Skip to main content
Skip to page content

Automotive terms explained

Man under hood reviewing car maintenance checklist
"What's a carburetor?" My teenager's question shocked me. Didn't he learn anything from all that time he spends on car websites? Then it struck me: He was an infant when the last carbureted new car was sold in the U.S. To him, carburetors are ancient technology, like flintlock muskets and Windows 98. When something's dead, it's dead.

Here are definitions for commonly used automotive terms that everyone might like to understand.

Carburetors were the analog version of fuel injection. Carburetors did a great job of mixing fuel and air to meet an engine's varying needs. They were relatively inexpensive and largely reliable.

However, they were made obsolete by electronic fuel injection, which allows engines to produce more power, better fuel mileage and, most importantly, cleaner emissions.

Carburetors are still used in some nonautomotive engines, such as lawn mowers, and are mandated in NASCAR®.

Torque and horsepower difference
Torque is a twisting or rotational force – no motion is required. Imagine trying to turn the pedals of a bicycle wedged in a bike rack with its rear tire held firmly to the asphalt. The pressure put on the pedals would be torque.

Torque definition: Torque is measured and expressed in foot-pounds. If the bike's crank is 1 foot long and my son stood on one pedal, that's 135 foot-pounds of torque. 

Horsepower definition: Horsepower is a calculation. The formula: engine revolutions per minute (rpm) multiplied by the torque at that engine speed, divided by 5,252.

In most passenger-vehicle engines, torque nears its peak early and remains fairly constant until it falls away due to friction and the weight of the moving parts. Horsepower rises with engine speed and hits its peak when increasing rpm no longer offsets falling torque in the math formula.

Determining which is better – torque or horsepower – depends on what the vehicle is designed to do. Here are two extremes:

  • Some 18-wheeler engines make about 1,300 foot-pounds of torque at just 1,200 rpm. However, they top out at only 300 horsepower near the engine's 2,100-rpm limit.
  • Recent Grand Prix race engines are reported to approach 900 horsepower at a mind-boggling 19,000 rpm and have a torque peak of 500 foot-pounds at around 14,000 rpm.

Without radically modifying the clutch, the Grand Prix engine couldn't move the 80,000-pound semi. The big rig's diesel would make for a very slow race car, partially because it's almost double the weight.

In comparable passenger vehicles, an engine with more torque at lower rpm will provide better acceleration from slower speeds. It's a different story on the racetrack. Take two otherwise completely identical cars (weight, suspension, gearing, tires) to the drag strip. Car T has an engine that makes more low-end torque, whereas Car H's engine builder traded low-rpm torque for more high-rpm horsepower. Car T will take an early lead, but the driver will have to shift to second gear sooner and, thus, lose the torque-multiplying effect of the lower gear. By the time Car T shifts to third (and maybe sooner), Car H will take the lead.

Here's another factor: Horsepower and torque reported by manufacturers are not exactly what every engine produces for its entire life. Production tolerances cause variations, while wear makes power rise and fall. But more importantly, the marketing department has a huge impact on the numbers reported. Sometimes the actual horsepower is higher than advertised – rev it a bit more and it'll yield more horsepower. Two years later, the marketers will be able to tout the car's 10 additional horsepower, which was there all along.

This is the device that opens valves to let the air/fuel mixture in and burned gasses out. On a camshaft, there are several egg-shaped lobes, one for each valve operated by that camshaft. When the camshaft is rotated, the pointed ends of the camshaft lobe push the valve open and the rounded end allows it to close. The cam lobe can work either directly on the valve stem or through a linkage.

Today, most camshafts are located on top of the engine, above the cylinder head. These are overhead camshafts. If one operates both intake and exhaust valves, it's a single overhead camshaft (SOHC). If there are separate camshafts for intake and exhaust, it's a dual overhead camshaft (DOHC).

A DOHC "V" engine has four cams, two over each of its heads. Traditional American V-8s, called overhead valve (OHV) engines, have a single camshaft located in the valley of the "V." This lone camshaft works all the engine's valves through a system of rods and levers (called pushrods and rocker arms).

The advantages of overhead cams are efficiency, precision and the ability to reach higher engine speeds more quickly. Disadvantages include cost, complexity and, with DOHC, weight.

What is a V engine?
Almost all of today's internal combustion auto engines come in two types: Inline and "V." Inline means the cylinders are in a straight row. "V" (commonly V-6, V-8, V-10) means the engine's cylinders are aligned like the letter V. The angle of the V can vary depending on a number of factors. Inline engines tend to be smoother and lighter. A "V" engine is usually shorter and slightly wider than an inline with the same number of cylinders.

This is the engine's internal size – the pistons diameter multiplied by how far they travel on each stroke multiplied by the number of cylinders. It's expressed in cubic centimeters (6,000 cc), liters or cubic inches (366 ci).

Everything else being equal, increasing displacement usually increases torque and horsepower. But if the increased displacement reduces the engine's rpm potential (bigger pistons are heavier), the rise in horsepower may be small.

Energy lives here