Where have all the brown plugs gone – long-time passing?

Spark Plugs

Long before MGs were fitted with V8 engines, when petrol cost 39p / gallon and you could spot motor cars that had just travelled up the motorway by the fawn / white colour of their exhaust pipes, you used to go to tune your car with a rich tea biscuit. No, not to eat, it was a colour comparison for your plugs. Fawn or light brown coloured plugs were an indication your engine was properly tuned. Where has it all gone?

No comment about the cost of petrol, now around £6.32 / gallon, the white exhausts disappeared when lead was removed from petrol. Interestingly it was not the lead that was responsible, but lead bromide. Many people think the lead in petrol protected the valves from wear. In practice, when tetra ethyl lead burns it produces lead oxide and some metallic lead which are deposited in the cylinder and can cause pre-ignition. Long ago, this problem was resolved by adding ethylene dibromide. During combustion, this reacted with the lead to produce the white lead bromide that is deposited in the exhaust. It is this lead bromide that acted to lubricate the exhaust valves.

What about the biscuit brown plugs, have they gone the same way as the white exhausts? Well yes and no. Look at the picture of a plug recently taken from my TC, just the right colour. Before you rush out, remove your plugs and start to worry when they are grey or some other colour, read on. The problem is due to petrol or to be precise different types of petrol. Firstly, in the UK there are three different grades of petrol sold over the year, winter, summer and transition. Buy Brand X on the motorway from a station with a fast sales turn-over, you could get summer grade, the same Brand X from a station in a sleepy village and you may be filling up with winter grade. These grades have a different mix of additives and volatilities and may colour your plugs differently.

Furthermore, pool petrol is supplied to all distributors from refineries around the UK with each distributor adding their own cocktail of chemicals. While it is very difficult to find the exact details, it appears that both the pool petrol and additives vary around the country. Brand X in the Midlands is not the same fuel as Brand X in East Anglia. One reason why I get brown plugs and you may not. Additionally, I believe that in some areas, premium brand fuels can give a pink or purplish colouration of the plugs.

Another affect on plug colour is washdown, liquid fuel entering the cylinder and washing the plug clean. This shows clearly on the pictures of the two-tone plugs from my TC. While all the plugs are basically the same colour, the photograph shows different sides. On one side they are brown, on the other white, almost as new.

Now-a-days, the only thing you can be really certain of when it comes to plug colours is if they are sooty black, you are running too rich.

Paul Ireland

Ed’s Note: Paul Ireland wrote this article for the V8 Register of the MG Car Club but he has given me permission to use it in TTT 2.

This seems to be a convenient slot to update you following a meeting the Editor had with his Member of Parliament. The M.P. promised that he would contact the Minister at the Department for Transport and true to his word I now have a letter signed by Norman Baker, Parliamentary Under Secretary of State at the Department. I reproduce a couple of paragraphs as follows:

“Mr James may be interested to know that no 10 per cent ethanol content petrol (“E10”) is supplied in the UK at present. Biofuel targets in the UK Renewable Transport Fuels Obligation (RTFO) only require fuel suppliers to increase the average biofuel content of road transport fuel to 5 per cent by 2013/14 and later years. At present fuel suppliers supply most biofuel as biodiesel where blends of up to 7 per cent (“B7”) are permitted in normal diesel with a lesser quantity of bioethanol being supplied as E5. At present, regulations require that Super grade petrol dispensed at high throughput filling stations contain no more than 5 per cent ethanol. The cessation of this requirement from the end of 2013 appears to have been widely misinterpreted as a general “phase-out” of E5. In reality, fuel suppliers will still be able to supply E5 and even fuel containing no ethanol (“E0”) provided that on average across all the fuel they supply they meet a 5 per cent biofuel uptake level.

Bearing in mind the bioethanol is more costly than petrol, current RTFO targets provide no incentive to supply E10 and fuel suppliers are certainly unlikely to introduce this fuel until a larger proportion of the fleet is compatible with it”.

The sentence beginning “The cessation of this requirement…” has been underlined by me as I think this is worthy of being highlighted.

The cost argument is also worth noting and with Governments under pressure to scale back expenditure one wonders how much longer the current subsidy regime can last. We live in hope!