TOP FUEL RINGS & BEARINGS
It’s simply amazing that any components are able to withstand this nightmarish environment.
by Mike Mavrigian
Note the missing plasma moly that was blown off of this top ring. This ring (and its companion set) saw a mere 24 seconds of running time, from startup to the end of the dragstrip. This particular engine powered the car to a 4.509-second run at 330 MPH. It's certainly no secret that Top Fuel drag engines produce horrific levels of power, to the tune of 7,000 to 8,000 HP and torque in the range of 6,000 lb-ft, currently shooting these cars to speeds in excess of 300 MPH. We thought that it would be interesting to dig into the particulars of the piston rings and bearings used in these over-the-top applications to gain an understanding of what these components are exposed to during a gut-wrenching 4.5-second blast down a quarter-mile stretch. (photo by author)
BRIEF ENGINE OVERVIEW
These engines, limited to 500 CID, basically operate in the 8,400 to 8,500 RPM range during the entire run, utilizing a slipper clutch system that allows engine tuning within this very narrow RPM band. Engine configuration is loosely based on the Chrysler 426 Hemi format with a 90-degree cylinder angle, two valves per cylinder and hemispherical combustion chambers. Bore is at 4.19" and stroke is at 4.500" (combos may vary). Blocks are machined alloy forgings with ductile iron cylinder liners. These engines run "dry" with no liquid coolant. The absence of water passages result in increased block strength with the need to add a block "filler" for added rigidity. Cylinder heads are also CNC machined from alloy stock, fitted with two spark plug ports per cylinder, titanium intake valves and solid Nimonic 80A exhaust valves. Intake valves are approximately 2.45" in diameter, with exhaust valves at around 1.925". The valvetrain includes a billet steel camshaft, solid roller lifters and full-roller rockers, titanium valve springs and retainers. Crankshafts are billet steel, fitted with five bearing shells. Connecting rods are forged alloy. Pistons are forge alloy, with a three-ring package. Piston grooves are hard-anodized to prevent microwelding. Piston pins are secured with alloy pin buttons to aid in quick piston changeovers. The supercharger is a 14-71 type roots unit, driven by a cogged belt. About 45.5 PSI boost occurs at wide-open-throttle. The engine oiling system is a wet sump setup with a capacity of about 16 quarts. Oil pressure is about 200 lbs. cold and about 160 lbs. during a run. The fuel mixture is primarily nitromethane, with about 15% methanol. Nitromethane features a high oxygen content, which means less air is required for combustion. Very rich mixtures are tuned. Coupled with nitromethane's ability to absorb heat as it vaporizes, this helps to promote engine cooling. In other words, these engines run on tons of fuel and little air (instead of "conventional" 14.7:1 air/fuel, mixtures can be as great as 2:1). Fuel consumption is about 1 gallon per second. Whoof.
Here's another view of a top ring's plasma facing damage from the same engine. Cylinder pressures are incredibly high, as much as 12,000 to 13,000 PSI (when detonation takes place, cylinder pressure is in the 16,000 PSI range). To put that into perspective, a Pro Stock engine may generate around 4,000 PSI of cylinder pressure. This is a major factor when considering the abuse experienced by the rings and bearings. (photo by author)
To find out what's happening in the Top Fuel ring arena, we spoke with Clevite's Don Sitter and Federal-Mogul's Scott Gabrielson, each of whom work hand-in-hand with teams in developing packages that perform. According to Sitter, Clevite offers a hardened ductile iron top ring (their PC479 material, tradenamed Firepower). This ring is also available with a moly face, but Sitter noted that some builders prefer not to use the moly since firing pressures in detonation instances can sometimes cause the moly to chip off, potentially contaminating the oil filter.
The second ring from the same engine features an inner chamfer. "There are two schools of thought with regard to top ring size," noted Sitter. "We introduced a 0.078" axial height ring with a 0.017" Dykes step. Prior to that the most common top ring was a 1/16" Dykes-style ring with a 0.017" step. In working with various teams, we found that distortional effects caused the 1/16" ring to reduce twist, essentially turning the ring into a beveled washer. It was obvious that substantial compression was getting past the top ring. To fight that, we went to the thicker ring, which worked out very well. A favorite saying of one of the Top Fuel builders we work with is ‘mass saves your ass,' which was certainly true in this case. We also changed the second ring to a 0.078" moly-faced version with a barrel (radius) face. "Considering the other variables involved, however, you can't look at the rings as a stand-alone of the overall equation," Sitter said. "You must also consider cylinder finish, as some builders run a 280 finish followed by a plateau finish, while others go with an as-cut 280 or 400 wall finish. There are substantial differences in the tuning of these engines, including blower overspeed, static compression, amount of fuel delivery, timing, clutch management, etc. One team may have problems with pan pressure and sealing, and by switching to dual Dykes (the Dykes ring has a dam cut about a third of the way across the radial wall, which allows pressure to get behind the ring and push it against the cylinder wall), but the same deal might not work for another team. "Top ring gap will range, depending on the builder's experience, from as tight as 0.028" to as much as 0.044". The enormous heat generated during combustion necessitates these larger gaps.
Heads and piston/rod sets are removed after every run. Rings and rod bearings are always replaced after every run. To save time, some teams replace the entire piston/rod/ring/bearing set. After examination, pistons and rods may be reusable. (photo courtesy Clevite)
Oil is also replaced after every engine shut-down, due to the excessive nitromethane fuel wash. (photo courtesy Clevite) "Some crew chiefs have told me that when we went to the thicker rings, pistons ran cooler due to the decreased thermal resistance in a dry block," Sitter said Sitter noted that taper-faced second rings, regardless of material, tend not to be effective because any compression that leaks past the top ring will load the face of the taper ring, causing the ring to be pushed away from the cylinder wall, increasing blowby and pan pressure. A barrel-faced ring fights this because the tangent point is closer to the top of the ring. Sitter also noted that Clevite is introducing a new second ring in 2007, a reduced radial wall 0.078" barrel-faced ring positive twist ring. This prevents the second groove from undercutting the top groove, allowing the No. 2 land to better withstand the primary firing pressure at the top ring. The reduced radial wall will also help to bring the mass back to close that of the original 1/16" barrel ring. "The laws of physics are routinely being broken in Top Fuel," jokingly noted Federal-Mogul's Gabrielson. "What these teams are accomplishing is nothing short of incredible." "Dykes rings seem to work very well in supercharged applications," Gabrielson said. In the beginning, top ring materials were generally chrome plated stainless steel, but about 15 years ago this progressed to the use of ductile iron with a plasma moly face. Stainless tends not to be very compatible, with the chrome plate tearing up cylinder walls quicker. Granted, the cylinder liners are replaceable, but since the teams have only about 75 minutes to service the engine between rounds, if a wall goes away, the team will usually just swap out the entire block to save time. "The most common top and second ring sizes have been 1/16". Our experience has been that some crew chiefs have used Dykes second rings in place of rectangular rings, but with mixed results," Gabrielson said. Some experimentation with wider Dykes rings (5/64" face) has taken place, with results showing superior resistance to twisting. We've seen mixed reviews with regard to the wider rings, with some teams claiming improvements and others saying that they saw no change. Top rings simply don't last, and are usually destroyed during the run, so ductile iron plasma-faced second rings are the norm, since you need a durable backup ring. Ring fracture isn't a big issue, since the incredible cylinder pressure seats the ring on the bottom of the groove, eliminating flutter. The top ring simply fails due to pressure and heat, leaving the second ring to serve as the essential backup compression ring." Oil rings, because of the pressure formed around the expander, can actually be smashed. Regardless of the brand, style or type of material, rings don't make it through more than one run, so they're replaced as a matter of course between rounds.
Detonation events will damage main bearings as well. These mains were exposed to a single run. Note the severe scoring, the result of enormous forces created by the astronomical cylinder pressures. (photo courtesy Clevite)
Wet sump oil systems are a must due to the need to change contaminated oil quickly. (photo courtesy Clevite)
"While rod bearings haven't changed much in the last eight years," noted Clevite's Sitter, "there was a series of changes in 2004 involving NHRA-mandated reduction of nitro, blower overspeed, etc., in an effort to regulate speed (since speeds were approaching 330 MPH). It's taken two seasons to get back to performance levels of 90% nitro. Now, we see more detonation and more damage to upper rod bearings. Again, because of the incredible cylinder pressures, upper rod bearings actually begin to extrude (about 0.080" at the ends), making them appear as flanged bearings. If materials change, moving to a mild grade steel, for instance, this would still present a liability to the crankshaft, since the harder extrusion would get into the journal fillet, possibly creating a stress area. At the present time, Top Fuel crankshafts are generally changed out anywhere from three to five runs (some teams stretch this to as many as 12 runs). Since these crankshafts cost around $3,500 each, and since the team may make about 200 passes each year, the difference between buying 20 cranks versus 60 cranks per year would present a real financial problem. "A number of detonation management issues need to be addressed before we blindly change rod bearing design," Sitter said. "The problem isn't the bearing ... rather, it's how the bearing is being treated. One analogy is that rod bearings can be viewed as the circuit breakers in the system." "Rod bearing clearances have decreased," Sitter said. "Years ago, it was not uncommon to see rod bearing clearances in the 0.005-0.006" range, with main bearings in the double digits. Today, rod bearing clearances range from 0.0035-0.0045", and main bearing clearances are in the area of 0.0045-0.006"." "Typically, Top Fuel bearings are not coated with a special anti-friction treatment.," Sitter said. We've tried Tri-armor, but there does not seem to be any big advantage. They work as expected, but don't offer the kind of benefits you need in a Top Fuel application. The issue focuses more on how to prevent the engine from shedding parts when detonation occurs. Our V bearing works great. This is constructed of lead indium plated overlay on a cast tri-metal lining with a conventional steel backing. This does a good job of managing the detonation environment. While the moly coatings, such as our Tri-armor treatment, survive very well, this offers no particular margin with regard to detonation events. "Top Fuel applications experienced thrust bearing problems a few years ago, which prompted us to make several improvements to the flanges, including the use of lead indium on the thrust faces, and to work with teams and crank manufacturers with regard to thrust face surface finish," Sitter said. "We did note that those teams having thrust bearing problems were all using a particular oil, which convinced me that some oil brands do perform better than others." By the way, most teams run 70-90 W synthetic oil. It's interesting to note that because of the excessive nitro fuel wash that runs through the engine, fuel dilution of the oil is so great that the engine oil must be changed every time the engine is shut off (even following a warm-up). As noted earlier, since these are dry engines with no liquid cooling system, this excessive fuel wash helps to manage temperatures, basically preventing the pistons, heads and valves from melting. The incredible cylinder pressures are so great that it's not at all uncommon for hefty wrist pins to bend. In addition, rod small ends are elongated and rod big end saddles can be pulled out. When you consider their environment, it seems a miracle that any rings or bearings can function and allow these engines to perform their jobs, yet they do, weekend after weekend, all year long. By the way: ever notice that the exhaust tubes are always turned up, with exhaust pulses exiting upwards? This isn't just for appearance. This is due to the enormous cylinder pressures. By directing the exhaust upwards, this helps to generate greater chassis downforce for better tire bite and stability. If the tubes aimed downward, the business-end of the car might very well be lifted off of the track surface.
Gone in about 4.5 seconds, currently reaching a trap speed of around 330 MPH. (photo courtesy Clevite)