Originally Posted by
rutan around
A.E.F.R says:
Really? If the lead salt deposit you're talking about is lead oxybromide how does it deposit on those mating surfaces? Lead oxybromide melts at 703.4*F. The engine exhaust is between 750*F and 1,650*F so it is in liquid form. The exhaust gas exits through the open valve at better than 600 km/h The valve head is heated to close to the exhaust gas temperature. Under those conditions I don't think anything would stick to to mating surfaces.
The most convincing argument for me that it is an old wives tale that lead is a lubricant and /or cushion is the result when natural gas or propane gas is used in internal combustion engines instead of leaded fuel. The engines last just as long if not longer and there is not an atom of lead in either gas.
I find it very strange that you think that. There is plenty of evidence that engines run on LPG need hardened valve seats and those that don't have them are likely to suffer valve seat recession and valve burning.
I first came across this in the early 1970s. My fiance's father had an almost brand new Leyland Mini van that he had professionally converted to LPG. Within weeks it began misfiring. The company who carried out the LPG conversion wouldn't take responsibility. I was asked to take a look. I removed the cylinder head and the valves and seats were already showing damage. The A series engine was one of those that had the seats cut directly in the cast iron head; it became a well known problem in later years on that type and on some other engines.
Valve Seat Recession Explained
Earlier this year I had to have a new exhaust valve seat fitted to a single cylinder motorcycle engine (Royal Enfield) because it had recessed; the valve was also damaged beyond further use. The only explanation I can think of is that the seat was too soft to be used on unleaded fuel; the exact type was never officially imported to my country (Indian home market bike) and it's possible the fuel formulation is different.
I can't explain the molecular effects because I'm not a chemist but I certainly do know what I've experienced first hand.