- Background: Application notes are provided on an application specific basis to provide background information as well as insight into the current state of the art in Carbon Raptor technology for each component.
- Application Note Subject: Carbon Raptor® as used on various parts of titanium valves.
- Customers and Applications: Nextel Cup, Nationwide, Truck, NHRA, IHRA and ARCA racing teams and other endurance or longevity critical engines, especially those that recently require use of unleaded fuel.
- History: Titanium is a strong but “gummy” material to machine and finish. This “gummy” quality also causes considerable trouble whenever titanium contacts another material in an engine. Excessive wear, galling, and sticking are just a few problems engine builders are concerned with when selecting suitably prepared titanium valves.
Material surface challenge #1:
Valve Stem Tips. Lash caps are typically used when hardened valve tip inserts are made impractical by relatively thin valve stems, such as those that are 6 to 7 mm diameter. When lash caps are used, titanium valve tips can gall to the inside of the cap. Titanium readily trades metal with other materials, and metal transfer (galling) will friction weld the valve tip by fretting to the inside of the lash cap. Material build up in the cap can change valve clearance at the head that interferes with a good seal, creates horsepower losses, and risks valve to piston interference.
Results and Benefits: Engine builders report Carbon Raptor prevents the lash cap from galling or sticking to the valve tip, minimizes tip rounding, makes disassembly easier and minimizes changes in valve clearance to the head.
Material surface challenge #2:
Valve Stems. Titanium valve stems will wear rapidly or gall against the valve guide. One solution valve manufacturers have used is to coat the stem with molybdenum or other materials that do not readily gall. In order not to change the stem diameter, the manufacturer machines down (undercuts) part of the stem, applies a coating, and re-grinds the stem to the required diameter, roundness and finish. Engine builders are concerned about an undercut in the valve stem. The undercut is perceived as a weak point where the valve could break. Typically the stem coating is a stressed, thick film, and it has been known to flake off into the engine possibly resulting in damage and/or engine failure.
Results and Benefits: Molybdenum wears more quickly and opens up clearances in the valve guide more quickly than Carbon Raptor®. Engine builders using Carbon Raptor on valve stems report using tighter clearances for tighter valve control, improved valve stem life resulting in longer lasting guide clearances and longer wearing valve seats. Valve reliability is significantly improved as a result of not undercutting the stem and by eliminating the chance of potentially destructive materials flaking from the stem. By eliminating the undercut, one manufacturer concludes there is enough wall thickness to hollow the inside of the stem to within 0.250” of the tip, making a stronger part with lower weight.
Material surface challenge #3:
Valve Seats. The causes of wear on the titanium valve seat can typically be traced to two conditions:
1. The valve material is softer than the seat material, and
2. The valve doesn’t hit the seat concentrically every time it closes or bounces.
This wear is evident on the head of the valve where it contacts the seat and will be seen as a depression in the contact area. This depression may also affect both angles adjacent to the valve seat. This wear causes a distinct change in the shape of the valve head and a corresponding change in the airflow rate of the intake port. These changes often reduce engine power output. In addition to flow rate, the seal at the seat will deteriorate over time (due to galling) and may also contribute to power loss. Valve angles are typically not as aggressive as they could be to try to help preserve surface integrity, which decreases flow and decreases horsepower.
Results and Benefits: Engine builders report that Carbon Raptor prevents galling between titanium valves and the seats in the head. The results of every material tested so far have shown an improvement if not the elimination of wear on the valve. Carbon Raptor makes possible more radical valve angles to increase flow and horsepower.
Other opportunities and observations: Teams report that Carbon Raptor has little or no wear when used on hardened valve tip inserts. Engine builders have tested Carbon Raptor on the valve stem area contacted by the valve locks against Carbon Raptor on the valve locks themselves, and report ease of disassembly, which is critical for drag racers, as time between rounds is very limited.
General considerations: Surface finish is important ensuring a good result when using Carbon Raptor on titanium valves. Finish of the area to be coated must be smooth, polished and machine mark free to ensure uniform coverage, adhesion and performance. Many customers report the best results when finishes are 2 microinches Ra or less.
NASCAR® and other engine builders use titanium valves to reduce mass in the valve train. However, valve train mass is still considerable, and spring rates required to reach engine speeds exceeding 9,000 rpm impose impact loads on valve seats that result in high wear rates. Soft alloys, such as copper-beryllium, may be used for cylinder head seat inserts. Soft, sticky titanium cycling against a seat is a poor match for wear purposes, and galling back and forth between titanium and copper is a common result. Carbon Raptor has been applied to titanium intake and exhaust valves to reduce galling and wear. The switch from a soft to a hard valve seat surface makes for a better match to the soft seat insert, but titanium still galls unless it is Carbon Raptor coated. Seat material and combustion by-products make less deposit on the coated valve, which may contribute to lower wear rates.
Wear life extension provided by Carbon Raptor is maximized when operating temperatures are less than approximately 760°C (1400°F). Trials on exhaust valve stems in a NASCAR and NHRA engines indicate that Carbon Raptor operating within the guide area has good life and provides the desired low friction surface. Ongoing trials of Carbon Raptor on intake valve stems also are producing positive results. Carbon Raptor, when used as a replacement to sprayed-on metals on the stem, allows tighter stem-to-guide clearance for better valve head control. Tips of titanium valves have been Carbon Raptor coated and reduced stem wear caused by lash caps galling to the tip.
Figure 2 presents identical titanium intake valves used in a NASCAR engine application, with similar usage, in excess of 1,100 miles, or 1.5 million cycles. Pitting and other visual indicators of damage to the seat surface are reduced with coated valves. Power output degradation due to valve wear and resulting sealing losses near the end of a 500 mile race has been reduced by 50 percent with the use of Carbon Raptor coated valves.
George Barr is president of Anatech Ltd. For more information, feel free to contact him at 704-489-1488, ext. 203 or email email@example.com.
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