Stainless-Steel Valves. Although stainless-steel valves may be offered in varying grades and alloy recipes, high-performance stainless-steel valves are most commonly made of material referred to as EV8 (a more expensive heavy-duty stainless alloy material), and are made from a one-piece forging. In addition, some valve makers offer a stronger stainless-steel formula that offers higher heat resistance. Some makers use EV8 only for their exhaust valves, while others utilize this material for both intake and exhaust valves.
High-quality performance stainless valves should feature hard stellite tips (since stainless is not hardenable, a hardened tip must be welded onto the stem) and hard chrome-plated stems (not cheap flash chroming) to reduce guide wear. Undercut stems contribute to a slight weight reduction and benefit flow characteristics. Note: If a particular brand of stainless-steel valves does not feature a hard tip, the use of lash caps will be required.
Titanium Valves. Titanium offers the highest strength-to-weight ratio of any known metal. In an unalloyed condition, titanium is as strong as some steel materials but about 45% lighter. When used to manufacture automotive valves, titanium is alloyed with small percentages of various materials, including copper and molybdenum. Titanium can be challenging to machine, as it can gall if tooling isn’t hard and sharp enough, and if the material isn’t cooled properly during machining.
Many titanium valves are produced by starting with a forging, then machined to final shape. But some are produced using a two-piece inertia-welded design. According to Xceldyne Technologies, this process is so effective that inertia-welded valves have been certified as having a superior grain structure than a one-piece forged design. The valve is then CNC-machined and in many cases undercut in the stem area to allow a bed for the inlay of a coating. The valve is then plasma moly-coated.
Specific sections of the valve are further machined and the stem is ground, leaving the plasma moly coating over only the desired stem area. The head, stem and keeper grooves are then final-machined. Stem grinding is finalized to establish dimensional tolerance to within .0002 in. A final precision-polishing reduces the potential for carbon buildup.
Three styles of valve tips are generally available, which include a hardened steel tip or a ceramic-coated tip (ceramic tips are to be used in conjunction with lash caps) and thin-film technology such as a plasma vapor deposition (PVD) coating.
Since titanium is a relatively soft material, it requires a protective contact surface at the stem tips, usually hardened lash caps. Xceldyne noted that when valves feature stem diameters smaller than 5⁄16 in. (7mm or less), a specialized hard coating is applied to the stem tip to protect it from lash cap friction.
The durable, hard ceramic coating is intended to protect the titanium from the friction caused by the lash cap. Other coatings such as a PVD treatment, a chromium nitride treatment, chemical vapor deposition (CVD), diamond-like carbon (DLC) or other highly specialized protective application may be applied to the tips. This hardened feature at the tip prevents material transfer or galling between the tip and lash cap.
Hollow titanium valves are also available, either with hollow stems or with a combination of hollow stems and hollow heads. Hollow-stem designs reduce valve weight by about 10%. The hollow-head design is a proprietary process that removes an additional 6 to 8 grams of weight (depending on valve size). As part of the proprietary process, the inside of the valve head may be reinforced to provide a support structure for strength and rigidity.
The following precautions should be taken during the handling and use of titanium valves:
•Do not touch the valve surface with your bare hands, since fingerprint acids may affect the coating. Use gloves or coat the valve with oil before handling.
•Never use a lapping compound or any abrasive material when the valve is coated with a PVD-style coating.
•Replace valve seats during each and every rebuild to ensure a proper valve-to-seat contact. The width of the contact zone (valve face to valve seat) should be at least 1mm.
•New valve seats should be a relatively soft material, such as bronze or nodular iron (heat-treated to Rockwell RC32 or less).
•Unless directed otherwise by the valve maker, always use hardened lash caps on titanium valves. Some makers offer valves built with friction-welded hardened tips. Bare, unprotected titanium tips will mushroom when exposed to rocker arm forces.
If the titanium valve features a stellite tip, the tips can be ground during valve service, but with caution. You should be able to safely remove a maximum of approximately .015 to .020 in.
As far as valve seats are concerned, traditional cast or hard seats can wear a groove into the valve face, so a nickel-bronze seat material is recommended. Other exotic-metal seats are also available.
Titanium valves are designed for applications where valve train weight needs to be reduced, for high-rpm and extended high-rpm applications, since titanium valves allow for higher engine speeds and will accommodate highly aggressive camshaft profiles. However, for extreme-temperature situations (blown, turbo and nitro engines), titanium may not be the ideal choice. Also, for many street applications, titanium may not be a good choice for an engine that doesn’t need to rev as highly, or for an engine that will be buttoned up and not torn down and serviced regularly. In other words, it’s probably best to reserve the use of titanium for naturally aspirated race or inlet-side forced- induction applications where valve train weight and sustained high-rpm use is paramount.
Inconel Valves. Inconel is a registered trademark of Special Metals Corp., and refers to a family of nickel-based superalloys. Inconel alloys are oxidation- and corrosion-resistant materials designed for use in high-heat environments. Inconel retains strength over a wide range of operating temperatures. As opposed to steel or aluminum, it doesn’t creep as much (change dimension) under high-heat use.
Inconel alloy makeup can include carbon, manganese, silicon, phosphorous, sulfur, nickel, cobalt, chromium, iron, aluminum, molybdenum, titanium, boron and copper, with the heaviest material concentration accounted for with nickel and chromium.
Five “grades” of Inconel are in common use—600, 625, 690, 718 and 939. Basically, the benefits of Inconel include light weight, resistance to extreme temperatures, high strength and resistance to thermal dynamics.
Inconel valves offer extremely high thermal resistance and are designed for high-heat use, such as found in turbocharged, supercharged and nitrous applications.
Nimonic 90 Valves. Nimonic is a nickel-chromium alloy, a specific grade of which—Nimonic 90—is used by some makers for producing high-performance valves. Nimonic 90 is a superalloy comprised of nickel-chromium-cobalt, which offers high strength and the ability to withstand extremely high temperatures, reportedly well within the 2000°F range, without distortion. Manley reports that they’ve seen success in such extreme applications as nitromethane and high-boost turbo applications such as multiple-turbo tractor-pull engines.
Inconel IS a "superalloy". Inconel 625 is an older alloy, though. It's not recommended for core metal temperatures over 1,200*F: http://www.specialmetals.com/products/i ... 625lcf.php
Nimonic Alloy 80A, used by Saab in exhaust valves is good to about 1,500*F of core temperature. Or, more simply, 300*F higher than whatever EGT you consider "acceptable" for Inconel 625.
Nimonic alloy 115 is good for an additional 350*F, or a core temperature of 1,850*F. (easily 2,000*F EGT)
Sodium-Filled Valves. Sodium-filled valves feature stems that are precision-gun-drilled and filled with a specially formulated sodium. This achieves weight reduction (the result of the gun-drilling to create a hollow stem) and better heat dispersion. There is some debate about the efficiency of this heat transfer, due to concerns that the heat transfer to the guides increases guide wear. Even given these concerns, it’s interesting to note that the Chevy LS7 engine features sodium-filled exhaust valves (along with titanium intake valves).
The hollow spaces in the head and stem of a sodium-cooled valve are filled to about 60% of their volume with metallic sodium, which melts at about 206°F. The inertia forces that result during valve opening cause the liquid sodium to migrate upward inside the stem, transferring heat to the valve guide and subsequently to the water jacket.