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8,000HP Top Fuel Engine

Among All Engines Of Drag Racing, The King Of The Hill Is The Supercharged And Nitromethane-Fueled Monster. So We Thought We'd Take An Inside Look At An . . .

By , Photography by Jon Asher

They are considered the kings of the sport-the supercharged and nitromethane-sniffing internal combustion beasts that literally stomp the ground and push a 2,300-pound land rocket on 1,000-foot runs eclipsed in 3.77 seconds at speeds approaching 325 mph. The violence that emits from the headers of these 500ci monsters is quite literally deafening. If you've never experienced the raw power of a pair of Top Fuel cars blistering the dragstrip, the initial sound pressure shock wave blasts you in the chest and assaults your eardrums. Standing within 30 feet of a Top Fuel car at WOT feels more like an explosion or some kind of uncontrolled crash of noise, pressure, and caustic, yellow-orange gas that instantly burns your nostrils. Top Fuel engines are power personified-a barely contained internal combustion evolution of mythical proportions that snarls and breathes fire and often wreaks havoc on those who presume to exercise control.

So let's take a look at what is responsible for all this violence. What you'll discover is that while a Top Fuel engine appears quite tame on paper, it's the synergy of fuel, spark, and boost that makes it violent.

Inside the Beast
There was a time when all you needed to race Top Fuel was a front-engine dragster chassis, a junkyard 392 Hemi, a Jimmy blower, and enough money to buy some nitro. There were very few limitations except on your nerve and creativity. Today, the kings of the sport are limited on almost every level with the intent to tame this very powerful beast. One of the first limitations was on displacement, now topped out at 500 ci. This creates the odd situation where many big-inch street engines would be illegal for use in Top Fuel. Ironically, our Kaase Boss engine in this same issue would not pass tech due to its 521 inches. In spite of the limitations, there's still plenty of room for tuning. Dale Armstrong has long since retired from active Top Fuel racing as crew chief of the famous Kenny Bernstein-owned Budweiser King Top Fuel car, but in the '80s there was still room for major innovation, and many of Armstrong's creations-such as the first use of onboard data logging-is still a Top Fuel standard.

Top Fuel is also a place where myths abound, especially in regard to ignition systems. Just 30 years ago, Armstrong says fuel engines ran on far less fuel, making them leaner, which turned spark plugs into glow plugs, which continued combustion even after the ignition system failed completely. But with today's fire hose-like delivery of fuel, Armstrong says, "You couldn't do any of this without a strong ignition system." Armstrong was in fact the man who pioneered the use of twin magnetos and two spark plugs per cylinder back in the '80s and who went so far as to add a third spark plug and mag that the NHRA immediately banned. The tremendous amount of fuel present in the cylinders demands an immense amount of current flow to spark the combustion process. The popular MSD Super 44 magnetos are so powerful that they deliver 1.2 amps of electrical energy across the spark plug gap each time a plug fires. This is four times the electrical energy delivered by a hot street ignition system at 300 milliamps (0.3 amp). Think of those MSD magnetos as the equivalent of an engine-driven arc welder and you get the idea.

How much liquid is injected into the cylinder at max revs? According to Bill Miller, owner of Bill Miller Engineering (BME) in Carson City, Nevada, his Top Fuel engines inhale as much as 80 gallons of fuel per minute at peak torque, which is equivalent to 1.3 gallons of fuel per second! "You can't pour it out of a gallon gas can that fast," Miller comments. After peak torque-between 6,000 and 6,300 rpm on a Top Fuel engine-fuel flow is reduced because the engine's volumetric efficiency suffers due to a lack of time. "At the higher engine speeds, all that liquid fuel acts like a choke that blocks the airflow," Armstrong says.

Typical Top Fuel Engine
Displacement: 496ci
Block and head: Aluminum Brad Anderson or Alan Johnson
Bore x stroke: 4.310 x 4.250
Compression ratio: 7.0:1
Cylinder head: Hemispherical
Intake valve: Titanium 2.45 inches
Exhaust valve: Inconel 1.92 inches
Rocker ratio: I = 1.73:1, E = 1.53:1
Camshaft: Mechanical roller
Duration: 290 to 300 degrees at 0.050
Roller lifter: 1.6875 inches in diameter
Valve lift: 0.800
Supercharger: Roots style, 14:71
Boost pressure: 65 psi maximum
Ignition: Twin 44-amp magnetos capable of 50,000 volts
Ignition timing: 52 to 55 degrees BTDC
Fuel pump: Mechanical, 100-plus gal/min
Injection: Mechanical, total of 42 injectors
Fuel: 90 percent nitromethane, 10 percent methanol
Oiling system: Wet-sump
Oil capacity: 12 quarts, 70W oil
Maximum rpm: 8,250 rpm
Maximum hp: 8,000 equal to 16 hp per ci

The Supercharger
Early Top Fuel engines all used the classic 6-71 GMC supercharger that has grown into the current 14-71. NHRA rules now limit virtually all aspects of the fuel superchargers, including the inlet and outlet dimensions. But that hasn't stopped racers from improving the breed. The latest iteration is the Gibson/Miller supercharger designed by aerodynamicist Tim Gibson and built by Bill Miller Engineering (BME). At 50 percent overdrive, Miller says the blower easily pushes 60 psi of boost pressure (that's four atmospheres), and at 12,450 rpm rotor speed, the Gibson/Miller supercharger is capable of 3,750 cfm. According to Miller, that's an air speed of "about 250" mph exiting the supercharger. Pushing this much air, you would think the blower would be plenty hot at the end of a 3-second pass, but with the 12 gallons per minute of fuel sprayed through just the hat nozzles, Miller says the blower is barely warm to the touch if you were to snuggle up next to it on the return lane. That's because the nitro works to pull the heat out of the air despite the immense power required to drive the blower. Miller estimates it takes between 900 and 1,000 crankshaft horsepower to drive one of his superchargers. Just let that last fact settle in for a moment.

Five Top Fuel Facts You Probably Didn't Know
• The noise from a blown nitro engine is due in part to the fact that you're hearing the tail end of the combustion process.
• A Top Fuel engine will consume more than 14 gallons of fuel over the entire time from start-up to the finish line.
• At peak power, each 62ci cylinder generates 1,000-plus horsepower, equaling 16 hp/ci.
• All 12 quarts of engine oil are changed after warming the engine due to fuel dilution that turns the oil a sickly mustard color.
• A Top Fuel driver would rate a Space Shuttle launch rather mild since he experiences in excess of 5 g's positive acceleration, while the Shuttle pulls a mere 3 g's.

Nitromethane
This slightly viscous liquid was originally created as a solvent for use in the dry cleaning industry. Its first use in competition is shrouded with various first-use claims, but among the accomplishments of record is nitro's first successful use in a circle track Midget driven at the famous Gilmore Stadium in 1950 by future Indy 500 winner Roger Ward and tuned by Vic Edelbrock Sr. The drag racers who experimented with the fuel in the early '50s, included Joaquin Arnett of Bean Bandits fame. The secret to power is hidden in nitromethane's chemical formula of CH3NO2. That O2 at the end is oxygen, which separates its chemical bonds to nitrogen during combustion and allows the oxygen to contribute to the combustion process. With gasoline, the air/fuel ratio for best power is 12.5 parts air to one part fuel (12.5:1). But because nitromethane contains its own oxygen, fuel racers long ago discovered that the more fuel you could feed the engine, the more power it made, bringing the air/fuel ratio closer to a 1:1 ratio.

Miller says if you compute the actual mechanical ratio of air and fuel with today's fuel engines, "the numbers get closer to 3 pounds of nitro for every 1 pound of air. But a lot of this fuel is used to cool the burn. People don't know this, but nitro, alcohol, and gasoline all have very similar burn rates," Miller says. "In fact, nitro is in between gasoline and methanol. All that lead [ignition timing] is due to the liquid fuel." In other words, the additional timing is necessary because, Miller says "only 10 percent of what's in the chamber is vapor; the rest is liquid." The vapor burns first, creating enough heat to begin to vaporize the rest of the liquid fuel. But this takes time, which means earlier ignition timing.

Armstrong reinforced this concept. He says, "We used to have problems with head gaskets on our old Top Alcohol engines that never happened with the fuel engines. We realized that nitro burned slower. The horsepower doesn't really come from peak cylinder pressure. It actually drives the piston farther down the hole because it [the fuel] keeps burning." This longer burn is also why the header flames are so easy to see even during daylight. Armstrong also noted that this has an effect on cam timing. "You have to be careful when opening the exhaust valve. You can't open it too early-like before 82 to 84 degrees before bottom dead center (BDC) or it will just break parts. The cylinder pressure is so high that the valve just won't open. That's also why they keep the exhaust valve small, so it's easier to open."

This tremendous volume of fuel is what drives the requirement for the equally serious amount of ignition power required to light all this fuel. In an attempt to minimize engine damage, the NHRA has mandated a maximum of 90 percent nitromethane chased with 10 percent methanol. In case you're wondering, nitromethane does not mix well with gasoline, tending to separate into layers much like water and oil. However, there is a fuel called nitropropane (C3H7NO2) that does mix with gasoline. For the record, it tends to break pistons unless you know how to tune.

Top 10 Advancements in Top Fuel History • Nitromethane fuel-first used successfully in circle track racing by Vic Edelbrock Sr. • Marv Rifkin's design of the first flexible-sidewall M&H drag slick. • The supercharger-first to be used were GMC diesel blowers. • The Paul Schiefer slipper clutch along with Bruce Crower and the Crowerglide. • The rear-engine chassis perfected by Don Garlits in 1971. • The Dale Armstrong data logger through Racepak. • Electric timers to engage lockup on clutches-Armstrong/Bernstein. • Monster fuel pumps-Sid Waterman • High-amperage magnetos. • Parachutes for stopping-Simpson innovation.

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