bit with an engine that has the tendency to preignite but the lead probably won't add much to the XK engine as David says above.
It does give the inside of your exhaust pipes a lovely pale/mid grey colour though :-)

"I understand an issue around lack of tax and therefore it should not be used exclusively as a fuel or on public roads"


In terms of taxing, it is not failing tax on the small added portion. The added small portion, however small, renders the whole batch taxable anew. This was years back and not in GB when I checked on this issue in a totally different matter.

This discussion comes up in discussions all the time on this side of the pond. 100LL has an extremely high lead content--much more than the premium grade ("Ethyl") of old. Many states here in the U.S. ban its use in anything but aircraft, and it's very difficult to find an airport willing to fill your jerry cans, let alone put it directly in the car.

As David says, the Jaguar's hardened valve seats don't require lead anyway. I did run a couple of tanks of 100LL through my '65 back in the day, and I didn't notice any real difference in performance, although it has a nice smell and the tailpipes acquired that lovely light-grey color.