The Amazing Engineering of Top Fuel Dragsters

The Amazing Engineering of Top Fuel Dragsters

In Vehicles, Videos by Paul ShillitoLeave a Comment


Top Fuel dragsters are the fastest accelerating ground vehicles in the world and have power outputs of upto 11,000 hp but how do they that and still stay in one piece and how do they put that amount of power down to the ground without destroying the tires in an instant, this is the amazing engineering behind the top fuel dragsters.

This video is sponsored by Brilliant.

If you ever wonder why top fuel is so-called, it’s because of what’s used to power these engines, a mix of up to 90% Nitromethane and 10% methanol and that’s what gives these vehicles the huge amount of power.

The acceleration they achieve at launch is in the order of 5G for the driver and after just 0.8 seconds and 60 feet or 18 meters they are doing 100mph with 200mph coming up around 2.2 seconds and 350 feet or 106 meters.

The so-far fastest 1000 foot run was set by Brittaney Force in her 2019 Carquest top fuel dragster when she ran a 3.659 second run at  338.17 mph.

But all that power comes at a very high cost with the engines needing to be completely stripped down and rebuilt after every race and that’s if your lucky and it hasn’t exploded going down the track.

And that whats give top fuel its attraction, not only is it the fastest, loudest motorsport on earth, you never quite sure if the cars will even make it past the burn-out.

In a world where we think of production vehicles with engine outputs of 1500 hp like the Buggati Chiron as being reliable and with the correct servicing capable of tens of thousands of miles and more, its easy to forget that when your really pushing the limits of the internal combustion engine things become a lot trickier but it’s amazing to see what is possible with a basic engine design that dates back to the 1950s.

The fuel used is Nitromethane which is widely used in industry as a reaction medium and solvent. This is what is called a mono fuel meaning that it carries its own oxygen unlike gasoline which needs to be mixed with air to combust but Nitro has a much lower energy density than gasoline at 11.2MJ/kg to gasoline’s 44MJ/kg.  

However, Gasoline needs 14.7kg of air with a 21% oxygen content to burn 1kg of gasoline, nitro, on the other hand, because it carries its own oxygen needs only 1.7kg of air to burn 1kg of nitro.

The upshot of this is that you can get 8.6 times nitro into the combustion chamber than gasoline and in general a supercharged top fuel engine makes about eight times more power than the equivalent supercharged gasoline-powered engine.

Now you would think that to handle all that power you would need some super high tech engine like they have in formula 1 cars but actually, all top fuel engines are based on a Chrysler V8 Hemi engine design dating back to the early 1950s.

Their size is limited to 500cu inch or 8.2 litres and they have pushrod operated two-valve cross-flow heads driven from a single centrally mounted camshaft, a far cry from the modern 5 valve double overhead cams or even computer-controlled pneumatic valves in the latest race engines.

This engine architecture and the type of supercharger used is part of the NHRA rules which might look arcane in this day and age but it keeps the tradition of the sport. It also keeps a more level playing field and the costs down with a typical complete top fuel engine including fueling and supercharger running at about $100,000. Sounds expensive but compared to the $7 -$10 million for an F1 engine it’s a bargain by comparison.

Whilst the engine format might be the same as the old hemi’s what they are made out of is very different. The engine block of a top fuel engine is machined from a solid piece of aluminium, as are the cylinder heads but there are no water cooling jackets in these engines, the fuel its self provides the cooling for the valves and pistons and don’t forget they only run at full power for less than 5 seconds on a run.

The crankshaft is billet steel and the connecting rods are forged aluminium and not titanium because aluminium has great shock absorbing qualities and lessens the combustion shock on the upper big end bearings, crankshaft and the block itself.

These aluminium rods dwarf the steel rods of a normal engine. The cylinder pressures in top fuel engines can be over 13,000 psi and this compressive load physically shortens the rod length over several runs which decreases the compression ratio, reducing the power output. The life of the connecting rods is about 8-10 runs or less but the bearings are changed after every run.

One of the main tuning devices in the engine is the head gasket. Depending on the weather and track conditions, a couple of hours before the race the crew will decide what thickness of the copper head gasket to use as this will affect the compression ratio and thus how much power the engine can make which is why the car arrives at the track with the heads off.

At launch, the engine revs to about 8500 rpm but once the car is travelling down the track this drops to 7900 rpm and this is done to try and limit the top speed to 330 mph. A couple of the main reasons why is partly down to keep the tires in one piece but also for the insurance purposes as insurance companies get panicky at speeds above this because most of the fatal crashes of the past have involved these very high speeds.  

To take the extreme heat, the exhaust valves are made from Inconel, the same superalloy used for the thrust chamber for the Rocketdyne F-1 engines of the Saturn 5 rocket. The spark plugs on the other hand face a much shorter life with most of the electrodes being burned off during the run. The twin MSD 44 amp magnetos produce a 60,000V 1.2amp spark to each of the twin plugs per cylinder but even that is not always enough to ensure combustion takes place.

The Nitro fuel is pumped in via 34 injectors, 16 in the heads, 8 in the intake manifold and 10 in the blower hat at 500 psi from a 17 gallon tank at the front of the vehicle via a 2.5” or 63mm pipe to a 100 gallon or 378 litre per minute rated fuel pump. Even the fuel tank has a vent pipe right at the front of the chassis where at high speed it has a ram air effect and helps pressurise the fuel system before it hits the fuel pump.

In a typical run of under 4 seconds, the engine will use about 20 gallons of nitro, thats about 5.5 gallons or 20 litres per second. A 50 gallon or 190 litre drum of Nitro costs about $1000.

All of this is forced into the engine at a pressure of 65 psi by the 14-71 roots-type supercharger. These were originally developed for diesel truck engines but are now the mandated supercharger type for top fuel.

The twisted lob design requires about 800 hp to drive them at full speed via a very large kevlar toothed belt from the crank and they have to be covered in a Kevlar blanket to protect against shrapnel as blower explosions are not uncommon if there is a dropped valve and a backfire into the supercharger. The charge in the combustion chamber is so dense that these engines run a very fine line between hydraulicking the cylinder with liquid fuel and the combustion of gas.

The exhaust from each cylinder then exits via the open headers which are pointed up and backwards, in fact, the force of the exhaust exiting upwards adds about 1000 lbs or about 4.5 kilonewtons of downforce and a bit extra forward thrust too. Each of the headers has a thermocouple that records the temperature of the exhaust gases which can tell how lean or rich the engine is running.

Because Nitro is a slow-burning fuel compared to gasoline some of it is still unburned as it leaves the exhaust and due to its high temperature it re-ignites as it mixes with the atmospheric oxygen creating the huge rooster tails of yellow flames you see from the exhaust.

Some Top fuel engines run very rich to help cool the valves and pistons and lessen the detonation. As this very rich air Nitro mix combusts it breaks down into hydrogen and carbon monoxide which leaves via the exhaust. This can be seen at night as a very bright white flame as the hydrogen burns off as mixes with the atmospheric oxygen in the air.

The exhaust from a top fueler is also the loudest of any motorsport at about 160 dB, certainly enough to burst your eardrums if you close by a run with no ear protection and vibrates your vision even if you are. Geologists once placed a seismometer on the start line between two top fuelers, the result was a tremor that registered 2.3 on the Richter scale.

Although the stated power of the most powerful fuelers is about 11 or even 12,000 hp the actual outputs are not measured directly on a dyno but calculated from the run time, weight of the vehicle and the track conditions on the run.

There are dyno’s which can handle this power but fueler engines can’t run at full power for more than 10 seconds without overheating or destroying themselves making it a very expensive exercise.

The amount of power the crew think is required to make the best run is dialled in on the start line just before each run and as such can vary depending on the conditions. Some have torque sensors that have recorded peak torque figures of over 12,000 ft/lbs or 16200 nm, roughly ten times that of the Bugatti Chiron.

Getting that power to the ground is done by a multiple plate clutch in a titanium housing which is connected directly to the rear axle. There is no gearbox, the engine is connected directly to the final drive through the clutch which slips in a timed manner from the launch up until about 280 mph where it locks up and feeds the full power to the wheels and after each run the all the clutch plates are replaced.

The live rear axle uses a 12 ¼” gear set with a 3.20 ratio and titanium axles. Carbon brake discs with carbon pads provide the main stopping power and even though parachutes are used the carbon-carbon brakes must be capable of stopping the one-ton vehicle from 330 mph without them and of course, there are no front brakes on the super slim front wheels.

Putting even just a few thousand hp through the tyres takes an enormous toll on them at the launch and they mustn’t fly apart through the centrifugal forces when they reach 330 mph at the finish line. They must then start to the job of slowing the one-ton vehicle by the end of the track assisted by parachutes.

The tyres which are made by Goodyear are called ripple wall slicks and have compounds that are very soft, much softer than F1 tyres. This allows them to have an incredible amount of grip on the already very sticky track but they only last for about eight 1000ft runs before they are replaced.

The tyres are clamped to the rims with 24 bolts to stop the wheel from slipping in the tyre and the tyres are inflated to between 7 and 10 psi. This and the very tall but thin sidewalls allow the tyres to twist and bunch up on the launch, this lets the tyres squat down and increase the contact patch during the launch to almost 250 square inches.

This also reduces the tyres diameter and effective gearing of the final drive. and. The twisting of the side walls acts to moderate the vicious delivery of torque and the energy built up in the twisted tyre then releases and helps propel the vehicle off the start line with a smoother acceleration and less wheelspin.

As the speed builds up the tyres grow through the centrifugal forces and become taller and thinner which decreases the rolling resistance and increases their diameter and the final gearing pushing up the speed even more.

Towering above the rear tires is the massive rear aerofoil which at 300mph can produce up to 12,000lbs or 5.4 kilonewtons of downforce which is needed to keep the tyres firmly planted on the track and reduces wheel spin as the full power of the engine is applied once the clutch locks up.

Meanwhile at the front end, the tiny by comparison wheels and tyres which are inflated to 90 psi and are held on the track by the front aerofoil which produces upto 2500lb or 1.3 tons of downforce at 300mph ensuring the steering works and stops the whole car from flipping up and over.

By the time you have added the cost of the chassis, rear end, engine, clutch, electronics & data logging you got somewhere between $250 to $300,000 for a complete car and that’s without transport & crew costs, about $5000 in parts on a normal run with no engine blowups plus $100 per second in fuel.

It certainly not cheap but it’s probably the biggest thrill any adrenaline junky can have this side of a rocket launch and still a bargain compared to Formula 1.

As the driver of a top fuel car you are putting you life in the hands of the engineers that made the parts and the mechanics that fitted them so it’s good to know that they know what they are doing and understand all the technical aspects of one of the most a fearsome of all racing vechicles.

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Paul Shillito
Creator and presenter of Curious Droid Youtube channel and website

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