Back when metals were soft and men were hard, we had a World War. The first one. One of the things that came out of the technological burst that inevitably seems to occur with major conflicts was a major leap in engine technology… mostly thanks to the introduction of aircraft into the European theater. Aside from the plethora of variations in basic layouts of airplane motors (like V8, V12, W3, X4, radial multis, 2-cycle, diesel and others) a lot was discovered regarding the biggest nemesis of sustained, powered flight. Would you care to guess?
Well, here’s a hint. Exhaust valve failure! (OK; that was more than a hint, but when you figure that effects from altitude, speed, air density and temperature were still undiscovered territory for internal combustion… let alone constructing reliable powerplants that could handle unknown and massive variations in the fuel… you begin to appreciate what those early engineers were up against.) To avoid digressing too far into other areas of progress towards overcoming these problems, suffice it to say, then, as now, the “Achilles heel” of any poppet-valve engine design is the almost unfathomable levels of, and fluctuations in, sheer heat the exhaust valves must be able to sustain to keep a plane in the air. To give you an idea of where the state of the art stood early on, one of the chief high mucky-mucks of the day talked in terms of “valves per hour” of flight time and no engine at the beginning of the war had more than a 50/50 chance of lasting 50 hours in the air. (Still think biplanes and the Red Baron are romantic figures? Ha!) All the same, by the time America got into it, things had improved greatly. No small part of the credit belongs to a Yankee biker and engine designer cum aviation expert nonpareil… Glenn Curtiss. This is the guy who went 127 mph on his V8 motorcycle in 1907… then set an airplane speed record of 47 mph two years later. (Think about it!) His OX5 engine was the backbone of our Air Force (such as it was) at the time and (getting back to cooked valves) in no small part because of the development of materials and designs for this humble but most essential component.
Caption: Here we see just a few variations of head shape, stem “tricks” and retainer groove designs that have been employed during and since the Great War. There are even more today, but all of them owe a debt to these giant leaps for valve kind.
The poppet exhaust valve has always been a critical item because it is subjected to such high gas temperature (up to 2,000 degrees Fahrenheit) and high gas velocity, with small areas available (stem and seat only) for heat dissipation through the heads (of iron, bronze or aluminum) to the cooling medium, be it air or liquid. One method of attack on this problem has been through the use of improved materials. By 1918 the ordinary steels used at first had given way to high-speed tool steel, which has a reasonably high degree of strength at elevated temperatures. (Tungsten is the chief alloying element in such steel.) Unfortunately, this type of steel also burns readily at the seat of a leaking valve. Since about 1920, austenitic (high chromium) steels have been successfully used in various forms, with several other alloying elements including principally silicon, nickel and cobalt. A further important improvement, from about 1934, was the use of Stellite facing on both valve seats and seat inserts.
Another critical design contribution to exhaust valve life and reliability has been the use of a hollow valve, drilled like the bore of a gun improving the conductivity of heat from head to stem. Then, somebody came up with the idea of filling the hollows in valves. Water was tried first in about 1913, but the high-steam pressure exploded the valve stem. (YEE-HAA!) Mercury was next tried, with some success, since its vapor pressure is lower. But mercury will not “wet” steel. So, by 1928 aviation had adopted liquid sodium as the internal coolant, still used in large exhaust valves and in many non-aircraft engines… including pushrod British motorcycles.
The automatic lubrication of valves by engine oil, introduced by Hispano-Suiza in 1914 and thanks to the inevitable “trickle down” of all that technology to motorcycles (and cars) shortly after, has also been a major contribution to the present long life and reliability of exhaust valves in general. The point is the giant leaps were all made before any of us were born, and since then, valve technology has been more about refinement than breakthroughs.
But what refinement! Subtle improvements to valve shapes, heads and stems in particular… and the advent of “space age” materials (like titanium)… not to mention the ability to blend different metals in the stem with those in the head, have led to a level of reliability and performance (assuming the right choice has been made for a given application) those engineering pioneers would deem science fiction. Anybody shopping for new hi-per valves might benefit from a quick review of the most basic variations of today:
Stellite is a hard coating applied to valve-stem tips and faces to provide a hard surface to minimize wear. Stellite alloy is a nonmagnetic and noncorrosive cobalt-chromium alloy that may also contain a tungsten element. It resists embrittlement and annealing at higher temperatures. Interestingly, the term Stellite was derived from the name of a Scottish racehorse (yeah, I know…who cares?). Stellite is often applied to steel or stainless steel valves.
Sodium-filled valves feature stems that are precision-gun drilled and filled with 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. The sodium turns from solid to liquid at operating temperature and travels up and down the hollow stem transferring heat from the valve head through the stem to the valve guide. This works extremely well in almost every application. The only caution is with temperatures above 1,650 degrees when the sodium liquid turns into gas and can become dangerous. (Oh… and they are NOT cheap!)
Caption: Monometallic Valves, as you might imagine, are made of a single material… except when they aren’t (as in the case of a hardened stem tip like this)
Hollow-stem stainless steel or titanium valves (no sodium fill) exist strictly for weight reduction (approximately 10 percent as compared to a comparable solid-stem valve) and that is reason enough! Quality versions have the stems micro-polished (reducing the risk of stress risers) and feature friction-welded tips, shot-peened and rolled lock grooves, “avionics” chrome-plated stems, and extreme face hardness to add to their usefulness.
Although stainless steel valves may be offered in varying grades/alloy recipes, high-performance stainless steel valves are most commonly made of material referred to as “EV8,” and are made from a one-piece forging. In addition, some valve makers offer an even stronger stainless steel formula that offers higher heat resistance (Manley’s XH-428 is an example). 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 that hard in itself, a hardened tip protects and preserves the stem) and hard chrome-plated stems (not cheap flash chroming) to reduce guide wear. Hugely popular “undercut” (also called “thin”) stems contribute to slight weight reduction and benefit flow characteristics.
Caption: This is essentially the design and construction of a stock Harley exhaust valve. (Kinda fun to take a magnet to a factory exhaust valve and see where the change from ferrous to “non” actually occurs.)
Titanium (chemical symbol Ti) offers the highest strength-to-weight ratio of any known metal. In an un-alloyed condition, Ti is as strong as some steel materials, but about 45 percent lighter. When used to manufacture automotive valves, titanium is alloyed with small percentages of various materials, including copper and molybdenum. Ti 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. (Just for the sake of trivia, titanium was actually discovered independently by a couple of guys in the late 1700s, who reportedly named the material after the Titans of Greek mythology.) There are a lot of caveats involved in using Ti, including compatible seat materials and coatings… since the stuff isn’t all that hard or heat resistant. Which leads to…
Nimonic is a nickel-chromium alloy. A specific grade of this material, Nimonic 90, is a “super” alloy comprised of nickel-chromium-cobalt, which offers high strength and, especially, an ability to withstand extremely high temperatures (reportedly well beyond the 2,000-degree Fahrenheit range) without distortion. This material is also widely used in aerospace industries for applications such as valves in turbo motors and blades and discs in gas turbines. They’ve seen success in such extreme applications as nitromethane and high-boost turbo applications like multiple-turbo tractor-pull engines… and drag racers
Inconel is a registered trademark of Special Metals Corporation, referring to a family of nickel-based super-alloys, which are oxidation and corrosion resistant, designed for use in high-heat environments since it also retains strength over a wide temperature range. As opposed to steel or aluminum, Inconel just doesn’t creep (change dimension) under high-heat use. As you’d imagine these valves are most useful in turbocharged, supercharged and nitrous applications.
Caption: Not shiny chrome… hard chrome… and one of the good (and common) solutions to long-term operation in iron guides and with unleaded fuels
The Bottom Line
High-quality EV8 stainless steel valves are a good choice for street and naturally aspirated race engines, while titanium valves accommodate high engine speeds in race engines (that don’t experience uncommon extremes in temperatures) and Inconel (as well as others with similar nickel content) valves are suggested for extreme cylinder pressure/extreme temperature applications. Although… certain extreme-temperature applications involving very high cylinder pressures (nitromethane, blown or supercharged) might just do even better with a combination of titanium intake valves and Inconel or Nimonic exhaust valves.
That said, the newest, and perhaps most promising, wrinkle involves high-tech coating options such as CrN (Chrome Nitride), TiAlCrN (Titanium Aluminum Chrome Nitride), DLC (Diamond-Like Carbon) and a:SiC (Amorphous Silicon Carbide), which are typically selected during the valve design process based on the suitability of the coating properties for the specific engine application. Coupla notes on that:
• DLC coating is a thin film applied via a plasma-assisted Chemical Vapor Deposition (Pa CVD) process. This coating combines very low frictional resistance and extreme hardness. The coatings are used to reduce wear and friction for rapidly reciprocating components, which aside from valves include finger followers, tappets and piston pins.
• CrN (Chromium Nitride) is a thin film applied using a Physical Vapor Deposition (PVD) process. A cathodic arc is discharged at the target to evaporate the chromium into a highly ionized vapor, which is done in a partial pressure of nitrogen. This provides a higher level of adhesion and is commonly used for titanium, steel and nickel-based valves.
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. Comes in real handy as a sort of “solid lubricant”… when you factor in the abuse involved in a red-hot sliding exhaust valve’s daily life and the lack of lubrication in modern “clean” fuels… particularly in air-cooled, high-performance engines.
Caption: Yeah… well… this might even be better! But as we’ve seen there are specific choices for specific applications that go far beyond the basics shown here. The thing to do if you want to pump up and/or perfect your poppets is to contact the experts at places like Kibblewhite, Manley, Ferrea and more… tell them in detail what you have in mind for your motor, and do as they say!