Leaded 4-star petrol was withdrawn from sale in 2000 and the product that replaced it – Lead Replacement Petrol (LRP) – went the same way in 2003 as sales were so low. What are the options if you drive a car designed to run on leaded petrol?
If you have a pre-1992 car it's important to find out if it can run on 95-octane Premium Unleaded Petrol, the normal grade sold in the UK. Check with a dealer or the manufacturer.
Converting your car
For pre-1992 cars designed to run on leaded petrol there are two possible problems if they are run on unleaded fuel.
There is the loss of about two 'octane numbers' from the older 4-star leaded and the LRP that replaced it. This should barely be noticed although ignition timing might have to be reset to avoid any tendency to detonation (or 'pinking').
Secondly, if the engine has valve seats cut directly into a cast-iron cylinder head or block, the loss of the protective effect of the lead compounds means that under conditions of hard, high-speed use, erosion of the seats can occur.
The long-term answer is to have hard-alloy valve seat inserts installed, or even an exchange head fitted.
Solutions for unconverted cars
If you don't want the expense of getting the head converted you have three basic choices:
1. Use a proprietary branded additive
In normal or moderately hard road use the valve seat protection afforded by these additives is perfectly satisfactory, and engine life will be just as good as with leaded petrol.
Take care, as mixing and dose-rates can be quite difficult to get right in small amounts. Once you've chosen an additive stay with that brand rather than swapping between products.
2. See what happens on unleaded only
Not completely daft, in that driven reasonably for limited mileages, the valve clearances will not be taken up between services, so with moderate care no harm should result.
Look out for loss of compression or reduced valve clearances, in which case you must take action right away.
3. Continue using leaded 4 star
When 4 star was withdrawn from sale a concession to the regulations allowed 0.5% of petrol sales to be leaded, for 'characteristic' vehicles. But there are now only very small quantities sold for historic vehicles by licensed garages who are members of the Federation of British Historic Vehicle Clubs (FBHVC).
It makes no sense to drive long distances to buy leaded, or to mix it with additised fuels if it's hard to find.
There are strict and sensible regulations controlling the storage of petrol: it may only be kept in the vehicle's normal petrol tank or as a few litres in approved spare cans.
Some specialist high performance cars need high octane fuel. These were made in the days of 100-octane fuels, but could get by on 97-octane leaded and then LRP. Going to 95-octane unleaded just possibly could cause trouble, even with the ignition retarded.
The long-term option is to lower the compression ratio – at some power loss – but with benefit to engine life. Usually a new set of pistons can achieve this; engine specialists can advise.
There are some proprietary 'octane boosting' fuel additives but their affect on different fuels can be unpredictable.
Two-strokes, without poppet valves, don't have a problem, and in fact lead replacement additives should not be used with two-stroke mixes. Sleeve valves have a different lubricating method, and will not be affected.
There are several proprietary lead-replacement additives available for 'DIY' dosing of unleaded petrol.
Additives may be phosphorus, sodium, potassium or manganese based – and are quite satisfactory used at the correct dosing rates in appropriate vehicles. It's best to choose one product and stay with it, though some inter-mixing will be inevitable at times.
The Federation of British Historic Vehicle Clubs has tested and endorsed a number of 'valve seat recession (VSR)' additives.
Why lead was used
Lead was used as an additive in petrol from the 1920s through to the beginning of 2000 when European legislation discontinued normal sale and distribution of leaded petrol (otherwise known as four star or BS4040).
Lead allowed the development of higher 'octane number' fuel (the higher the number, the greater the resistance of the fuel to uncontrolled burning in the engine, or 'detonation'), and was also discovered, later, to have the property of protecting valve seats from wear.
Leaded fuel was withdrawn because lead is a cumulative toxin, found to affect human health adversely. Modern refining methods can provide the necessary fuel quality (octane) without added lead.
(Page updated 22 March 2013)