the Story on Lead
Substitute Gas Additives?
still add lead substitute to most of my older cars as I have not pulled
heads and replaced the valves and valve seats with those suitable for
gas. I always heard that the lead added to our gas left a thin layer of
the valves and seats. This improved the heat transfer and thus kept the
from burning. Reading more on the subject, I find this is not the
in gas had two effects: one was to slow the burn rate of the gas and
the octane rating. This prevented pre-ignition known as knocking or
permitted the use of higher compression ratios. The second was to
against very rapid wear, or “recession” of the
exhaust valve seats.
lead additive of choice was tetra-ethyl lead (TEL). TEL was first added
during the 1920's as an anti-knock additive. It was not well understood
later that it formed deposits which acted as a solid film lubricant on
valves and seats. These deposits were useful when the engine was run
hard and the
valve temperature rose. Apparently at very high temperatures the valves
micro-weld themselves to the seats. When they open a fraction of a
later, they pull iron particles from the valve seat. These particles
exposed to hot exhaust gas and are not well coupled to bleed off the
exposed iron particles heat to extreme temperatures and form iron oxide
particles which become imbedded in the valve. Iron oxide is harder than
metal in older valves or valve seats. This hard coarse surface causes
to start wearing into the valve seat. As the valve rotates the imbedded
oxide causes increased wear in the valve seat. This is called valve
recession. The TEL additive formed a lead compound that coated the
seats. This coating both conducted the heat well and prevented the
micro-welding and thus the oxides from forming. Without the TEL and
engine is run hard, the valve temperature rises. The longer and harder
engine is run the more rapidly we get rapid recession of the valve
yet, as the valve recesses, and assuming you do not reset the tappet
the valve spends less time seated and heats even more. This causes more
to form and the wear rate to increase. The seat continues to wear and
eventually the valve becomes so hot it burns.
So how do we
prevent our older cars from damage? We
have several choices.
not drive on extended runs or drive the car hard. Well that leaves out
in Texas and participating in autocross so that is not really an option
Ray would have to park his Spitfire and not do any 100 mph testing on
Rebuild the head and install new valves and valve seats. This is a good
if you are in need of a head rebuilding anyway and it is completely
It is, however, an expensive option for a car otherwise running well.
It is not
an option for some of the older cars which will not accept replacement
seats or require custom produced valves.
lead substitutes and add some to each tank of gas. These lead
additives usually contain some combination of phosphorus,
sodium or potassium compounds. This
is a low cost
option and lead substitutes are readily available at local auto stores.
Hopefully this will be the case for years to come. However there is one
that should be added. According to some testing done in Australia and
UK, the effect of lead substitute additives is very
temporary. While the effect
of TEL in protecting your valves lasts
several tanks of gas past the last use, the effect of
or potassium additives is gone almost immediately when gas without the
is burned. So if you use any of these additives, be sure to be regular
the original reason for adding lead based TEL to our gas, we need to
worry about replacing the anti-knocking qualities of leaded gas. The
rating of our gas now is boosted by the addition of ethanol. I just
replace fuel pump diaphragms, rubber fuel lines, all the rubber gaskets
32 and 49 and any paint on which I spill this new ethanol laden gas.