What the Numbers Mean
By Aaron Gold
When you’re reading about cars, you’re going to run into engine specifications, i.e. a 2.0 liter 4-cylinder turbo producing 160 horsepower and 175 lb-ft of torque. What do all those numbers mean? That’s the subject of this Vroom Girls University lesson.
A cylinder is the power unit of an engine; it’s the chamber where the gasoline is burned and turned into power. (For more on what goes on inside the cylinders, see How Engines Work.) Most cars and SUV enginess have four, six, or eight cylinders. Generally, an engine with more cylinders produces more power, while an engine with fewer cylinders gets better fuel economy.
Cylinders will either be arranged in a straight line (an inline engine, i.e. “inline 4”, “I4” or “L4” ) or in two rows (a V engine, i.e. “V8”).
DISPLACEMENT (Liters and Cubic Inches)
Engines are measured by displacement, usually expressed in liters (L) or cubic centimeters (cc). Displacement is the total volume of all the cylinders in an engine. An engine with four cylinders of 569cc each totals 2276cc, and will be rounder off and referred to as a 2.3 liter engine. Larger engines tend to produce more power — specifically more torque (see below) — but use more fuel.
Up until the early 1980s, engines were measured in cubic inches. One liter equals about 61 cubic inches, so a 350 cubic inch engine is about 5.7 liters.
A turbocharger is a device that is used to boost the power of an engine. A four-cylinder engine with a turbocharger can produce as much power as a six-cylinder engine, but uses less fuel when driven gently. (For more information, see How Turbochargers and Superchargers Work.) Engines with a turbo sometimes get a T after their displacement; “2.0T” denotes a 2-liter engine with a turbocharger.
HORSEPOWER AND TORQUE
Horsepower and torque measure the amount of power an engine develops, with horsepower being the most commonly-used measurement. The difference between horsepower and torque is widely misunderstood (and difficult to explain).
Torque, which is measured in pound-feet (lb-ft or ft-lbs), measures pulling power; when you step on the gas pedal and the seat pushes into your back, you are feeling torque. Trucks need lots of torque to get their heavy loads moving. Horsepower is a function of torque and engine speed (RPM), and indicates how much sustained work the car can do. Racing cars need high horsepower to maintain high speeds. Generally, bigger-displacement engines develop more torque, but small engines can spin faster, which increases their horsepower output.
A car with high horsepower but low torque may feel sluggish from a stop, but will feel stronger as the engine spins faster and faster. A high-torque, low-hp engine will accelerate strongly from a stop, but will trail off as the engine speeds up (until the transmission shifts gears).
Horsepower and torque measurements are “peak” numbers; a 180 horsepower engine will only produce 180 horsepower at a certain engine speed — say, 6,000 RPM. At other speeds, the engine develops less horsepower. The same goes for torque, although some engines (especially those with turbochargers) have a sustained peak-torque range, developing their rated torque between, say, 1,800 and 4,000 RPM. An engine with strong mid-range torque (peaking between 2,000 and 4,000 RPM) will have good passing acceleration, while lot of low-end torque (below 1,500 RPM) is useful for towing trailers or driving off-road. However, cars with high-torque engines are more likely to slip and slide in rain and snow.
All that said, other factors, such as how much the car weighs, will affect acceleration. How the vehicle feels when you drive it is more important than the horsepower and torque ratings.