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 concerning the efficiency of this heat transfer due to concerns that the heat transfer to the guides increases guide wear. Even with these concerns in mind, it's interesting to note that the Chevy LS7 engine features sodium-filled exhaust valves (along with titanium intake valves). The hollow space in the head/stem of a sodium-cooled valve is filled to about 60 percent of its volume with metallic sodium, which melts at about 206 degrees 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.
HOLLOW STEM VALVES
Hollow-stem stainless steel or titanium valves (no sodium fill) features gun-drilled stems to create hollow stems strictly for weight reduction (this reduces valve weight by approximately 10 percent as compared to a comparable solid-stem valve). Citing Ferrea as an example, its hollow stem valves are gun-drilled and micropolished, and feature friction-welded tips, shot-peened and rolled lock grooves, "avionics" chrome-plated stems and feature face hardness up to 42 HRc. This micropolishing reduces the risk of stress risers in the I.D. walls of the stem.
VALVE MATERIALS & SELECTION FROM A CAM MAKER'S POINT OF VIEW
Since valve weight in particular naturally relates to valve spring force and cam profile selection for given race applications, I contacted the folks at Comp Cams for their input. For the majority of street engines, a quality stainless steel valve is recommended. Billy Godbold of Comp Cams noted that the company prefers titanium for most race applications but that some engine builders that specialize in turbocharged applications prefer a high nickel Inconel valve. "We have to leave the final decision up to the engine builder, but it does limit the cam designs we can choose from when going to a heavy valve." Hollow stem valves tend to work great on the intake side but they are much more difficult to manufacture and to inspect for defects on the I.D. surface. Many of the upper-echelon engine builders shy away from hollow valves for that reason in endurance (NASCAR or 24-hour style) racing. Comp Cams' Thomas Griffin noted that stainless steel valves are most common in street and mild-performance racing. Titanium is used when valve weight is important and when budget is not a consideration. Inconel is used when exhaust gas temps get really high. Stainless steel (for street performance) has much better durability characteristics than titanium and the street guys won't usually see the real benefits of titanium. In racing, use titanium when you want to lose weight and spend a lot of money. If durability is a concern, you're already making as much power as you want and are already turning the engine as high as you want, then you need to use a stainless steel material. If you're running nitromethane, then an Inconel exhaust valve material will be your best bet if you want to finish a race. NASCAR engines use a variety of titanium materials because of the temperature and impact-related issues associated with their severe applications. "On the exhaust side, sodium-filling is the best way to increase the head capacity of a hollow exhaust valve," Godbold noted. "If stock diameter steel valves are required, but a valve weight is not mandated, going to a hollow intake and sodium filled exhaust is certainly a major advantage. "From our point of view," Godbold continued, "the most critical point is to get mass out of the valve. The lighter the valve, the stiffer the valvetrain system is in relation to the mass it must move. Also, as the valve mass is decreased, you can reduce the spring force needed to control a given valve motion and/or go to a more aggressive cam design that can make more power." To prevent excessive tip wear on titanium valves (especially the small O.D. non-hardened tip variety), using a lash cap provides an excellent wear surface for the roller tip, sliding contact or cam follower. While you always want the rocker to push down on the section of the cap directly over the stem, using a lash cap certainly gives you a better sense of safety as you approach the edge of the valve in high lift applications with a very small (5-7 mm) stem. Comp Cams' Griffin noted that since engine builders and cam designers push limits, the stems get smaller and the lift gets higher. As a result, lash caps are used with smaller valves to better distribute the load across a larger area than the base stem tip area. They are also used with higher lift because when the lift is increased, the rocker arm sweep length usually increases across the valve or lash cap as well, compounding the issues caused by the smaller valve stem diameter. We only provide solid stem designs through Comp Cams, but the hollow stems certainly provide lower mass. Theoretically, these would be the best way to go, but there are very serious manufacturing and inspection hurdles to jump when producing a hollow valve. The question becomes, "Where is the safest and best way to invest my money when building this engine for this specific application and within this budget?" Sometimes your answer will be a hollow valve, but in most cases it would probably be a solid valve stem, unless we see a major technical jump on the manufacturing side. As the OEMs start pursuing that route on the mass-market side, we could find new technologies available to make these parts on the performance and racing side. We have certainly seen that effect with the availability of several new Beehive valve springs and now nitrided flat tappet cams that we offer. Just a few years ago, we could not provide either of those technologies dependably, at a high level and for a reasonable cost, but lower machine cost (although the cheapest machine was still on the order of a quarter-million dollars plus) for the tools to manufacture these parts became available in recent years. Per Griffin, solid stems are stronger, hollow stems are obviously lighter, but the quality control of an inside stem surface is very difficult to control. Because of the pounding of the valve upon closing, the sensitivity to failure is compounded if there are machining marks that can neither be controlled nor removed because they cannot be seen. While 95 percent of our market uses a square groove lock, the stresses in the valve are minimized with a single round groove. The lowest stress system is a top lock design with a small round groove at the top of the lock, and the lock is designed with a slightly smaller angle than the retainer so that the valve is held by the collet force squeezing more at the bottom region of the lock-to-valve interface. "Round grooves are best because they address the issue of stress concentration zones associated with a very small radius of the inside corner of a square groove lock. In high-end racing with any material valves, retainers and locks," Griffin stated. "I would use only a single groove because it forces the lock to grip the valve stem and hold it in place." Many OEM engines feature multiple-groove steel locks and valves that allow the valve to spin in the locks, which is fine for street and low-end performance. Because there is a loose fit between the valve and locks, it could cause an over-stress condition if used in severe racing. Godbold noted that because Comp Cams only offers a street valve, we tend to defer many of these questions to the experts at the valve engineering and manufacturing end. We have worked very closely with Del West and Xceldyne on projects in the past and will continue to do so in the future.
As a quick summary, high-quality EV8 stainless steel valves are a good choice for street and naturally-aspirated race engines, while titanium valves benefit high revs in race engines that don't experience extremes in temperatures and Inconel (and other similar nickel content) valves are suggested for extreme cylinder pressure/extreme temperature applications (primarily exhaust). For extreme-temperature applications such as very high-cylinder-pressure nitromethane, blown or supercharged use, a combination of titanium intake valves and Inconel or Nimonic exhaust valves are used.
EXPLAINING VALVE COATINGS
Instead of simply listing the names for the various valve coatings, we wanted to provide a bit of information about each of these specialized coatings. The information that follows was provided courtesy of Joe Rogers of Xceldyne. The primary role of the tribological coating is the reduction of friction to enable increased service life and functional reliability. At Xceldyne Technologies, an advanced in-house precision finishing technique referred to as micro-polishing is directed upon critical valves surfaces prior to PVD or CVD thin film (2-4 micron thick) coating to meet stringent coated surface finish requirements such as 0.1µm Ra or RpK. Physical Vapor Deposition (PVD) occurs because of a physical reaction. Inside a vacuum chamber plasma environment, metals are deposited via evaporation, sputtering or arcing fragments of the metals which are physically moved on to the substrate. In other words, there is no chemical reaction which forms the coating on the substrate. Chemical Vapor Deposition (CVD) occurs because of a chemical reaction. The process exploits the creation of solid materials directly from chemical reactions in gas and/or liquid compositions or with the substrate material. The product of that reaction is a coating material that condenses on all surfaces of the part to be coated and inside the vacuum chamber plasma environment. Thin-film coating options such as CrN (Chrome Nitride), TiAlCrN (Titanium Aluminum Chrome Nitride), DLC (Diamond-Like Carbon) and a:SiC (Amorphous Silicon Carbide) are selected during the valve design process based on the suitability of the coating properties for the specific engine application and with reference to historical post-engine teardown feedback and analysis. In certain applications, a combination of coatings may be selected for an individual valve. For example, the "ductile" properties of a CrN coating (hardness 1,600 HV) will be selected for application to the valve tip, while the "low friction" attributes of a DLC or a:SiC coating (friction coefficients 0.1 or less) will be chosen for application to the critical valve seat head region. Dry fuels, such as those with low-sulfur content or alcohol-based, are suitable environments for certain low-friction and inert thin-film coatings. The application of a coating upon the valve head and valve stem can be exploited as a solid lubricant minimizing adhesive wear between the valve-seat or valve-guide interface. Adhesive wear, also known as scoring, galling or, worst case, seizing, results when two solid surfaces slide over one another under pressure. Surface projections, or asperities, plastically deform and eventually weld together under the high localized pressure. As sliding continues, these bonds break. This creates cavities on one surface and projections on the other. Tiny abrasive particles can also form, causing additional wear. Specific to applications associated with excessive exhaust gas temperature, hybrid coatings (Pt, Pd, Nb based) have been examined as a means to retard embrittlement of the base Ti material by minimizing the ingress of oxygen through the coating and represent novel strategies to yield robust coatings for ultra-high temperature environment applications.
A Sample Listing of Performance Valve Manufacturers
Comp Cams 3406 Democrat Rd. Memphis, TN 38118 (800) 652-0406, ext. 13411 www.compcams.com Crane Cams Inc. 530 Fentress Blvd. Daytona Beach, FL 32114 (800) 652-0406, ext. 13412 www.cranecams.com Del West Engineering Inc. 28128 W. Livingston Ave. Valencia, CA 91355 (800) 652-0406, ext. 13413 www.delwestusa.com Electronic Chrome & Grinding 9128 Dice Rd. Santa Fe Springs, CA 90670 (800) 652-0406, ext. 13414 www.ecgrinding.com Elgin Industries 1100 Jansen Farm Dr. Elgin, IL 60123-2555 (800) 652-0406, ext. 13415 www.elginind.com Ferrea Racing Components 2600 NW 55th Ct., No. 234 Ft. Lauderdale, FL 33309 (800) 652-0406, ext. 13416 www.ferrea.com KPMI (Kibblewhite Precision Machining) 580 Crespi Dr., Ste. I Pacifica, CA 94044-3426 (800) 652-0406, ext. 13417 www.blackdiamondvalves.com Manley Performance Products 1960 Swarthmore Ave. Lakewood, NJ 08701 (800) 652-0406, ext. 13418 www.manleyperformance.com Milodon Inc. 2250 Agate Ct. Simi Valley, CA 93065 (800) 652-0406, ext. 13419 www.milodon.com Pioneer Inc. 5184 Pioneer Rd. Meridian, MS 39301 (800) 652-0406, ext. 13420 www.pioneerautoinc.com Racing Engine Valves 4704 NE 11th Ave. Ft. Lauderdale, FL 33334 (800) 652-0406, ext. 13421 www.revalves.com RPM International Inc. 16313 Arthur St. Cerritos, CA 90703 (800) 652-0406, ext. 13422 www.racingpartsmaximum.com SI Valves 4477 Shopping Ln. Simi Valley, CA 93063 (800) 652-0406, ext. 13423 www.sivalves.com Supertech Performance Inc. 3580 Charter Park Dr. San Jose, CA 95136 (800) 652-0406, ext. 13424 www.supertechperformance.com Xceldyne Technologies 37 High Tech Blvd. Thomasville, NC 27360 (800) 652-0406, ext. 13425 www.xceldyne.com Zanzi Spa Corso Vercelli 159 10015 Ivrea (TO), Italy +39 0125251540 www.zanzi.com