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Valve Seat Technology for Stock and Performance Applications
Valve seats are a critical
engine component because they are the foundation of the valvetrain. The seats
provide a surface for the valves to seal against when they close so there’s no
loss of compression or pressure from the combustion chamber. The seats also
help cool the valves by conducting heat away from valves into the cylinder
head.
The seats also support the valves and determine
installed valve height, which affects valvetrain geometry and valve lash. The
diameter of the seats and the contour of the angles that are machined into the
face of the seats also limit how much air the engine can flow at any given
valve lift and rpm. Because of all these factors, the valve seats deserve a lot
of attention whether you are rebuilding a stock engine or building a
performance engine.
What’s hot on seats?
Though
the basic metallurgy of valve seats hasn’t changed a great deal over the years,
there have been some significant developments in the past year or two that is
changing the way many performance engines are built. Racers love titanium
valves because of their light weight. Titanium valves are typically 40 percent
lighter than steel valves, but they are also very expensive. The price of
titanium has soared to record highs due to rising demand and a limited supply
of metal, but for serious racers there is no other alternative (except maybe
hollow stem stainless steel valves).
One of the drawbacks of using titanium valves is
that titanium does not conduct heat as well as steel. Consequently, the valve
seats should have good thermal conductivity to pull heat away from the valves.
If the exhaust valves get too hot, they can cause preignition and detonation
that can destroy an engine. Excessive temperature can also cause the valves to
burn, and the seats to pit and erode, either of which can cause compression
loss in a cylinder.
When
titanium valves first became popular, tool steel alloy valve seats were often
used. The seats were durable enough but proved to be too hard for the titanium
valves, and they didn’t pull heat away from the valves fast enough to provide
adequate cooling at high rpm. Cast iron seats were tried and worked well enough
on street engines, but they lacked the durability required for NASCAR and other
high rpm, high horsepower engines.
The seat
material that proved to work best with titanium valves turned out to be
beryllium copper (Be-Cu).
The amount of beryllium, copper and other
ingredients in the alloy also affect the thermal conductivity of the seat and
how quickly it can transfer heat from the valve to the cylinder head. Some 2
percent beryllium alloys have a thermal conductivity rating of up to 140 BTUs
per foot per hour per degrees F, while others that contain less beryllium are
in the 60 to 68 BTUs per foot per hour per degrees F.
Most tool
steel and iron alloy valve seats, by comparison, are rated much lower at 20 to
22 BTUs per foot per hour per degrees F. This is good enough to provide
adequate cooling for stainless steel valves in an aluminum or cast iron
cylinder head, but not for titanium valves in a high horsepower (over 600 hp)
performance engine.
Many
racers have been using a softer beryllium copper alloy for the intake valve
seats, and a harder, more thermally conductive alloy for the exhaust seats.
Intake valves run much cooler than the exhaust valves, but also tend to have
more radical lift profiles and slam shut harder than the exhaust valves.
Consequently, a softer seat material has more of a dampening effect when the
valve closes to reduce the risk of valve bounce when the valve closes. It’s
also kinder to the valve face and helps extend valve life – which is important
when you’re paying up to $100 or more each for titanium valves!
Though
beryllium copper seats have been the alloy of choice for racers using titanium
valves, one of the drawbacks to using seats made of this material is that
beryllium is a toxic metal. There’s no risk in handling the seats, but the dust
that’s given off when cutting or grinding the seats can be dangerous because of
the beryllium it contains. The danger is in inhaling dust that contains
particles smaller than 10 microns in size. Beryllium may cause a lung disease
called berylliosis or other allergic reactions. Dust can be minimized by using
a liquid coolant while machining the seats. Wearing a dust mask that meets HEPA
standards is also a good idea.
Occupational
Safety and Health Administration (OSHA) regulations say workers should be
exposed to no more than 2.0 micrograms of beryllium dust per cubic meter of air
during an 8 hour shift. But these regulations are over 50 years old, and some
are calling for much more stringent regulations that would reduce exposure to
0.2 micrograms per cubic meter. Because of these concerns, other valve seat
alloys are now being used in place of beryllium copper.
Rule Changes Bring in New Seats
The 2007
rule change in NASCAR that finally did away with leaded gasoline forced many
teams to take a second look at the valve seat alloys they were using in their
engines. Tetraethyl lead is an excellent octane booster for high compression
racing engines, and it also forms a protective coating on the valve seats that
acts like a lubricant to extend valve and seat life. But lead is a toxic heavy
metal. Because of this and the fact that lead poisons catalytic converters and
oxygen sensors, tetraethyl lead was phased out of most motor fuels back in the
1970s.
Even so,
NASCAR continued to use leaded fuels because there were no rule requirements
for emission controls, and electronic engine controls were forbidden. NASCAR
engine technology was essentially frozen in the pre-fuel injection era, so
leaded racing gas lived on.
When
NASCAR finally succumbed to environmental pressure to get the lead out, some
teams found the seats they were using didn’t provide enough cooling for the
titanium valves in their engines. Other alloys were tried, and some new
beryllium-free copper-nickel based alloys were found to provide even better
cooling and durability.
One of
these new alloys is a product called “Moldstar 90,” a patented and proprietary
copper-nickel alloy with a thermal conductivity rating of 90 BTU per foot per
hour per degrees F. Tom Malaska of CV Products said many NASCAR teams are using
this new alloy because it is safe to work with (no beryllium dust hazard in the
machine shop), and it cools better than Be-Cu seats. Malaska said the new alloy
is being used mainly by NASCAR teams, and that availability is limited. The
alloy is currently only produced in bar stock rather than tube, which means
there’s a lot of waste when valve seats are machined out of the bar stock.
Steve
Erickson of Winsert, a supplier of unfinished valve seats to other aftermarket
valve seat suppliers, said his company is introducing a brand new
beryllium-free copper alloy seat called “Velocitor” at the Performance Racing
Industry (PRI) Show in Orlando, Florida. The alloy is intended for racing applications
that currently use beryllium copper seats. Erickson said testing has shown
their new Velocitor copper alloy outperforms beryllium copper valve seats in
both thermal conductivity and durability. It is also easier to machine and
poses no health risks whatsoever.
Erickson
also said the new alloy should give engine builders more freedom to maximize
horsepower by opening up the inside diameter of the seats and changing the seat
angles to optimize airflow. He said some tests have shown as much as a 10 percent
gain in horsepower is possible over beryllium copper seats!
Higher Metal Prices
Another
factor that is driving change in valve seat alloys is the soaring cost of
nickel, cobalt and many tool steel alloys. Two years ago, nickel was selling
for $6 to $7 a pound. Today, the price has soared to $32 a pound. Some blame
the Chinese for driving up prices with their exploding economy. Others blame
the ongoing war in Iraq. Some claim metal suppliers are creating artificial
shortages to manipulate the market and drive up prices.
Whatever
the causes are, the fact remains that many metals today are much more expensive
than they used to be.
Brian
Bender of SB International said some OEMs are now substituting seats made with
new iron-based alloys for ones that were formerly made of nickel or cobalt
alloys. These changes are affecting mostly heavy-duty diesel engines, and
industrial engines that run on natural gas or propane. He said the new alloys
are working well, and are less expensive than the nickel and cobalt alloys they
replaced.
Bob
McBroom of Dura-Bond said his company sells a lot of tool 70000 series powder
metal high alloy tool steel seats for dry fuel engine applications. He said the
price of their raw materials have doubled, but thinks the price increases have
leveled off. The company also sells a 30000 Series powder metal valve seat that
works well with stainless steel or titanium valves, and is used in many racing
heads. Dura-Bond also has low alloy 15000 and 25000 Series seats for stock
passenger car and light truck engines.
“We have
a new high performance seat that contains 15 percent copper to provide higher
thermal conductivity,” said McBroom.
Powder Metal
Most late
model aluminum heads are fitted with either cast iron or iron alloy seats, or
powder metal (PM) seats. PM seats have become very popular with the vehicle
manufacturers for a variety of reasons. PM seats are less expensive than iron
seats, and they are proving to be very durable. PM seats often show little wear
at high mileages. Consequently, if you are rebuilding a head with PM seats, the
seats may only need a light touch-up.
Simple,
right? Well, PM seats tend to work harden as they age, and can be be very hard
(up to Rockwell 40 to 50) making them difficult to machine. As long as you have
equipment that can cut hard powder metal seats, remachining the seats should be
no problem. But if you don’t have equipment that is designed for this kind of
work, you may be better off replacing the seats with new ones to get restore
the seats. New powder metal seats are much softer (typically around Rockwell C
25) when they are initially installed, and easier to machine than aged seats.
They also require less force to press into the cylinder head than iron or steel
valve seat inserts.
One
difference between cast alloy seats and powder metal seats is the way the seats
are manufactured. Cast alloy seats are made by melting and mixing different
metals together so they combine chemically. This molten soup is then poured
into a mold and cast to shape. The rate of cooling and subsequent heat
treatment of the metal determines its microstructure, hardness, strength and
other physical properties.
Powder
metal seats, by comparison, are made by mixing together various dry metal
powders such as iron, tungsten carbide, molybdenum, chromium, vanadium, nickel,
manganese, silicon, copper, etc.). The powder is pressed into a die mold, then
subjected to high heat and pressure (a process called “sintering”) to bond
together the metals and form a solid composite matrix with very uniform and
consistent properties.
One of
the advantages of powder metal sintering is that materials that are difficult
or impossible to mix together in a molten state can be blended together and
bonded to create totally unique materials. Another advantage of the powder
metal process is that parts can be manufactured very close to final tolerances,
reducing the amount of machining that’s needed to finish the part to size. This
lowers production costs and boosts manufacturing productivity.
The main
reason why vehicle manufacturers have switched from cast alloy seats to powder
metal seat inserts, however, is to extend engine durability. Most late model
engines have to be emissions-certified to 150,000 miles or higher depending on
the application and model year. If the valve seats can’t go the distance during
durability testing, the vehicle manufacturer can’t certify the engine.
Powder
metal seats are very good at handling thermal stress as well as impact stress,
and typically show minimal wear after tens of thousands of miles of use. The
homogeneous consistency of a powder metal seat also improves heat transfer,
which is good for the valves, too. Powder metal seats also tend to experience
less micro-welding between the seat and valve even at high combustion
temperatures, which helps extend the life of both components. Yet some engine
rebuilders are leery of powder metal seats, and prefer to replace PM seats with
conventional iron seats.
Reconditioning The Seats
In cast
iron heads with integral seats, the sealing surface on the seats is restored by
cutting or grinding. How the seats are cut also affects valve height. Changes
in valve height can be compensated for by grinding the tip of the valve stem to
reduce its overall length. But there are limits as to how much you can grind
off the hardened tip of a valve stem before you grind away the case hardened
layer. Replacing the stock valves with ones that have thicker heads is one
alternative.
In
situations where an integral valve seat is damaged, the head can often be
salvaged by cutting out the old seat and installing a seat insert – provided
the casting is thick enough to accept a seat.
With
aluminum heads, badly worn or damaged seats can be removed and replaced with
new seats. But as one supplier said, the demand for replacement seats has been
declining because of the longevity of the original equipment powder metal seats
that are used in many late model engines.
When
seats are replaced, the amount of interference fit is critical for proper seat
retention and heat transfer. The seats in some OEM heads may have as little as
.002 inch of interference fit – but keep in mind these seats were installed in
brand new heads. Engine rebuilders tend to use more interference fit to
compensate for any distortion in the seat recess that occurs when the old seat
is removed. Some use as much as 0.005 of interference in cast iron heads, and
up to 0.007 of interference fit in aluminum heads. Additional peening or
staking of the seats should not be necessary if the correct amount of
interference fit is used.
For best
results, the valve seat inserts should be chilled in a freezer prior to
installation, and the cylinder heat heated to reduce the amount of force needed
to install the seats. The outside diameter of the seats can be chamfered to
ease installation, and a lubricant also helps. No sealer should be needed for
seat retention, and some sealers may actually inhibit heat transfer by forming
a thin barrier between the seat and cylinder heat.
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