Anatomy of a Top-Fuel Dragster

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No other ground vehicle can out-accelerate a top-fuel dragster. The fastest class in the National Hot Rod Association (NHRA) drag-racing series, these cars can rocket to 300 mph in less than 4 seconds. That seemingly physics-defying performance is why Jim Oberhofer, the VP of operations at Kalitta Motorsports, calls top-fuelers violence on wheels.

Add expensive too. One run down the strip consumes fuel, bearings, valve springs and other parts to the tune of $5800. Stand nearby when a “rail” car lights off and you can feel the ground shake. It’s unnerving.

These machines are so fast—topping 330 mph—that the quarter-mile track (1320 feet) was shortened to 1000 feet after Scott Kalitta was killed in 2008 when his car exploded at the finish and crashed into the catch fence. That tragedy did not deter the Kalitta crew. As the team prepares for the 2011 season, which starts Feb. 24, we peel off its top-fueler’s bodywork to learn the secrets of its insane acceleration.

By Larry Webster, January 24, 2011 6:30 AM

Popular Mechanics article

Popular Mechanics article

1 – Fuel Top-fuelers burn a mixture that’s 90 percent nitromethane and 10 percent alcohol. Interestingly, a gallon of nitromethane, which costs $58, has less energy content than gasoline (14 kilowatt-hours versus 34). But nitro is an oxygen-rich compound that requires less air to burn, so the engine can consume more of it, thereby producing greater power. The 58 nozzles in the intake tract are always open, dumping about 5 gallons of fuel in a 4-second run. That kind of flow requires a firehose-like fuel line.

2 – Clutch A five-disc dry clutch is the only link between the engine and the locked rear end—there’s no transmission. “The clutch is the lifeline of the car,” Oberhofer says, because it regulates wheelspin by gradually engaging and slipping as the car moves down the track. A hydraulically motivated throw-out bearing operates off a simple timer (computer controls are illegal). The clutch is tuned according to track conditions. If it engages too quickly, the tires will spin, but if it’s too lazy, the car won’t accelerate as fast as possible. The discs get so hot that at least two of them are welded together by the run’s end.

3 – Tires Specially constructed Goodyears have the intimidating task of transferring all that rotational energy to forward speed. The bias-ply tires also dramatically change diameter over a run, which has the effect of altering the overall gearing. At the start, the 36-inch-diameter tires squat as the sidewalls wrap around the wheels’ bead locks. With increasing vehicle speed, the tires expand to a final diameter of 44 inches. The special blend of tread rubber is designed to adhere to the adhesive that’s applied to the track. The resulting grip is akin to driving on flypaper.

4 – Engine A top-fueler’s exact horsepower is a mystery—there isn’t a dynamometer that can handle one. Current estimates are in the 8000 neighborhood, and, no, we didn’t mistakenly add a zero on the end. The basic layout is very similar to the 1964 Dodge Hemi 426 V8—16 pushrod-activated valves—but with two spark plugs for each cylinder and a total displacement of 500 cubic inches. The supercharger, which is just a belt-driven air pump that force-feeds the engine, is so massive that it takes 700 hp to run it. The extreme internal forces literally flatten the crankshaft bearings and destroy valve springs during a pass. So the engine is rebuilt after every run by a team of eight mechanics. They perform this harried rebuild in just 40 minutes.

The Race On March 13, 2010, in Gainesville, Fla., Kalitta driver David Grubnic set a top speed record on the 1000-foot track. Here’s a breakdown.

0.50 Sec 10.27 ft — 73.89 mph

The motor screams at its 8300 rpm redline, and the tires buckle under the load of getting the 2300-pound dragster moving.

1.00 Sec 51.94 ft — 113.82 mph

To reduce power and the acceleration of the driveshaft and tires, the ignition advance is retarded from 56 degrees to 27—a strategy to keep the tires gripping the track surface instead of spinning.

1.50 Sec125.09 — 162.45 mph

The ignition timing is advanced back to the maxi­mum value. The clutch’s throw-out bearing has moved through three of its five stages, increasing the pressure on the discs.

2.00 Sec231.91 ft — 213.09 mph

The fuel flow ramps to 95 gallons per minute. Aerodynamics increase tire traction so the throw-out bearing tightens its grip. The engine slightly bogs to 7200 rpm.

2.50 Sec379.86 ft — 248.50 mph

In the time required to take a sip of coffee, Grubnic’s car reaches 248 mph. Some of the clutch plates begin welding together, pulling the engine to its lowest rpm, 6500.

3.00 Sec566.16 ft — 271.62 mph

Thanks to 5000 pounds of aero downforce, the tires have incredible traction, but wind resistance slows acceleration from the maximum 5 g’s to about 4.

3.50 Sec783.62 ft — 293.41 mph

Late in the run, many spark plugs have burned away, so the engine is dieseling, and some cylinders simply don’t fire. “This is the engine’s toughest zone,” Oberhofer says.

3.83 Sec 1000 ft — 321.58 mph

The mechanical abuse finally ends as the dragster clears nearly 500 feet per second. Next, the driver pulls the chute and the crew then feverishly ­prepares for the next run.

Read more: Top-Fuel Dragster Diagram – Top-Fuel Drag Racing Car Video – Popular Mechanics
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