No lead needed

Posted by Bob Johnson on August 06, 1998 at 08:49:29

In Reply to: gasoline
posted by Al Downs on August 05, 1998 at 19:25:45

Al,
Ok here is what I know.
Lead was introduced in gas to increase the octane rating. It was first used in the 1920's but was not widespread until the 1950's. This is when the real horse power wars started. As the auto manufacturers raised the compression ratio to increase the horse power higher and higher octane gas was needed to prevent preignition. It was observed that the lead in the gas would coat the exhaust valves and seats. This coating 'softened' then blow when the valves closed. Because of this the manufacturers found that they could use softer metal for the valves and seats. When the lead was removed from the gas the 'lead coating' started to wear away from the valves and seats. Now there was metal to metal contact which resulted in burnt exhause valves.
As has been said before lead was not normally found in gas in the 1930's so the Model A engine does not need it. It should also be noted that originally the Model A came with two different type of valves. One for the intake and a harder one for the exhaust. However in the 50s, 60s and 70s many Model A engines were rebuilt with inferior exhaust valves (really intake valves). One could have problems with these.
Lastly it should be noted that the problem with burnt exhaust valves increases with the engine rpm. I read one study that said the problem occured above 3000 rpms. This is above the redline of most Model As so there is not a problem here.
Bob

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