How to Calculate Horsepower

Horsepower is characterized as work done after some time. The correct meaning of one horsepower is 33,000 lb ft/minute. Put another way, if you somehow managed to lift 33,000 pounds one foot over a time of one moment, you would have been working at the rate of one horsepower. For this situation, you’d have exhausted one horsepower moment of vitality.

Difference between horsepower and torque for vehicles

Horsepower

Horsepower is known for speed and is measured at high revolutions per minute (RPM). Horsepower is what makes a vehicle manufacturer determine the tachometer max specifications and also determines the type of tires and suspension is to be used on vehicles. The horsepower sets the limits of how fast an engine can propel a vehicle during the drive cycle.

Torque

Torque is known for strength and is measured at the low-end (grunt) and is determined at low revolutions per minute (RPM). Torque is what makes a vehicle move from a standstill to a full movement of motion. Manufacturers determine what type of differential and transmission to use based on torque. Horsepower will just speed up a transmission; however, torque is what will make the gears come in contact with much strength.

Part 1 of 4: Measuring horsepower on an engine in a vehicle

Materials needed to perform the job

  • Pen and paper
  • Vehicle owner’s manual

Step 1: Get the vehicle's torque numbers. You can find this in the index of the owner’s manual and the book will tell you the torque values.

Step 2: In the owner’s manual, look up the engine’s speed values.

Step 3: Multiply the torque value by the speed values of the engine. You will be using the formula (RPM x T)/5252=HP, where RPM is the engine speed, T is the torque, and 5,252 is radians per second.

  • Example: A 2010 Chevrolet Camaro 5.7 liter produces 528 foot pounds of torque at 2650 RPM. You would calculate 2650 x 528 first. You would get 1,399,200. Take 1,399,200 and divide it into 5252 and you will get the horsepower. You would get 266 horsepower.

If you do not have an owner’s manual and you want to know how much the engine’s horsepower is, then you can check to see what the engine is in the vehicle. You can look at the engine and determine how many cylinders are on the engine based on how many injectors and spark plugs there are.

Then check to see what type of engine is in the vehicle. Look at the door placard, the label on the door jam of the driver door wall. This placard will display the year of the vehicle, loading specifications, and the engine size. If you do not have a door placard, then look at that vehicle identification number. Take the number and do a VIN break down. Once you have the VIN breakdown, you will know what size the engine is.

Take the engine size and times it by the number of cylinders. Then take that number and times it by the outcome of the number of cylinders divided by the size then times by 3 for standard engines or 4 for an engine with a torque package. Then times the answer by pi. This will give you the torque of the engine.

  • Example:

5.7 x 8 = 45.6, 8/5.7 = 0.7125, (0.7125 x 3 = 2.1375 or 0.7125 x 4 = 2.85), 45.6 x 2.1375 x 3.14 = 306 or 45.6 x 2.85 x 3.14 = 408.

The torque is 306 for standard engines and 408 with a torque package. To determine the horsepower, take the vehicle and determine the RPM values.

Automatic transmission

  • Warning: Make sure that the brakes work prior to the test. The vehicle will be at a state of full acceleration and faulty brakes will make the vehicle move.

Step 1: Set the parking brake and start the engine. Step on the service brakes all the way. Put the transmission shifter in drive and step on the accelerator for about 3 to 5 seconds at full throttle.

Step 2: While at full throttle, watch the RPM gauge. Record the readings that the gauge says. For example, the gauge may read 2500 RPM. This is the max that the torque converter can produce for full torque from the engine.

Manual transmission

Step 1: Take the vehicle for a test drive. When shifting, do not use the clutch, but rather rev up the engine until the shifter slides into gear.

**Step 2: As the shifter slides into gear, monitor the RPM gauge and record what is shown.

Once you have the RPM that is for stall check or for slide check, then take your RPM and x by the torque then divided by 5252 and you will get the horsepower.

  • Example:

Stall speed 3350 RPM x 306 standard engine specs = 1,025,100/5252 = 195. For engine with torque package: stall speed of 3350 RPM x 408 = 1, 366, 800/5252 = 260

So the engine could have 195 HP for standard engine package (3 inch bore depth) or 260 HP for a torque package (4 inch bore depth).

Part 2 of 4: Measuring horsepower on an engine on engine stand

Materials needed to perform the job

  • Breaker bar 1/2 drive
  • Depth micrometer or caliper
  • Inside micrometer
  • Micrometer set
  • Pen and paper
  • SAE/metric socket set 1/2 drive
  • Telescoping gauge

If you have an engine on an engine stand and would like to determine how much horsepower the engine is capable of producing, you would need to perform the following steps:

Step 1: Remove the intake and cylinder heads to the engine. Be sure to have a drip pan in case there is any unexpected coolant or oil drainage under the engine.

Step 2: Get an inside micrometer or telescoping gauge. Measure the bore of the cylinder around the top, just under the ring ridge.

  • Note: Ring ridge is where the piston stops and forms a ridge above the piston as the piston rings wear into the bore.

Step 3: After measuring the bore, get a micrometer set and find the micrometer that will fit the dimensions of the tool used. Measure the tool or read the inside micrometer to see what the bore size is. Read the micrometer and write down the measurement. An example would be checking the bore on a 5.7 liter Chevrolet block would be reading around 3.506 on the micrometer.

Step 4: Get a depth micrometer or caliper and check the distance from the locations where the piston stopped at on the top and bottom of the bore. You will need to measure the piston at bottom dead center (BDC) and again at top dead center (TDC). Read the depth gauge and write down the measurements. Subtract the two measurements to get a distance in between.

Now that you have the measurements, you would need to make a formula to determine the proper amount of horsepower the engine is going to produce.

The best formula to use is:

Cylinder size times the depth of the cylinder times the number of cylinders time pie

  • Example:

3.506 X 3 X 8 X 3.14 = 264.21

This example is given for a 5.7 liter Chevrolet engine with a bore of 3.506, a depth of 3 inches, with a total number of 8 cylinders and multiplied by pie (3.14) to give a horsepower of 264.

Now the longer the piston stroke is on an engine, the more torque the engine has and also the more amount of horsepower. With long rods, an engine will rotate the crankshaft very quickly making the engine spin up very quickly. With short rods, an engine will rotate the crankshaft more moderate to slower making the engine spin up in a longer timeframe.

Part 3 of 4: Measuring horsepower on an electric motor for electric vehicles

Materials needed to perform the job

  • Pen and paper
  • Vehicle owner’s manual

Step 1: Locate the vehicle’s owner manual. Go to the index and find the electric motor specifications. If you do not have an owner’s manual, then look for the label plate on the electric motor and write down the specifications.

Step 2: Write down the amps used, voltage used, and guaranteed efficiency. Then use the formula ((V * I * Eff)/746=HP) to determine the electric motors horsepower. V = voltage, I = current or amperage, and Eff = efficiency.

  • Example:

300 x 1000 x 0.80 = 240,000 / 746 = 321.715

The electric engine would produce about 322 horsepower for a continuous time. Diesel and gasoline engines are not continuous and require fluctuating speeds.

Part 4 of 4: If you need help

If you are in need of help determining the specifications of the engine in your vehicle or if you need assistance in how to calculate the horsepower for your engine, then you should seek out help from one of our certified mechanic that can help you with your vehicle.


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