We all know about the dangers of high manifold temperatures, and the destructive effects of detonation. It’s the primary reason for the use of an intercooler with most forced induction systems, including superchargers and turbochargers. But there is another benefit to cool air: it is denser, and contains more oxygen than warm air. An increase in the oxygen density of the intake air will allow for the use of more fuel. This not only increases the available horsepower of an engine, but allows for a more complete burn, which results in greater overall efficiency.

There is a general rule among hot rodders that ten degrees of temperature is equal to about 1 percent of horsepower. While that may provide for a quick estimate of the effects of temperature on horsepower, it isn’t a very accurate measure. Part of the reason for this is that the density of air at 90 degrees is lower than the density of air at 60 degrees, so changing the temperature by ten degrees has a different effect on 90 degree air than it does on 60 degree air.

Horsepower change is dependent on the square root of the change in air temperature expressed as “absolute temperature.” On an absolute temperature scale, there is no possibility of a negative measurement because the value of zero represents the lowest temperature possible for any matter. On such a scale, zero is known as “absolute zero.”

When using the Fahrenheit scale, absolute zero is equal to approximately -460 degrees. So in order to convert degrees Fahrenheit to absolute degrees, all that is need is to add 460 to the temperature on the thermometer. 40 degrees Fahrenheit is thus equal to 500 degrees on the absolute scale; 75 degrees Fahrenheit is equal to 535 degrees on the absolute scale, and so on.

In physics terms, the temperature coefficient of horsepower is a negative one. That is, the higher the temperature, the lower the air density and as a result, the lower the horsepower. Because of this, we can’t simply establish a ratio between the beginning and ending temperatures, we will have to use the inverse ratio instead. So, the formula for calculating a change in horsepower as a result of a change in temperature is as follows, where HP = horsepower and T= temperature:

HP2 / HP1 = square root of (T1 / T2)

The horsepower at temperature 2 divided by the horsepower at temperature 1 is equal to the square root of the result of temperature 1 divided by temperature 2.

Example 1

An engine with 200 horsepower at 75 degrees will have how much power at 55 degrees? (Remember to express both temperatures in absolute terms by adding 460)

HP2/200 =square root of (535/515)

HP2/200 =square root of 1.0388

HP2/200 =1.0192

HP2 =1.0192 x 200

HP2 = 203.85

In other words, reducing the intake air temperature by 20 degrees gives this engine an extra 3.85 horses.

Example 2

Same 200 horsepower engine, cooled to 50 degrees.

HP2/200 = square root of (535/510)

HP2/200 = square root of 1.049

HP2/200 = 1.024

HP2 = 1.024 x 200

HP2 = 204.80

Example 3

Same 200 horsepower engine at 95 degrees F.

HP2 /200 = square root of (535/555)

HP2/200 = square root of 0.964

HP2/200 = 0.982

HP2 = 0.982 x 200

HP2 = 196.4

So raising the temperature of the intake air by 20 degrees reduces our horsepower by 3.6.


By Vanessa Wilson