These cars compete in a 1/4 mile (0.4 km) race and complete it in less than 4.5 seconds at upwards of 330 mph (530 km/h). A Top Fuel dragster accelerates from 0 to 100 mph (160 km/h) in less than 0.8 second, subjecting the driver to a force about 5.7 times his weight. This acceleration takes less than a tenth of the time needed by a production Porsche 911 Turbo to reach the same speed. A fuel dragster can exceed 280 mph (450 km/h) in just 660 feet (0.2 km). The full race distance is 1/4 of a mile, 1320 feet. For further information and standards for drag-racing, including safety requirements, see NHRA
The engine used to power a Top Fuel drag
racing car has its roots in the second
generation Chrysler Hemi 426 "Elephant Engine"
made 1964-71. Although the Top Fuel engine is
built exclusively of aftermarket parts, it
retains the basic configuration with two valves
per cylinder activated by pushrods from a
centrally-placed camshaft. The engine has
hemispherical combustion chambers, a 90 degree V
angle; 4.8" bore pitch and a 5.4" camshaft
height. The configuration is identical to the
overhead valve, single camshaft-in-block "Hemi"
V-8 engine which became available for sale to
the public in selected Chrysler Corporation
(Dodge, DeSoto, and Chrysler) automotive
products in 1952.
The NHRA competition rules limit the
displacement to 500 cubic inch (8193.5 cc). A
4.19" (106.4 mm) bore with a 4.5" (114.3 mm)
stroke are customary dimensions. Larger bores
have been shown to weaken the cylinder block.
Compression ratio is about 6.5:1, as is common
on engines with overdriven (the supercharger is
driven faster than the crankshaft speed)
superchargers.
The block is CNC machined from a piece of forged
aluminium. It has press-fitted ductile iron
liners. There are no water passages in the block
which adds considerable strength and stiffness.
Like the original Hemi, the racing cylinder
block has a long skirt (to reduce piston
"rocking" at the lower limit of piston travel);
there are five main bearing caps which are
fastened with aircraft-standard-rated steel
studs; with additional reinforcing main studs
and side bolts. There are three approved
suppliers of these custom-made after-market
blocks, from which the teams may choose.
The cylinder heads are CNC-machined from
aluminum billets. As such, they have no water
jackets and rely entirely on the incoming
air/fuel mixture for their cooling. The original
Chrysler design of two large valves per cylinder
is used. The intake valve is made from solid
titanium and the exhaust from solid Nimonic 80A
or similar. Seats are of ductile iron,
beryllium-copper have been tried but its use is
limited due to cost. Valve sizes are around
2.45" (62.2 mm) for the intake and 1.925" (48.9
mm) for the exhaust. In the ports there are
integral tubes for the push rods. The heads are
sealed to the block by copper gaskets and
stainless steel o-rings. Securing the heads to
the block is done with aircraft-rated steel
bolts.
The camshaft is billet steel, made from 8620
carbon steel or similar. It runs in five oil
pressure lubricated bearing shells and is driven
by gears in the front of the engine. Mechanical
roller lifters, steel push rods and steel
rockers are used to actuate the cams. The
rockers are of roller type on the intake side,
high pressures on the exhaust limits its use to
the intake side only. The steel roller rotates
on a steel roller bearing and the steel rocker
arms rotates on a titanium shaft within bronze
bushings. Intake rockers are billet while the
exhausts are investment cast. The dual valve
springs are of coaxial type and made out of
titanium. Valve retainers are also made of
titanium, as are the rocker covers.
Billet steel crankshafts are used; they all have
a cross plane a.k.a. 90 degree configuration and
runs in five conventional bearing shells. 180
degree crankshafts have been tried and they can
offer increased power, even though the exhaust
is of open type. A 180 degree crankshaft is also
about 10 kg lighter than 90 degree crankshaft,
but they create a lot of vibration. Such is the
strength of a top fuel crankshaft that in one
incident, the entire engine block was split open
and blown off the car during an engine failure,
and the crank, with all eight connecting rods
and pistons, was left still bolted to the
clutch.
Pistons are of forged aluminium, 2618 alloy.
They have three rings and aluminium buttons
retain the 1.156" x 3.300" steel pin. The piston
is anodized and Teflon coated to prevent galling
during high temperature operation. The top ring
is an L-shaped Dykes ring that provides a good
seal during combustion but a second ring must be
used to prevent oil from entering the combustion
chamber during intake strokes as the Dykes-style
ring offers less than optimal combustion gas
sealing. The third ring is an oil scraper ring
whose function is helped by the second ring. The
connecting rods are of forged aluminium and do
provide some shock damping, which is why
aluminum is used in place of titanium, because
titanium connecting rods transmit too much of
the combustion impulse to the big-end rod
bearings, endangering the bearings and thus the
crankshaft and block. Each con rod has two
bolts, shell bearings for the big end while the
pin runs directly in the rod.
The supercharger is a 14-71 type roots blower.
It has twisted lobes and is driven by a toothed
belt. The supercharger is slightly offset to the
rear to provide an even distribution of air.
Absolute manifold pressure is usually 3.8-4.5
bar, but up to 5.0 bar is possible. The manifold
is fitted with a 200 psi burst plate. Air is fed
to the compressor from throttle butterflies with
a maximum area of 65 sq. in. 45.5 Maximum boost,
in PSI, produced by the supercharger at
wide-open throttle.
These superchargers are in fact derivatives of
General Motors scavenging-air blowers for their
two-cycle diesel engines, which were adapted for
automotive use in the early days of the sport.
The model name of these superchargers delineates
their size; i.e. the once commonly used 6-71 and
4-71 blowers were designed for General Motors
diesels having six cylinders of 71 cubic inches
each, and four cylinders of 71 cubic inches
each, respectively. Thus, the currently used
14-71 design can be seen to be a huge increase
in power delivery over the early designs.
Mandatory safety rules require a secured
Kevlar-style blanket over the supercharger
assembly as "blower explosions" are not
uncommon. The absence of a protective blanket
exposes the driver, team and spectators to
shrapnel in the event that nearly any
irregularity in the induction of the air/fuel
mixture, the conversion of combustion into
rotating crankshaft movements, or in the
exhausting of spent gasses is encountered.
The oil system has a wet sump which contains 16
quarts of SAE 70 mineral or synthetic racing
oil. The pan is made of titanium or aluminium.
Titanium can be used to prevent oil spills in
the event of a blown rod. Oil pressure is
somewhere around 160/170 lb during the run, 200
lb at start up, but actual figures differs
between teams.
Fuel is injected by a constant flow injection
system. There is an engine driven mechanical
fuel pump and about 42 fuel nozzles. The pump
can flow 92 gallons/minute at 8000 rpm and 500
PSI fuel pressure. In general 10 injectors are
placed in the throttle horn above the
supercharger, 16 in the intake manifold and two
per cylinder in the cylinder head. Usually a
race is started with a leaner mixture, then as
the clutch begins to tighten as the engine speed
builds, the air/fuel mixture is enriched. As
engine speed builds pump pressure the mixture is
made leaner to maintain a predetermined ratio
that is based on many factors, one of which is
primary one of race track surface friction. The
stoichiometry of both methanol and nitromethane
is considerably greater than that of racing
gasoline, as they have oxygen atoms attached to
their carbon chains and gasoline does not. This
means that a "fueler" engine will provide power
over a very broad range from very lean to very
rich mixtures. Thus, to attain maximum
performance, before each race, by varying the
level of fuel supplied to the engine, the
mechanical crew may select power outputs barely
below the limits of tire traction. Power outputs
which create tire slippage will "smoke the
tires" and the race is often lost. However, as a
testament to never giving up, Cruz Pedregon, a
Funny Car driver has won two races, years apart,
when his car spun the tires during the run, when
the OTHER driver also smoked his tires, and they
both proceeded down the track, smoking the tires
side by side. It was an amazing spectacle to see
once, let alone twice. (This is not to be
confused with pre-race tire-spinning which heats
that tires and smokily lays down molten rubber,
which cools slightly, allowing a maximum
traction "run" just moments later.)
The air/fuel mixture is ignited by two 14 mm
spark plugs per cylinder. These plugs are fired
by two 44-amp magnetos. Normal ignition timing
is 58-65 degrees BTDC. (This is dramatically
greater spark advance than in a gasoline engine
as "nitro" and alcohol burn far slower.)
Directly after launch the timing is typically
decreased by about 25 degrees for a short time
as this gives the tires time to reach their
correct shape. The ignition system limits the
engine speed to 8400 rpm. The ignition system
provides initial 50,000 volts and 1.2 amps. The
long duration spark (up to 26 degrees) provides
energy of 950 millijoules. The plugs are placed
in such a way that they are cooled by the
incoming charge. The ignition system is not
allowed to respond to real time information (no
computer-based spark lead adjustments), so
instead a timer-based retard system is used.
The engine is fitted with open exhaust pipes,
2.75" in diameter and 18" long. These are made
of steel and fitted with thermocouples for
measuring of the exhaust temperature. They are
called "zoomies" and exhaust gases are directed
upward and backwards. Exhaust temperature is
about 260 °C at idle and 980 °C by the end of a
run. A night run provides visual excitement with
slow-burning nitromethane flames many feet above
this screaming spectacle of acceleration. A
"good run" is over in just 4.5 seconds, the
noise ends, and braking parachutes are seen in
the distance, after a speed of over 325 miles
per hour has been reached.
The engine is warmed up for about 80 seconds.
After the warm up the valve covers are taken
off, oil is changed and the car is refueled. The
run including tire warming is about 100 seconds
which results in a "lap" of about three minutes.
After each lap, the whole engine is taken apart
and gone through, and much of it is replaced.
Performance
Power output of these engines is most likely
somewhere between 6000 and 8000 horsepower
(approximately 4500-6000 kilowatts). This is
calculated from performance as these engines
aren't tested on a dynamometer. This would
suggest a torque output of 5100-6750 Nm
(3760-4980 lb-ft) and also a brake mean
effective pressure of 80-100 bar.
Engine weight
Block with liners 85 kg
Heads 18 kg each
Crankshaft 37 kg
Complete engine 225 kg.
