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Converting Vintage Cars to Unleaded Petrol

By: Lucy Debenham BA (hons) - Updated: 19 Sep 2018 | comments*Discuss
Converting Unleaded Petrol Valve Lead

It is largely accepted by scientists worldwide that the acceleration of global warming and the increasingly common occurrence of extreme weather conditions are by and large, as a result of human activity. At present there are upwards of 560 million cars currently in use around the world. So it is no wonder that the motorcar is considered to be a significant factor in contributing to the potentially devastating effects brought about by climate change.

In 1920, leaded petrol was introduced to the motoring market as a main fuel for running cars. In the 1980s, it was decided that the increasing collective lead content was too toxic, and as such unleaded petrol was introduced in 1986, and leaded four star petrol was subsequently phased out and withdrawn from the fuel market in 2000. Only a small percentage of garages still offer leaded replacement petrol (LRP), and this potentially poses a problem for the vintage car owner.

The proportion of vintage and classic cars that once ran on leaded fuel only account for a somewhat inconsequential amount of toxic emissions and CO2. However, as a vintage car owner, you may be still want to play your part and make your vintage car as environmentally friendly as possible, or simply make filling up more convenient by converting to unleaded petrol.

Converting the Engine

As a vintage car driver, if you are not prepared to drive for miles to find a garage offering LRP, there are a few “greener” options open to you. Previously, the leaded compounds present in the fuel provided the valve seats in the engine with a level of sustained protection. As unleaded petrol does not provide this protection, problems begin to arise due to what is known as ‘valve seat regression.’ This is where the valve seats of the cylinder head are slowly worn away, causing the valves to regress back too far into the cylinder head.

The valves need to lose heat extremely rapidly, and they normally do so via conduction with the cylinder head when they close. If they regress too much, there is less contact surface area between the valve face and the valve seat of the cylinder head and so the valves are unable lose heat fast enough. This can result in the valves becoming “burnt out” - literally having holes and indentations burnt out of them. This can cause considerable damage to the engine.Interestingly it is often possible to run unleaded petrol in an unconverted engine for short periods of time, as the valve seats have in the past already been protected by the lead that was present in the fuel. This is known as ‘lead memory.’ However, be aware that you if your valves have been replaced at some point in the past, and new valve seats have been cut for them, then this ‘lead memory’ may have been removed.

If you wish to run on unleaded petrol for sustained periods of time, then the best option is to have a machine shop perform an ‘unleaded conversion’ on your cylinder head. This will normally involve them removing the old valve seats and replacing them with special hardened alloy valve seat inserts – which tough enough not to suffer from valve seat regression.It should however be noted that, due to a change in octane number, when converting to an unleaded fuel, your engine’s timing will need adjusting. This is quite important as otherwise detonation can occur instead of a controlled combustion of the fuel and air mixture in the combustion chamber. Detonation, sometimes called ‘pinking’ due to the sound it makes, can literally ruin an engine.

The success of this sort of conversion can be subject to the configuration of the vintage car engine, and should not be undertaken lightly. It is advisable to seek professional advice if you are seriously considering doing this. If you are concerned with reducing toxic car emissions, you might be able to retrospectively fit a two-way catalytic converter - but again this is subject to the type of engine and exhaust line in your vintage car.

Just Unleaded Please

If drastically converting your car engine is not an option, then there are other steps that you can take. The simplest option by far is to opt to run your car on unleaded fuel only. Vintage cars are able to run for a while on purely unleaded petrol, but if the car is frequently used, as aforementioned, eventually the cylinder block or head will become vulnerable to corrosion. For this reason, if you choose to run your vintage car on just unleaded petrol and want to drive it frequently, then you will need to use a lead replacement additive. They are designed to offer the protection that four star leaded fuel once did, and are quite easy to add, if you are adept at measuring and following instructions! Most petrol filling stations or motorist centres stock a range of these additives. As there are a range of alternative additive ingredients on the market, it would be advisable to contact a specialist or your car club to find out which is the best for your vintage car engine.

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[Add a Comment]
Hi If i change my 1929 Peugeot pick up from unleaded to leaded will it change the value of it.
Papa - 19-Sep-18 @ 2:02 PM
I have a 302 Windsor of 1970s vintage I want tn make it leaded is it financially viable or should I just buy another engine? Cheers
Jimbo - 19-Aug-18 @ 9:16 AM
Starting an article off with fake news doesn't help your credibility.
Wookiee - 28-Oct-17 @ 9:39 PM
I have a 1968 Austin 1100 and was wondering how much it should cost to convert from four star to unleaded??
Scouse - 7-Jan-17 @ 7:53 PM
Hi Moggy owners ! What make of fuel additive is recommended for a 1968 A-series fully restored 1098cc engine ? Thanks
fasthamster - 9-Dec-16 @ 8:25 PM
I have had a 1275 allegro engine head changed to run on unleaded. I believe I have to alter the timing, can you give me any measurements please.
Catchunky - 17-Jun-16 @ 7:13 PM
I have a 1985 Citroen 2cv that runs on leaded petrol what additive can I use
Joe - 11-Jun-16 @ 10:35 AM
I have just converted a 1980 mini clubman but not too sure what fuel too use leaded or unleaded could someone help me out please
marti - 12-May-14 @ 10:06 PM
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