I have recently (late 1980s) had a number of enquiries about the advisability of using unleaded petrol in engines used in the A30/A35. With the increasing environmentalist pressure to eliminate lead in petrol, many more members may be wondering if their cars can be run on unleaded fuel. A further encouragement to use unleaded petrol is that it cheaper than leaded, and it is likely that this difference in price will increase further in future.
I have now made enquiries of authoritative industry sources, and I acknowledge their help with the information on which this article is based, but they are not responsible for the content: my Sources, the Club and its officials cannot accept any legal liability for any loss injury or death which might arise from the adopting the advice which follows.
The major reason why older engines with cast iron cylinder heads cannot run safely on unleaded petrol is the vulnerability of the valve seats. The lead content in leaded petrol protects them against wearing away, wear that leads to valve sinkage.
To make cast iron cylinder heads compatible with the use of unleaded petrol requires the fitment of special hardened valve seat inserts during production of the head.
Although it is impossible to obtain lead-free compatible cylinder heads that are intended for our cars it should be possible to achieve satisfactory valve seat protection for a car driven gently and for low mileages by using one leaded fill up in every four. This is not a mixture of 1 part leaded to 3 parts unleaded each time you fill up, but a full tank full of leaded petrol on every fourth fill up. This will enable the layer of lead lubricant to be reestablished. It is essential to follow these requirements very closely if you are to ensure adequate engine protection.
When lead-free compatible cylinder heads are fitted to present day engines the manufacturers also have to develop a specific tune for each engine to allow it to run on unleaded petrol. Fortunately the relatively low compression engines in the A30/A35, ranging from 7.2:1 for the 803cc engine to 8.5:1 for the 1098cc engine should not require retuning because the octane rating of the petrol should be adequate. While there should not be any tune problems with unleaded petrol in standard engines, it may be necessary to make minor adjustments to the idle mixture, or retard the ignition slightly if pinking develops.
This advice is not suitable for engines which are driven hard, as in these conditions the protective layer will soon be eroded, nor for tuned engines, because the lower octane rating combined with a high compression ratio could result in detonation and possible piston failure. Finally the advice is not necessarily suitable for engines other than the A-series 803, 850, 948 and 1098cc engines as originally fitted in the A30/A35.
For those determined to go "Lead free" at any cost there are other possibilities that might be considered, but they could give you more trouble or expense. If you have a large stock of cylinder heads which you don't mind burning your way through (and it might not take very long!), you could use unleaded petrol all the time, and change the head when the valve sinkage becomes excessive. Because of the rapid valve seat wear, you would have to return to the pre-war practice of decarbonising and grinding in the valves every year!
An expensive alternative would be to obtain a lead-free compatible cylinder head intended for a Mini or Metro or other A-series engine of similar engine capacity to the one you have (ie that for the modern 998cc engine for engines up to 948cc, and a 1275cc head for 1275cc engines. For 1098cc engines, at least, the combustion chamber of the 998cc head would need to be enlarged to reduce the compression ratio. The main difficulty would then be to obtain a suitable advance curve for the distributor (the distributor intended for use with the head would be a starting point), and the advance curve would really require to be calibrated on a rolling road. Needless to say this option would be expensive, and the reliability uncertain.
A cheaper but almost certainly unreliable alternative would be to fit valve seat inserts made of a suitable alloy. Retrofitted valve seat inserts have been around for a long time, and they have the reputation of being unreliable, being inclined to fall out, particularly in an engine driven hard.
If anyone does try any of these options I would be interested to hear whether the conversion is satisfactory, so that we can pass on the accumulated knowledge as it develops.
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(c) R H Johnston, 1998. Material may not be published for profit
without permission.
Whilst great care is taken to ensure that the technical information and advice offered is of the highest standard, neither the author, nor The Austin A30/A35 Owners Club, or the Officers of the Austin A30/A35 Club accept any liability at law for any death, injury or loss whatsoever arising from the use of advice contained in this article.