Crank vs. Wheel Horse-Power
Due to the increasing amount of customers reaching out to us with questions regarding the different ways power outputs on cars are measured. For that reason, we decided to create this section to bring some clarification to this topic. If there are still questions, please feel free to email us at This email address is being protected from spambots. You need JavaScript enabled to view it..
Crank Horse-Power
First, we would like to explain power ‘measured at the crank’, also called ‘measured at the engine‘ or ‘engine horse-power’. This method is mostly used by vehicle manufacturers. To measure engine horse-power, the engine is connected to an engine dyno before being installed in a vehicle. It typically will have the intake and exhaust systems attached as well as the engine accessories. Manufacturers use this method for multiple reasons. For one, it gives them the possibility to measure the power output of an engine and apply the result for multiple models, so that all cars equipped with this engine can have the same rated power. Another reason is that it provides a cost effective way for them to tune the engine. They can also run durability and load tests that will isolate any potential issues to the engine. If this were the only method available to measure horse-power, the cost would be too high for most consumers due to the need to remove the engine from the vehicle. Therefore the aftermarket industry measures the power at the wheels.
Wheel Horse-Power
The second, and most commonly used, method in the automotive aftermarket industry, is measuring an engine’s power output at the wheels. Here the power is measured while the entire powertrain is installed in the vehicle. The vehicle is attached to a dynamometer and the power is measured with the car applying power to rollers. One major advantage of measuring wheel horse-power is that it allows tuners to look at the affects that increasing power and torque will have on the whole powertrain rather than just the engine. The power measured at the wheels is lower compared to the power measured at the crank, due to the fact that the engine has to rotate everything between the crankshaft and the rollers. This difference between the horse-power measured at the wheels and at the crank is called the ‘drivetrain loss’. Wheel horse-power shows the power that arrives at the wheels and is most commonly used in the automotive aftermarket industry.
Why publish both?
As most other companies in the industry, RENNtech had only published their performance numbers measured at the wheels. Especially since that is power numbers received during the measurement on their in-house dyno. However, since Mercedes-Benz and AMG only ever published power numbers measured at the crank/engine, customers wanted to compare their upgrades to the stock performance published by the manufacturer. There have also been differences between what Mercedes-Benz and AMG rate their engine and what RENNtech has measured in the real world. Often the vehicles make more power than Mercedes claims. RENNtech then decided to publish both the power at the wheels and at the crank, with the goal to be transparent and giving customers the ability to compare true power gains.
Drivetrain Loss
The drivetrain loss is the power lost between the engine and the wheels. The drivetrain includes parts such as the transmission, torque converter, driveshaft, differential, axle, etc. Different vehicles have dramatically different components used to get the power to the ground. For example, one car with a particular engine may get a different transmission halfway through the model production run. Therefore, the drivetrain loss can vary dramatically between cars. Sport cars with rear-wheel drive have a more efficient drivetrain than all-wheel drive off-road vehicles. Based on our experience through our 28+ years in the industry and measurements from a huge variety of cars we have developed a system to calculate crank horse-power numbers based on a known drivetrain loss.
- 12% - Rear-Wheel Drive Sportscars
- 14% - Rear-Wheel Drive Cars
- 14% - All-Wheel Drive Cars equipped with 4MATIC+
- 17% - All-Wheel Drive Cars equipped with 4MATIC
- 18% - SUV’s equipped with 4MATIC
- 20% - Off-Road vehicles equipped with 4MATIC
Gains at the Peak vs. Peak Gains
There are two methods to calculate power gains achieved though performance upgrades. Most commonly used are the gains at the peak. Here the gains are calculated based on the difference between the peak performance stock and the peak performance with upgrades, independent from the rpm they are reached at. The peak gains are the maximum difference between the stock and the upgraded power at any point on the power curve. For example, a bad tuner may make 30hp over stock low in the RPM range, but lose 13hp from the maximum stock power. They may claim peak gains of 30hp, but actually make less maximum horse-power than stock! In another effort for more transparency, RENNtech always calculates power gains based on the difference between peak performance stock and peak performance with upgrades.
Average vs. Peak Power
In recent years Mercedes-Benz started to release their performance numbers based on an average during a rpm range, instead of the peak performance on a specific rpm value. This shows that an engine sustains an average power output during a range of rpm, which can indicate better perfromance if it extends over a wider range. This, however, leads to different performance numbers when only peak numbers are measured on the dyno. To help customers compare their performance with RENNtech upgrades to their factory performance, RENNtech calculates the average power output during the same rpm range to make power numbers compareable and keep transparency.