Auto Tech: Horsepower and torque

By Jim Kerr

Comparing horsepower and torque ratings between vehicles can be confusing, partly because when manufacturers tune their engines, they are after specific engine characteristics to match the weight, size and purpose of the vehicle. The numbers vary from engine to engine and typically only the peak ratings are given, which can also be misleading. Let’s take a look at what horsepower and torque really are and how they are measured.

Horsepower is related to torque, and torque is turning force. Place a lever on the engine’s crankshaft and measure the twisting force produced, and you have torque. Note that time is not a factor – torque can be constant over time. Horsepower, on the other hand, is the rate at which the torque is produced.

Horsepower is a measurement of the engine’s ability to do work. One horsepower can lift 33,000 pounds up one foot in one minute. Horsepower is measured over time. The more horsepower a vehicle has, the more mass it can move in the same amount of time, or it can move a fairly constant mass (the weight of the car) in less time. In simple terms, to get a vehicle accelerating quicker, we need to produce torque faster.

Until 1971, horsepower was reported in SAE (Society of Automotive Engineers) gross ratings. This measured power at the flywheel but without any accessories, exhaust or air filters. In 1972, SAE ratings were now given as net horsepower. This was with all accessories and components as would be found when the engine is installed in a vehicle. As a result, the power ratings were significantly lower.

A couple years ago SAE announced a new J1349 power and torque certification procedure. This J1349 standard not only specifies new test procedures for measuring engine horsepower, but also requires that an SAE qualified observer witness and verify the actual test. This enables consumers to compare the horsepower of one certified engine to another. But there is much more to vehicle performance than just horsepower numbers.

That brings us back to torque. Diesel engines typically have high torque ratings compared to their horsepower output. Some of this is provided by the higher compression used in diesel engines but most of it is created by the diesel combustion process. Diesel combustion starts when the fuel is injected into the cylinder. The burning fuel forces the piston down and the crankshaft to turn. Torque is the twisting force of the crankshaft. When we want to accelerate or pull a heavy load with a diesel powered vehicle, more fuel is injected to provide additional force. Unlike gasoline engines, where the fuel is delivered into the cylinder before the spark plug ignites it in a big bang, diesel injection continues to spray the fuel into the cylinder as combustion takes place. This keeps the pressure and force on the pistons higher for a longer time – hence more torque.

Horsepower is the rate at which torque is produced. Rev the engine higher and more horsepower is produced. There are limits to when maximum horsepower is developed and this is usually created by engine mechanical limitations or restrictions in the airflow through the engine. Gasoline engines typically have higher horsepower ratings than torque ratings, because the lighter components (compared to diesel engines) allow them to accelerate rpm quicker so they can produce the power quicker and accelerate the vehicle quicker.

Almost all horsepower and torque ratings are given in “peak” numbers at a specified rpm. This maximum rating may not mean much to your driving unless the specified rpm is within your normal driving speeds. For example, a race engine might produce a lot of horsepower at 7,000 rpm but very little at 2,500 rpm. This wouldn’t make for a pleasant car to drive on the street. Variable cam timing, variable intake manifold tuning and turbocharging are all technologies used to broaden the power and torque range of an engine.

For pulling power, you want lots of torque at low r.p.m. Some engines like Ford’s Ecoboost use twin turbos to help produce peak torque from 1,500 rpm all the way to 5,250 rpm. For acceleration, look for higher horsepower numbers. If peak horsepower is delivered at lower rpm, the vehicle will be more relaxing to drive.

Manufacturers try to match horsepower and torque output of an engine to the vehicle purpose: most of the time they do a very good job.