Thread: lead additive

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This subject is fraught with wives tales and superstition. Valve recession (the valve "sinking" into the head as the valve/valve seat errodes), most specifically the exhaust valve, is a real issue but doesn't happen automatically by using unleaded fuel. It's a matter of heat at the valve face/seat interface. The mechanism is that at the "right" temperature the valve micro welds to the seat and then tears the weld the next opening. Repeated often enough and errosion occurs. Why leaded fuel "prevented" this (although in the "right" conditions even leaded fueled engines needed valve replacement) was that it deposited lead oxide on the valve and seat as the burned fuel left the combustion chamber. As you who weld have learned, it's difficult to weld dirty surfaces together. Some have come to referring to this oxide deposition as "lubrication". I don't care for that term as most car guys think of lubrication as an oiling function, so in the mythology it's believed that oil in the process will help the valves. Anyway, the operating conditions that typically contribute to valve recession are severe duty cycles; e.g. towing, racing, heavy hauling and so on. Those conditions that contribute to higher operating temperatures. In "normal" driving conditions where there isn't any particular heavy loading of the engine valve recession may not occur with the use of unleaded fuel. So, you need to evaluate how you plan to use the car. If you're going to really hammer it then you should upgrade the seats/valves. If you're going to use it like a commuter car then you most likely won't have a problem, and you can at least put off upgrading til some later time when you have to get into the engine for other reasons. The valve/seat errosion doesn't occur instantly, it occurs over time, the rate dependent on metallurgy and operating conditions. The drawback to using an additive (as is almost always the case with any "magic in a can") is unintended consequences. Mainly, in this case, increased deposits in the engine (particularly the top ring), shorter spark plug life, downstream accelerated corrosion of the exhaust system being the most likely.