Most of the new cars’ engines come with an HP rating, and the reseller also provides the information with their used one.
But can you believe them blindly? Is there a formula to determine the actual horsepower of an engine? How to calculate horsepower from engine displacement?
It’s a bit tricky to determine engine horsepower from displacement as you need some data. Plus, the formula isn’t universally accepted but still gives a way to make the solution.
In the following, we have broadly revealed the formula along with the easiest one to help you be aware of an engine’s power and performance.
How to calculate horsepower from the engine displacement?
A common rule is that 15 CC is equal to 1 HP. Calculate the engine’s capacity in cubic centimeters first, and then you will get the value of horsepower. For a 750CC engine, the horsepower will be = 750/15 = 50 HP.
What are horsepower and engine displacement?
To determine your car’s horsepower, you should have a clear idea of both the horsepower and the engine displacement.
When power needs to move 550 pounds across one foot per second or 33,000 pounds per minute, it’s defined as horsepower.
Conversely, engine displacement is the cylinder volume that is covered by pistons in a single revolution.
How to calculate engine horsepower from a displacement- Brief Discussion
A general rule of thumb is that you will get 1 HP from 15 CC. unfortunately; there is no formula to measure horsepower from displacement.
In fact, engine displacement is one of the determining factors of an engine’s torque and horsepower. Other factors are the ignition system, forced induction, fuel delivery, and the valve train layout.
A bigger engine offers more powerful performance. It means a higher displacement engine delivers more power.
The bigger engine has more space to hold a large volume of fuel. It also accommodates a larger amount of air inside the combustion chamber to burn to produce more power.
The formula to calculate horsepower
Let’s start with the easiest way.
The formula is horsepower = torque x RPM/5,252.
Torque is a force that is related to the driveshaft and expressed in pound-feet or Newton meters.
RPM means revolutions per minute. It’s the number of times an engine’s crankshaft makes a complete rotation every minute while the piston goes up and down in the cylinder.
5,252, on the other hand, is a radian per second. It’s the change in the engine’s orientation.
The constants in the formula are:
- Torque measure in ft-lbf
- Speed express in RPM
- 63,025 is also constant especially if the torque is measured in lbf-in
Let’s calculate the horsepower of an engine
Suppose your engine has
- Speed = 600 RPM
- Torque = 150 pound-foot
The horsepower of your engine will be
Horsepower = Torque x RPM/5,252
= (120 x 600) / 5,252
= 13.7 HP
To calculate horsepower from engine displacement, you have to first determine the engine’s displacement.
How to calculate the displacement of an engine?
The formula for calculating engine displacement is= the area of the engine cylinder bore X the stroke of the crankshaft X the number of cylinders.
The result you’ll get will be the volume of air that your engine can display.
To determine the engine cylinder bore’s area, you have to measure the diameter of one cylinder. And you will get a radius that you can use in the calculation.
The stroke of the crankshaft stands for the distance that pistons travel.
In most cases, the volume of air displaced by the engine will remain the same unless the volume of air inside the cylinder changes during the piston’s movement up and down.
You can measure displacement in cubic centimeters (cc), cubic inches (CI), or liters (L).
The formula is:
Engine displacement = π x (Radius) 2 x Stroke x Number of cylinders
Let’s calculate the engine displacement in cc so you can convert it into HP.
Suppose your engine has
- Bore = 90 mm
- Stroke = 100 mm
- Number of cylinders = 1
- Radius = Diameter/2 = Bore/2,
- So, Radius=90/2= 45
Displacement = π x (radius) 2 x stroke x number of cylinders
= 3.14 x (45)2 x 100 x 1
= 635850 mm3
Convert to cubic centimeters or CC:
1000 mm3 = 1 cm3,
= 635850 / 1,000
= 635.85 cm3 or 635.85 cc
Converting in HP:
15 CC= 1HP
635.85/15 = 42.39 HP
How to find displacement from horsepower?
A general rule is that you will get 1 horsepower for every 15 CC. The engine displacement is calculated in cubic centimeters (CC). And you already have the formula to measure the engine displacement.
But the truth is that there is no exact formula to convert engine horsepower into displacement. So, if you want to figure out the amount of HP, determine your regular driving requirements.
A small engine is perfect for driving in the city, and it offers improved fuel economy. This engine isn’t too expensive to maintain either. But if you require more speed and grunt, consider a bigger engine. Remember, it will be costly to maintain.
Is displacement the same as horsepower?
You have to calculate the volume of every chamber to determine displacement. Conversely, it does require two numbers to calculate horsepower. They are torque and 5252 radians.
How does displacement affect the performance of the engine?
Engine displacement determines the torque and horsepower that an engine delivers. It also helps figure out the amount of fuel an engine consumes. A general rule of thumb is that the higher the engine displacement, the more powerful the engine is. Conversely, lower displacement means lower fuel consumption.
By now, you know how to calculate horsepower from engine displacement. It’s a slightly trickier process than the toque/RPM method. It is still helpful to let you realize the power or performance of your car’s engine. So, whether you’re looking to buy a new or used engine, this formula will help you get what you pay for.