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Most mechanics are familiar with the idea of "torque" but outside of setting a wrench or tool to a torque specification or as an arbitrary drive train output, they have little idea what torque really is. Yet it's important that mechanics understand what torque is so that it can be applied as needed, especially when no spec or manual is available.

For fasteners in automotive, we've talked about torque spec before. That covers how it applies to nuts and bolts specifically.

Now, what is torque itself and why is it a measurement for both tightening bolts and engine output?

Moment of Force

Force is defined as a push or a pull measurement. The moment of force is the same measurement applied to twisting rotation on an axis. The terms "torque" and "moment of force" (or just "moment") are interchangeable. Mechanics and engineers usually use the term torque while most physicists use moment instead, but both are referring to the same thing.

Measuring Torque

Torque is measured by calculating three quantities:

The force applied
The length of the lever or arm
The angle between the vector and lever

The equation for measuring torque is T = r x F (the "T" is a Greek symbol) or torque = displacement vector times force vector. For most auto mechanics this means that the amount of torque is equal to the length of the wrench handle times the amount of force being applied.

Understanding Torque in Practical Application

Think of your torque wrench on a bolt head. As you pull it (apply force), the arm of the wrench (handle) multiplies that force to create a certain amount of torque. A short-handled wrench may have a multiplication vector of 1.5, so you're applying 150% of the pressure you put on the handle to the bolt head (i.e. 10 pounds on the handle means 15 pounds on the bolt).

Similarly, a car with the same engine may have a different torque output from its twin if its drive shaft is longer. This is why the torque at the wheels of a front wheel versus rear wheel drive vehicle of the same make and model will be different.

Usually, however, torque is measured at the engine's output point rather than at the wheel. The engine's output is a measurement of its torque (valves turning the shaft) and its rotational speed (rotations per minute). Most internal combustion engines have a useful torque inside a limited range of RPMs, which is why this is the preferred measurement method for engine power.

For a gasoline vehicle, for instance, the range may be 1,000 to 6,000 RPMs so the current RPM times the output torque is charted to give a power to speed curve. Since the engine and transmission are usually measured separately and the entire drive train of the vehicle is tuned to a specific output capability, understanding torque outputs is vital.

It's for this reason that pairing a Ford V8 with a Nissan small car transmission is a bad idea: the engine will produce more torque than the transmission can handle, leading to failure. Automotive engineers put a lot of calculation into engine-transmission pairings. If one or the other is off, the entire setup will either fail or be woefully under-efficient.

Wait, what about horsepower?

Horsepower (U.S.) is actually a calculation utilizing torque. HP = ((lb-ft torque) x 2pi x RPM) / 33,000. So HP is measured by multiplying the foot pounds of torque with 2Pi (2 Pi units, or 180-degrees) and the rotations per minute speed of the shaft. Dividing all of that gives the horsepower.

It appears very complicated, but is a way of measuring the practical output of the vehicle versus its theoretical torque. It's possible for one 350hp engine to have a much smaller or larger torque than another 350hp engine. In practical use, however, both vehicles are likely much the same in similar use situations with their main difference likely to be fuel economy, speed-from-stop capability, and pulling power. A 350hp Mustang, for instance, will not pull a 2,500 pound trailer like a 350hp Sierra might and conversely, that Sierra is severely handicapped against the Mustang in a 0-60 speed test.

In both cases, however, the engine is producing the same overall amount of power.

Head Bolt Torque Specs

Torque Specs for Bolts

Intake Manifold Torque Specs