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Truck Performance: The Execution of an Action

Shop Class: Performance

Alex Steele
Nov 27, 2019
Performance can be a broad term when talking about cars and trucks. It's often used when evaluating different categories like top speed, acceleration, fuel economy, ride, handling, and so on. And let's not forget about high performance, the heavy-duty stuff that performs better than the rest.
More often than not, folks labeling the performance of sports cars—and trucks—narrow down the conversation to everyone's favorite word: horsepower. And once in a while, torque.
Driver A's car puts out 325 hp, driver B's only 300, so driver A has the faster, more powerful ride; end of story. But it's never that simple.
Let's look at some rudimentary values that determine automotive performance.
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Horsepower vs. Torque
A short history on horsepower: The unit of measurement was established in the 1700s by Scottish engineer James Watt. The idea was comparing work done by steam engines to that of the horses they ultimately replaced.
Horsepower is a measurement of the rate (speed) of work done. The perfect example is a horse pulling a rope, which is routed over a pulley to lift an object off the ground. The horse lifting a 550-pound weight off the ground a distance of 1 foot in 1 second is considered 1 hp.
Today's common hp standards calculated by watts are mechanical or imperial horsepower (1 hp = 745.5 watts), and metric hp (1 hp = 735.5 watts). And, of course, we're now dealing with pistons driven up and down by combustion.
Torque is a measurement of force using leverage on something at a distance. In this case, a piston pushed down by combustion using leverage on the crankshaft to produce rotation, or a twisting force. An example is tightening a bolt using a 1-foot wrench, then tightening another identical bolt using the same force with a 2-foot wrench. The second bolt will be tightened to a greater torque using the same force due to the added leverage of the longer wrench. Using this principle, an engine's torque is greatly affected by crankshaft design.
Crankshaft throw is the distance from the centerline of the crankshaft to the centerline of each piston's connecting rod journal. This distance also determines stroke; the distance each piston travels up and down. Increasing throw/stroke in turn increases the leverage the piston applies to the crankshaft, and therefore boosts low-rpm torque.
A race engine, on the other hand, typically uses larger pistons with a shorter stroke to increase power at high rpm.
Manufacturers publish engine performance specifications, including peak horsepower and torque (lb-ft), established by SAE protocol.
The bottom line in a perfect world is more horsepower means more speed, but horsepower can also be converted to higher torque at the drive wheels using aggressive gearing.
As we know, truck and industrial applications require more torque at lower speeds to pull heavy loads. This is where diesel engines burning a higher-energy-density fuel, with a longer piston stroke, earn their money with exceptional low-end torque.
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Power-to-Weight
But what about driver A's bragging rights over driver B going by horsepower alone? Increasing or decreasing vehicle weight has a significant effect on acceleration. An extreme example is a 325hp, 3,500-pound car against a 100hp, 500-pound motorcycle in a quarter-mile race. Power-to-weight ratio is calculated by dividing horsepower by curb weight to determine horsepower per pound. Here, the 325hp car comes in at .093 hp/lb while the bike is closer to .20 hp/lb.
The motorcycle takes the win by a large margin, with less than one-third of the horsepower due to its vast power-to-weight advantage.
Power-to-weight ratio determines how quickly you get there but has less of an effect on top speed. Horsepower, gearing, traction, and aerodynamics play more dramatic roles when you're flying down the road at the outer limits.
 
Ride and Handling
Performance discussions can also apply to suspension, chassis, and body designs. Some vehicles are equipped with technology enabling on-the-fly adjustable suspension damping, drivetrain, traction, and stability control systems. This can make the ride adaptable to various road conditions or the driver's mood.
But the old standards still hold true. In general, stiffer suspensions provide better handling, softer a more comfortable ride, and a lot of main-line vehicles are based on a happy medium.
When talking high speed in sports, super, and race cars, the main concern is keeping the wheels stuck to the ground. That's why these machines make use of stiffer suspenders that better resist centrifugal forces in corners, keeping the wheels pressed into the pavement.
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Top Speed
This is something nobody should be doing outside of a racetrack or salt flats. When traveling at super-high speeds, traction is do or die. Aside from frame and suspension, the quality, width, and temperature of tires must be ideal. After that, success is determined by weight and aerodynamics.
Automotive body designs are focused on reducing a vehicle's drag coefficient; basically, making it sleeker so there's less resistance cutting through the air. This enhances fuel economy and high-speed efficiency. The problem at really high speeds is the creation of lift, or high air pressure beneath the vehicle. This lift reduces the vertical force on the tires and, consequently, traction on turns.
The counteraction is reducing lift or increasing downforce (pressure pushing the car down to the pavement). A spoiler is commonly used on sport production vehicles. It reshapes the front or rear of the body to disrupt (spoil) airflow and decrease potentially dangerous lift.
A wing or airfoil is a different animal. It's basically an upside-down airplane wing. Instead of creating positive lift (up), it builds negative lift (downforce). Along with downforce, a wing creates a certain amount of unwanted drag that can be reduced, but never eliminated.
Both spoilers and wings can be used front and rear to help balance lift and downforce. This increased vertical force increases the frictional limit at the tires, meaning the vehicle can handle more lateral G-force on a turn before it loses grip and slides out.
To put things in perspective, a Formula 1 race car produces three to four times the vehicle's weight in downforce at high speeds. This allows the car to withstand 4 to 5 Gs on a turn. Theoretically, an F1 car can drive upside down.
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Sport Buying?
We've figured out that making a decision based on the horsepower spec alone is not the best way to go—for most anyway. And keep in mind the majority of driving takes place on public roadways, where you can't do continuous wide-open-throttle passes on a regular basis without getting caught.
Compare power-to-weight ratios. This is really important if you enjoy quick throttle response and tight handling. Too much power and weight may take you in the wrong direction while increasing cost. Better power-to-weight on a smaller, lighter car may be what you're looking for.
Raise your hand if you break 160 mph on the way to work every day. If not, ignore the top speed numbers.
Everybody has an opinion about the takeover by electric vehicles in the near future. Be it good, bad, or indifferent, the acceleration from high-voltage electric motors is extraordinary. As compared to the increased use of turbochargers, which increases horsepower while initiating a characteristic lag in throttle response, electric motors pin you to the seat big-time when you hit the switch.
The muscle car era fans have spoken, and the American retro-car phase is holding strong. Pick a Challenger Hellcat or Mustang GT500 for supercharged horsepower, or the V-6 version of either one. In this category, a lot of "reasons not to" don't matter anymore.

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