Cars are typically defined by the amount of horsepower they have. However, there are often two types of horsepower numbers tossed around in discussions - engine horsepower and wheel horsepower.
The two are measured on a dynamometer or dyno as it’s commonly referred to in the industry. We like to call the dyno in our Houston-area shop “The Heartbreaker” because sometimes people will come in expecting the engine horsepower number when what they really get is the wheel horsepower number.
Engine horsepower is the amount of power produced by the motor measured at it’s flywheel. This is sometimes referred to as brake horsepower or power “at the crank” meaning the crankshaft of the engine. This horsepower number is what car manufacturers will advertise that a vehicle has.
Wheel horsepower is the amount of power measured at the wheels of the vehicle. This is also called “xxx horsepower to the ground” as it is the amount of power made by the vehicle as a whole and not just the engine. The wheel horsepower number is usually lower than the engine horsepower number by about 20%-45%. This is because the power made by the engine is influenced by the transmission, drivetrain, vehicle weight, tire and wheel size, gear, and other external variables such as weather, altitude and what type of dyno the vehicle is on. In other words, WHP is the amount of horsepower that a vehicle actually makes on the ground.
A 5.0L Coyote Mustang versus a lifted 4WD F-150 with the same 5.0L Coyote engine. Naturally, the truck is going to be heavier than the car, but because the truck is lifted and has larger wheels and tires it’s substantially heavier. All of these variables play a factor into the WHP number. As you probably guessed, the Mustang would have a higher WHP number than the truck due to less weight even though the two vehicles have the same engine.
There’s factors outside of the vehicle itself that also affect horsepower. Humidity, altitude, and temperature all have an effect on engine performance.
At the most basic level, humidity is water vapor suspended in the air. This water vapor displaces the amount of air molecules present. Engines need air and therefore oxygen to create the internal combustion to create power. The same goes for altitude. A racecar in Houston will perform differently than a racecar in Colorado because of the difference in air density due to different elevations. When there’s less air to feed the motor there’s less power to be made.
Cool air is dense air which means that there’s less space for water vapor. Less water vapor means more air in the atmosphere which leads to more power. This is why cool weather is often referred to as “boost weather”. Cool days make for great track days!On the other hand, hot air is heavy and usually consists of more water vapor. This is why a vehicle’s performance declines in the heat.
Now you know the difference between engine horsepower and wheel horsepower (whp) and the various factors that can effect it.