Unleaded
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The addition of tetra ethyl lead (t.E.L.) has a more significant role to play than raising octane ratings. That light grey coating that you used to see on the tail pipe of a correctly set up engine is partly caused by the lead deposits in the exhaust. This coating also forms on the exhaust valve, which is significant. When you realise that the only way the valve transfers it's heat is via the seat and this only happens when the valve is shut, you come to understand at what temperature the valve head operates at, which is at dull red heat. That is why they are so expensive in turbo engines, because of the high quality of the stainless from which they are manufactured, in order to cope with the higher temperature of the exhaust gas due to the forced induction.
If we go back to basic metalwork, one of the ways of joining two pieces of metal is to heat them up to red-heat and hammer them together on an anvil. Every time the exhaust valve closes it does just this as it is clamped tight to the hot valve seat. When the valve opens, a few molecules of the seat suface adhere to the valve and are then seared off by the exhuast gasses. This is the cause of valve seat recession. The lead in the petrol provides a sacrificial coating, preventing v.s.r. You will however be pleased to know that because lead has been banned from petrol in Germany for so long, any vehicle of German origin is likely to feature valve seats hard enough to resist v.s.r., but for the benefit of the club I will ring the OPC and get some details.
If we go back to basic metalwork, one of the ways of joining two pieces of metal is to heat them up to red-heat and hammer them together on an anvil. Every time the exhaust valve closes it does just this as it is clamped tight to the hot valve seat. When the valve opens, a few molecules of the seat suface adhere to the valve and are then seared off by the exhuast gasses. This is the cause of valve seat recession. The lead in the petrol provides a sacrificial coating, preventing v.s.r. You will however be pleased to know that because lead has been banned from petrol in Germany for so long, any vehicle of German origin is likely to feature valve seats hard enough to resist v.s.r., but for the benefit of the club I will ring the OPC and get some details.
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