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Top Fuel Racing Engines – Containing the Explosions

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It’s been over 60 years since Vic Edelbrock starting experimenting with nitromethane as a racing fuel in the early 1950s. In the 60 years of hands-on education since, along with the help of modern technology, there have been some amazing results.

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Earlier this year, I wrote a Memory Lane column titled “Going Faster in 1954” (March 2016 Engine Builder, page 6). In it, I explained how nitromethane was developed and how early race teams learned how dangerous – and powerful – it could be.

A Short Review

Nitromethane is known as a monopropellant fuel, which means it has the potential to combust without any air at all. That’s why nitromethane was once used as a rocket fuel. Fortunately for hot rodding, nitromethane also has industrial-world uses-primarily as a dry cleaning solvent, which makes it readily available.

Nitromethane has a lasting harmful affect on everything it touches. It is also very toxic to humans. Care should be exercised to avoid exposure to the skin and lungs.

According to the 1954 “Hot Rod Yearbook,” an excess of nitromethane is needed within the cylinder so that enough water is created during the combustion process to cool the valves and pistons. Not enough nitromethane will cause an engine to melt down…literally. It is also common to see the tachometer read 2000-2500 rpm for up to 10 seconds after the fuel is shut off. This is common until the cylinders cool down enough to stop igniting the leftover nitromethane.

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Some is Good…

More is Better?

Now to this already violent racing fuel what if you added a supercharger to the engine to force yet more fuel and air into the engine to develop even more horsepower? As the old saying goes, if some is good, more is better, at least in theory. So let’s test that theory in the real world and tell you how Top Fuel engines may differ from the ones you normally build.

The engine in a modern top fuel dragster is made of solid billet aluminum (engine block and cylinder heads) and the oil pan is made of titanium (see photo above). It is based on a Chrysler Hemi block of about 500 cubic inches. (NHRA rules limit size). It has no radiator – the excess nitromethane fuel that does not burn helps cool the cylinders as Vic Edelbrock also learned many years ago.

The supercharger will build approximately fifty pounds of manifold pressure. With 3,000 CFM of air being forced into the cylinders along with the fuel, the mixture is compressed into almost a solid form before ignition. At full throttle the cylinders are on the verge of hydraulic lock.

That is where the ignition comes into play. Modern top fuel dragsters run two magnetos. The output of each magneto is 44 amps to each cylinder. That is about the same amperage as an arc welder. Should ignition fail the resulting violent explosion will literally blow the cylinder heads right off of the engine block and often will break the engine block in half sending very expensive parts and pieces flying everywhere.

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Meanwhile…

Initial spark advance at the starting line is 56 degrees. At about 100 feet from the starting line the computer controlled ignition reduces the advance to about 27 degrees to help with traction.

A top fuel engine develops more than 10,000 horsepower (though they were never able to be accurately tested on a dyno, recent tests have shown that they can actually exceed 11,000 hp). It takes about 700 of those horsepower to turn the supercharger the 12,000 rpms at engine redline of 9,500 rpms. Maximum boost from the supercharger at redline is 45 psi.

NHRA rules say a maximum of 85 percent nitromethane is allowed. The rest is alcohol that also helps cool the cylinders. You will need to be at least 85 inches away from the header pipes to avoid getting burnt by the nitro flames exiting those pipes.

By the 500-foot mark about half of the spark plugs will be melted (just as Vic experienced all those years ago). It is not uncommon to see part of the cylinders not firing past the 500-foot mark. You will see the telltale white smoke instead of the bright yellow burn of firing cylinders at the exhaust. Most of the other cylinders are dieseling from the heat inside of the cylinders. The temperature of the exhaust valves at this point is about 1,400 degrees F.

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Under full throttle a modern top fuel dragster engine burns approx.11 gallons of fuel a minute. The fuel pump is capable of delivering 77 gallons per minute through a 2.5˝ diameter fuel line. The fuel tank holds 17 gallons of fuel. A 55-gallon drum of nitromethane today costs about $900.

The cost of a complete top fuel dragster engine is in the $58,000 range depending on options. The engine will be stripped down to the bare block after every run and be rebuilt in a span of 40 minutes. The engine will be torn down and rebuilt 184 times in a year if the car makes it to the finals at every race.

It costs about $5,000 in replacement parts and fuel for every run if nothing major breaks.

The $5,000 includes things like fuel, engine bearings, spark plugs, piston rings and valve spring things that are replaced after every run. If the ignition happens to misfire you could end up with something like this (see photo above)…with the price increasing in proportion.

The wheelbase of a modern top fuel dragster is 300 inches. There is about 250 feet of chrome moly tubing in the chassis which remains unpainted due to the constant need to repair cracks. There are 58 Bead Lock bolts used to retain each rear tire to the wheel. Tire pressures are set between 6.5 and 7.5 psi.

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The slicks will grow 8.0 inches in height from 36” to 44” from the starting line through the first fifty feet. The width of the slicks will also be reduced by a third as they grow in diameter (see photo on page 32). By the way a pair of slicks only lasts about four runs and tires cost $700 each, so add that expense into the budget.

Here is a sneaky trick…

12,000 pounds is the amount of downforce generated by the rear wing of a top fuel dragster at 325 mph. The downward thrust made by the exhaust gases escaping the headers generates 800 pounds of that down force. Pretty sneaky, eh?

All of the ignition and related functions are controlled by computer and are programmed at the starting line using a laptop computer (see photo at right). The computer in the car is set based on watching the run of the previous car. The computer can control things like timing advance, how fast the clutches engage to control traction and at what point, along with fuel delivery. The computer also records things like top engine rpms, wheel slippage, axle revolutions and a host of other details.

To illustrate how much valuable data is available from the onboard computers in such a short time, race teams have learned that a top fuel engine will turn only 540 revolutions light to light. Including the burnout, the engine only has to survive 900 revolutions under load before it is disassembled and all of the broken or stressed parts replaced.

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So there you have it, from the past to the present in the nitromethane story. The real thrill would have been the first time a supercharger was added to an engine running on nitromethane. No doubt the learning curve was a little steep but oh, what a thrill it must have been when the driver nailed the throttle for the first time! ν

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