§ How it works

Torque is the twisting force at the crankshaft. Power is how quickly that torque does work. They are not two separate things an engine makes: power is simply torque multiplied by rotational speed.

P = T \cdot \omega \qquad \omega = \frac{2\pi\, n}{60}

With $T$ in newton-metres and $n$ in revolutions per minute, $P$ comes out in watts. Folding the constants together gives the two workshop versions:

P_{\text{kW}} = \frac{T_{\text{N·m}} \cdot n}{9549} \qquad\qquad P_{\text{hp}} = \frac{T_{\text{lb·ft}} \cdot n}{5252}

Why the curves cross at 5252

One horsepower is 33,000 ft·lb per minute, and one revolution is $2\pi$ radians, so the constant is $33000 / 2\pi \approx 5252$ . At exactly 5252 rpm the fraction $n/5252$ equals one, which means horsepower and pound-feet are numerically equal. Switch the sidebar to Imperial to see the crossover line on the chart.

The curve on this sheet

The torque curve is a simple parabola centred on the peak-torque speed, with $k$ as the fall-off setting:

T(n) = T_{\text{pk}} \left[ 1 - k \left( \frac{n - n_{\text{pk}}}{n_{\text{red}}} \right)^{2} \right]

Set fall-off to 0 and the torque line goes flat. The sheet then works as a plain converter: enter a torque, pick an RPM, read the power.

§ How it works

Torque is the twisting force at the crankshaft. Power is how quickly that torque does work. They are not two separate things an engine makes: power is simply torque multiplied by rotational speed.

P = T \cdot \omega \qquad \omega = \frac{2\pi\, n}{60}

With $T$ in newton-metres and $n$ in revolutions per minute, $P$ comes out in watts. Folding the constants together gives the two workshop versions:

P_{\text{kW}} = \frac{T_{\text{N·m}} \cdot n}{9549} \qquad\qquad P_{\text{hp}} = \frac{T_{\text{lb·ft}} \cdot n}{5252}

Why the curves cross at 5252

The curve on this sheet

The torque curve is a simple parabola centred on the peak-torque speed, with $k$ as the fall-off setting:

T(n) = T_{\text{pk}} \left[ 1 - k \left( \frac{n - n_{\text{pk}}}{n_{\text{red}}} \right)^{2} \right]

Set fall-off to 0 and the torque line goes flat. The sheet then works as a plain converter: enter a torque, pick an RPM, read the power.

Horsepower & Torque

Inputsdrag · tap · type

Fig. 01Modelled dyno sheet

§ How it works

Why the curves cross at 5252

The curve on this sheet

Horsepower & Torque

Inputsdrag · tap · type

Fig. 01Modelled dyno sheet

§ How it works

Why the curves cross at 5252

The curve on this sheet