Re: a really good question

[Menko Johnson <menkoj@xxxxxxxxx>]

I will unabashedly steal this from a friend of mine who is quite
experienced in mopars and automotive technology in general.  I had
asked him the same question since I was rebuilding my head recently.

Dan Stern writes: 

"The mechanism by which Lead prevented valve seat recession is
commonly misunderstood. It's nothing to do with lubricating anything.

With an unhardened exhaust valve and seat, the valve and seat can
micro-weld to each other if they get hot enough. Lead acts as a buffer
to prevent this happening. The important thing is that exhaust valve
and seat recession ONLY takes place when the valve gets hot enough to
undergo localised welding. 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.

The main thing to remember is that this bad stuff *cannot* happen If
the valve never reaches the crucial temperature. Whether the valve
reaches the crucial temperature depends mainly on how the car is
driven and used (Towing, drag racing or pedal-on-the-floor hauling
WILL heat the valves--driving down the highway at a constant 120
klicks won't, and neither will hopping from traffic light to traffic
light in the city or running down to the local grocery for a carton of
ice cream.) 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. The Slant-6 has ample
exhaust valve seat cooling and stout valve material, and the valve
itself is small enough relative to the combustion chamber area that
you really have to abuse the engine before things heat up to the
danger point.

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. The majority of the Lead
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. 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. Because for quite a while only unleaded fuel of
subregular octane was available, plenty of people experienced these
effects from using unleaded. While many of those engines that suffered
under this low-octane unleaded really *DID* need the lead (high load
and/or high-RPM engines), the bulk of the failures were due to the low
octane increasing combustion chamber temperatures (see above). And so
the myth was born that old cars' engines "WILL DIE" if run on
unleaded.

These days we have wider availability of high-octane unleaded fuels,
which obviate the insufficient-octane cause of valve heating and
subsequent localised welding.

if you have an old car that is a low-stress application , used in
daily-driver service, then you need have no qualms about using
whichever octane grade of lead-free fuel your car runs well on and
drive it for a Loooooooonnnnnng time with nary a valve or seat
problem. Many US-based 6 and 8 cylinder engines fall into this
category in normal daily driving
service. 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). This is utterly
standard practice in the rebuilding of cylinder heads, and has been
for
years, as for instance when doing a gas conversion.

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. 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. Drive and
enjoy! You likely won't experience any problems for a LONG time, if
ever. If you ever do, have a head job done.
"