Quote: I use lead substitute (CD2). One ounce treats 10 gallons. You can get it from most "big box" retailers.
Waste of money, time and effort.
The additives available on shelves vary widely in what they do. Some of
them use a sodium salt and claim to duplicate the buffer effect of lead.
Some of them use "MMT" (methylcyclopentadienyl manganese tricarbonyl),
which is not a very nice substance at all and is of questionable benefit
in buffering exhaust valves. Regardless of whether or not any human-health or environmental risk is posed by MMT, the stuff causes hard red deposits on your spark plugs that will cause you to need to replace them more often. Other additives are simply octane boosters of varying effectiveness and varying side effects.
Some history and tech:
With an unhardened exhaust valve and seat, the valve and seat can micro-weld to each other if they get hot enough. Then, when the valve opens next, the metal pulls apart like taffy. This roughens the meeting surfaces, and they become quite abrasive. The pounding/turning of a valve with such "pulled" metal on it creates a nice grinding wheel effect on the seat. In addition, the roughened surfaces no longer seal against each other properly, which eventually allows still-burning combustion gases to flow through the "closed" valve, causing a blowtorch effect on the poor valve and depriving it of any prayer of a chance to cool while it's on the seat. The blowtorch effect rapidly deteriorates the seal further, snowballing the seat recession.
Lead acted as a buffer to prevent this localised welding. Very *VERY* little lead is required to prevent the localised welding and
"taffy pull apart" effect that leads to the abrasive surface which,
through incidental or positive rotation of the valve, eventually
grinds-down an unhardened seat. I'll emphasize that again: VERY LITTLE
LEAD. The remainder was in the fuel as an octane booster, that's all. It
was widely used because it was a very cheap and very effective octane
booster. The important thing is that exhaust valve and seat recession ONLY takes place when the valve gets hot enough to undergo localised welding.
Whether the valve reaches the crucial temperature depends mainly on how
the engine is loaded and run. Other factors in the margin of safety
include the size of the exhaust valve, its material, and the efficiency of
valve seat cooling in that particular engine design. If you have an old engine that is in a low-stress application (such as virtually any street-driven Slant-6), not heavily loaded or run at high RPMs, then you need have no qualms about using whichever octane grade of lead-free fuel your engine runs well on and drive it for a Loooooooonnnnnng time with nary a valve or seat problem.
When unleaded fuels were first widely introduced (which introduction was
brought about by legislation) , there was generally only one grade of
unleaded available, and the octane was *quite* low--less than that of
leaded regular. We all know that when you use a fuel of insufficient
octane, your engine pings (detonation, pinking, pinging, spark knock--call
it what you will.) This phenomenon creates *tremendous* heat in the
combustion chamber--certainly enough heat to push the exhaust valves to
the crucial temperature.
These days we have universal availability of high-octane unleaded fuels,
which obviate the insufficient-octane cause of valve heating and
subsequent localized welding.
The way to eliminate even the *possibility* of valve heating causing
localized welding and subsequent seat recession is to install hardened
exhaust valve seat inserts and exhaust valves of upgraded material
(typically 21-4N stainless instead of 21-2N) when an engine rebuild is
undertaken.
Hard seats and valves are readily available for just about anything you
want to put them in. It's a very common operation and a competent machine shop can handle it, and it adds very little to the cost of a cylinder head reconditioning job. But the main thing here is that there's absolutely no reason to tear into the engine solely to install hard seats. There is no
collateral damage from seat recession; the engine simply runs poorly once
the recession advances.
It's worth noting that on the East Coast of the US, Amoco marketed
unleaded high-octane gasoline for *decades* before the EPA decided to
"unlead" the country's fuel by regulation. That Amoco high-test unleaded
was widely regarded as quite a fine fuel indeed.
Some feel that lead-substitute additives will "give you peace of mind".
That's about all the good they'll do. They're expensive, messy to work
with, often hazardous to store, are a hassle to handle, and really
aren't necessary on a street-driven Slant.
Of greater concern to you should be the growing use of "reformulated" and
"oxygenated" fuels in many areas of the country, either year round or
seasonally. These fuels can attack and degrade some parts of your fuel
system and can cause running problems. There's a lot you can do for very
little money and effort to proof your old engine against these new fuel
formulations, including replacing rubber and plastic materials in the fuel
system with parts made of modern materials.
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