What is top fuel and how is it different from gasoline?

"" Nitromethane is a little like gasoline that has been pre-mixed with nitrous oxide; it comes with its own oxygen atoms to help it burn.

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If you want to make a high-power 4-stroke engine, there are a couple of different ways to accomplish your goal. One way is to increase the displacement. Another is to stuff more air into the engine with a turbocharger or a supercharger. If you want to go to extremes, you would replace gasoline altogether and use a more energetic fuel. Top fuel dragsters do all three.

A "nitro-burning" engine and a "top fuel" engine are the same thing -- engines designed to burn nitromethane rather than gasoline. Gasoline is a hydrocarbon, and the common chemical formula for gasoline is C8H18. Nitromethane has the formula CH3NO2. Nitromethane is a little like gasoline that has been pre-mixed with nitrous oxide. The fuel comes with its own oxygen atoms to help it burn.

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The big advantage of nitromethane is that you can get a lot more power from each explosion inside the engine. Pound for pound, nitromethane is less energetic than gasoline, but you can burn a lot more nitromethane in a cylinder. The net result is more power per stroke. You typically need about 15 pounds of air to burn 1 pound of gasoline, whereas you need only 1.7 pounds of air to burn 1 pound of nitromethane. This means that, compared to gasoline, you can pump about 8 times more nitromethane into a cylinder of a given volume and still get complete combustion.

Since nitromethane is not as dense as gasoline in terms of energy, you do not get an 8-time improvement in terms of power. It is more like a 2.5-time improvement (see this page for a comparison). Still, you can double or triple your engine's horsepower simply by changing the fuel. That's a huge improvement!

A typical drag-racing engine has a displacement of 8.9 liters, is supercharged and produces about 6,000 horsepower. It can burn close to a gallon (4 liters) of nitromethane per second! To put that in perspective, there is something like 2 teaspoons (10 cc) of nitromethane being poured into each cylinder per intake stroke.

An interesting thing about nitromethane is that it does not burn as quickly as gasoline. In fact, there is not enough time to burn all of the nitromethane between when the spark plug fires and when the exhaust valve opens. So the engine is pumping still-burning nitromethane into the exhaust pipe. That's why you see flames shooting out of the exhaust of a drag-racing car.

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TOP FUEL

Among the fastest-accelerating machines in the world, 7,000-horsepower Top Fuel dragsters are often referred to as the Kings of the Sport – and with good reason. They can cover 1,000 feet in less than 3.8 seconds in excess of 300 mph. 

Powered by a supercharged and fuel-injected 500-cubic-inch adaptation of the famed Chrysler Hemi engine, Top Fuel dragsters can burn up to 15 gallons of nitromethane fuel during a single run. Constructed of chromoly steel tubing and carbon-fiber composite, Top Fuel cars are 25 feet long and weigh 2,300 pounds in race-ready trim.

FUNNY CAR

Like their Top Fuel counterparts but with a shorter wheelbase and a carbon-fiber body that loosely resembles a production-based automobile, Funny Cars routinely run in the 4.0-second range and are capable of speeds more than 300 mph to 1,000 feet.

Funny Cars are powered by the same supercharged and fuel- injected 500-inch engines as Top Fuel dragsters. Funny Cars are also like Top Fuel dragsters in that they do not use a transmission but rather transmit power to the huge Goodyear rear slicks through a multistage clutch assembly that is activated by timers.

PRO STOCK

Often called “factory hot rods” because of their resemblance to production-based automobiles, Pro Stock cars are some of the most technologically advanced machines in drag racing. Built around a sophisticated tube chassis and four-link rear suspension, Pro Stock cars must conform to precise measurements and weigh no less than 2,350 pounds.

Pro Stock engines use two carburetors and spec gasoline, and are restricted to a maximum of 500 cubic inches. They can rev to more than 10,500 rpm and make more than 1,300 horsepower. A competitive Pro Stock car can run in the 6.6-second range at nearly 210 mph on the quarter-mile.

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PRO STOCK MOTORCYCLE

These highly modified vehicles, which can run under seven seconds at more than 195 mph on the quarter-mile, featuring a purpose-built tube chassis and a lightweight, aerodynamically enhanced replica of original bodywork. The class features a wide variety of makes, models, and engines, including V-Twin entries from Harley-Davidson and Buell and inline 4-cylinder-equipped Suzuki and Kawasaki models.

To ensure a level playing field, the pushrod-equipped V-Twin engines are limited to 160 cubic inches, and the high-winding 4-cylinder engines cannot be larger than 101 cubic inches. Fuel injection is permitted, and spec gasoline is the only fuel allowed.

TOP ALCOHOL DRAGSTER

Top Alcohol Dragsters may look like Top Fuelers, but they have significant differences. Whereas Top Fuelers use supercharged, nitro-burning engines, Top Alcohol Dragsters may use a supercharged methanol-burning engine or an injected nitromethane combination.

The injected nitro cars do not use a transmission, and the supercharged cars have three forward speeds. Weights vary according to combination, but are generally between 1,975 and 2,125 pounds. Like Top Fuelers, Top Alcohol Dragsters are restricted to a maximum wheelbase of 300 inches. A typical quarter-mile run is in the 5.2-second range at more than 270 mph.

TOP ALCOHOL FUNNY CAR

Similar in physical appearance to their nitro-burning Funny Car counterparts, Top Alcohol Funny Cars are restricted to the use of methanol fuel and have a three-speed transmission. Top Alcohol Funny Cars feature basically the same chromoly steel chassis as the nitro cars and are fitted with the same carbon-fiber replica bodies, though the Top Alcohol Funny Car bodies do not need as much downforce and use a much smaller rear spoiler. Top Alcohol Funny Cars are capable of quarter-mile performances in the 5.5s at more than 260 mph.

COMP

No category in NHRA competition features more variety than Comp. Dragsters, altereds, street roadsters, coupes, sedans, front-engine nostalgia dragsters, sport compact cars, and trucks race in 87 classes. The engine combinations are just as diverse as the vehicles, from turbocharged 4- and 6-cylinder engines to Pro Stock-style V-8s and nitrous-oxide-equipped mountain motors.

Most cars are classified using a formula that divides total car weight by cubic inches. Each class is assigned an index based on what a well-built car should run, and races are handicapped according to those indexes.

SUPER COMP

The quickest of the heads-up Super classes(8.90 index), Super Comp is composed primarily of dragsters. Engine, chassis, and body modifications are virtually unlimited, though all entries must adhere to NHRA safety standards. Four- and 6-cylinder-powered entries may have a minimum weight of 1,000 pounds; all others cannot weigh less than 1,350 pounds. Most Super Comp cars can run well under the 8.90 index but use a number of electronic aids, including a timer and adjustable throttle, to run close to the index without running quicker than it, or breaking out.

SUPER STOCK

Super Stock vehicles may look like ordinary passenger vehicles, but they are highly modified race cars. The category features primarily late-model sedans and vintage muscle cars, and entries are classified using a system that divides factory shipping weight by NHRA-factored horsepower. Significant engine modifications are permitted, but the vehicle must retain the correct engine block, cylinder heads, and carburetor.

The top class is SS/AH, which is exclusively for ’68 Dodge Dart and Plymouth Barracuda factory race cars. Cars are handicapped using an index system, and the breakout rule is enforced.

STOCK

Stock cars are like Super Stockers, but the rules regarding everything from engine modifications to body alterations are much stricter. Virtually any car is eligible to compete, and entries are classified using a system that divides factory shipping weight by NHRA-factored horsepower.

Bodies must be unaltered and retain a full factory interior. Tires are limited to a maximum 9-inch-wide rear slick. Engines must be correct for the make and model vehicle and must retain stock cylinder heads, intake manifold, and carburetor. Modifications are limited to a basic balancing and rebuild with only a few performance enhancements.

SUPER GAS

Super Gas entries, which run on a 9.90 index, are primarily full-bodied cars and street roadsters. No dragsters or altereds are permitted. Rules regarding engine and chassis modifications are extremely liberal, though the use of exotic fuels is prohibited. The minimum weight is 2,100 pounds except for 4-cylinder-powered cars, which may have a minimum weight of 1,200 pounds.

As in Super Comp, competitors use electronic timers and throttle stops to run as close to the class standard without going under. Also, as in Super Comp, races are staged using a four-tenths Pro start.

SUPER STREET

At NHRA Lucas Oil Drag Racing Series races and select NHRA Camping World Drag Racing Series events, Super Street racers compete on a fixed 10.90-second index. All vehicles must be full-bodied cars and weigh no less than 2,800 pounds except for 6-cylinder cars, which may have a minimum weight of 2,000 pounds, and 4-cylinder- and rotary-powered cars (1,200 pounds). Engine and chassis modifications are virtually unlimited.  Racers compete on a five-tenths Pro Tree.

PRO MOD

With an eclectic mix of vehicles that range from ’41 Willys coupes to ’63 Corvettes to late-model Ford Mustangs and Dodge Vipers, few classes offer as much diversity and excitement as Pro Mod. The engine choices are equally as diverse; competitors can use superchargers, turbochargers, or nitrous-oxide injection and run into the high-five-second zone at speeds of more than 250 mph.

TOP DRAGSTER & TOP SPORTSMAN

Essentially a pair of categories for fast E.T. racers, Top Dragster (open-wheel entries) and Top Sportsman (full-bodied cars) feature a qualified field of 32, 48, or 64 entries. Competitors may choose their own dial-in for eliminations provided it is not slower than 7.99 seconds (8.19 in Divisions 6 and 7). Eliminations in both classes are run using a traditional bracket racing format with a full three-amber Christmas Tree start, and the breakout rule is enforced. It is not uncommon for entries to run in the sixes at more than 200 mph.

ET BRACKETS

Virtually any vehicle — from a dragster to a muscle car to a daily street driver or snowmobile — can compete in the most popular form of drag racing, E.T. bracket races, which are contested weekly at most of NHRA’s 140 member tracks in North America. E.T. bracket racers can select their own dial-in or handicap based on the performance of their vehicle, which puts a premium on driver skill rather than vehicle performance. Other than safety requirements, few rules govern the construction of E.T. bracket vehicles.

JR. DRAGSTER

Created as a way to introduce kids ages 8 through 17 to drag racing, the NHRA Jr. Drag Racing League has become a training ground for NHRA’s future stars. Jr. Dragsters are constructed using a steel frame and an aluminum or fiberglass body. The power comes from an engine based on a five-horsepower, single-cylinder engine, which is carbureted and burns alcohol. Power is transmitted using a slipper clutch. Competitors can run as fast as 85 mph and as quick as 7.90 seconds on the eighth-mile.

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