Torque and BHP explained
An engine’s power is measured by running the engine against a load on a dynamometer. The braking effort needed to hold the engine at a steady speed on full throttle gives the torque. The power can then be calculated by multiplying the torque by the engine speed.
An engine which produces a lot of torque over a wide range of engine speeds
will be relaxing to drive because fewer gearchanges are needed: the engine’s
torque is often sufficient to accelerate the car without changing down. At
cruising speeds a lorquey’ engine will not need to be turning over very quickly
because it can pull at high gearing, which makes for good
economy.
Engines that produce a lot of power for their size do not usually produce so
much torque, and what torque there is is often produced at higher engine
speeds. It is also likely that the engine will be producing usable torque and
power over a smaller range of engine speeds; this narrow ‘power band‘ makes the
engine less suitable than a torquey or ‘lazy’ engine for jobs such as towing,
and the car will be less relaxing to drive.
Typical figures
A fairly typical small family car engine puts out, say, 60bhp (brake horse
power) at 5000rpm. The same engine can be tuned or modified so that it gives
80bhp at 6000rpm. But although the power is greater, the peak torque can
actually be less, as well as occurring at a higher engine speed. There will be
less torque at low and medium engine speeds.
In other words, though a car with the tuned engine would have a higher
maximum speed, it would only accelerate better as long as the gearbox was used
to the full to keep the engine speed up, assuming the gearing remained the
same.
In practice, the highly tuned car would almost certainly need to be
differently geared to remain drivable – the gears would have to be more closely
spaced and the overall ratio slightly lower.
Measuring power
The usual engine test procedure is to run the unit on a ‘brake’ or
dynamometer which measures torque over a large range of speeds by seeing how
much braking effort is needed to keep the engine at a steady speed on full
throttle.
The torque times the engine speed then gives the power output, called brake
horse power (bhp). Power measured like this, with the engine on a test bed, is
expressed as a power output at the flywheel.
It is possible to run the car on a `rolling road’ dynamometer to measure the
power output at the driving wheels instead. This is less than the power at the
flywheel because of frictional losses in the car’s transmission system, but it
gives a more realistic idea of how the car will perform as it shows how much
power reaches the road.
Torque/bhp balance
Every engine designer has to bear in mind the balance between power and
torque. He might even move the balance a little away from power and towards
torque if enough drivers understood the importance of torque and the
generalization that power versus aerodynamic drag determines maximum speed, but
torque versus weight determines acceleration.
As the car speeds up, forces other than weight, such as aerodynamic drag,
rolling resistance of the tyres, and the friction within the engine and
transmission, act on it to try to resist this acceleration. At a certain speed,
these drag forces equal the car’s driving force, or torque, and there is no
excess power left for further acceleration.
Gearing
Changes in gearing are important when looking at power and torque,
because the gears act as torque multipliers.
If first gear has a ratio of 3:1, it multiplies the engine’s torque
output by three when passing it on to the final drive. Similarly, the final
drive ratio, typically around 3.5:1, multiplies the torque from the gearbox
by that much again.
In first gear, therefore, the torque delivered to the driving wheels can
be around ten times greater than the engine’s torque output, while speed of
rotation will have reduced by a similar factor. This gearing down is
necessary because one of a piston engine’s biggest drawbacks is its poor
torque at low speed.
Torque and brake horsepower curves
The amount of power an engine develops can be measured on a dynamometer
and the results plotted on a graph. Shown here are typical curves for an
engine in what an engine tuner would call ‘road tune’ and ‘fast road tune’
states.
Road tune (near right) is the compromise between power/torque and fuel
economy that a car manufacturer builds into a typical car engine when
designing it.
A fast road tune engine (far right) sacrifices some fuel economy for
increased power. The amount of torque is overall slightly less, and the
maximum torque occurs at higher revs. Such an engine develops more top-end
power which would give a higher top speed, but its decreased overall torque
requires higher revs for the same power output and more gearchanging — a
less ‘lazy’ drive.
Above fast road, it is possible to tune the engine to increasingly
higher levels known as ‘road/rally’, ‘rally’ and ‘race’. But the greater
power output is paid for, apart from decreasing fuel economy, by the torque
band moving to higher revs and becoming narrower—the car increasingly loses
its flexibility.