The Difference Between Rear Wheel And Flywheel Power

When it comes to optimizing the performance of your BMW through tuning, understanding the difference between rear wheel and flywheel power is crucial. In the world of automotive performance, horsepower numbers are often thrown around, but how are these numbers measured, and what do they really mean? Let’s dive into the intricacies of rear wheel and flywheel power to shed light on this important aspect of BMW tuning.

Flywheel Horsepower (FWHP) vs. Rear Wheel Horsepower (RWHP)

Car manufacturers typically measure power at the flywheel, which is the most accurate way to gauge the power an engine produces. This measurement, known as Flywheel Horsepower (FWHP), is obtained with the engine removed from the vehicle. However, this process is both expensive and time-consuming, involving the creation of a complete exhaust system for the dyno room, programming a stand-alone Engine Control Unit (ECU), and custom heat exchangers to simulate real-world driving conditions.

Chassis Dyno: A Practical Approach for Aftermarket Tuning

It is very expensive and time-consuming to remove the engine and dyno it. You must:

• Make a complete exhaust that will fit into the dyno room.
• Make/program a stand-alone Engine Control Unit since the OEM ECU will not work without the other vehicle hardware attached to it.
• Custom heat exchangers to match the temperature you would normally see with the engine inside the car on the road. 

Given the challenges and costs associated with flywheel power measurement, aftermarket tuners often rely on chassis dynamometers for tuning and development. This involves driving the car with the engine installed on a rolling dyno. However, measuring power on a chassis dyno introduces drivetrain losses due to friction in various components such as the gearbox, differentials, driveshafts, and half shafts, as well as tire slippage and roller resistance.

Real-World Considerations and Drivetrain Losses

When measuring power on a chassis dyno you have drivetrain losses from friction in the:

• Gearbox
• Differentials
• Driveshafts 
• Half shafts
• Slippage from the tires
• The dyno rollers themselves (brand and user settings make a difference!)

Drivetrain losses can vary depending on the type of dyno used and the configuration of the vehicle. Axle-based dynos tend to be more accurate, with drivetrain losses typically around 12% for two-wheel drive cars and 18% for all-wheel drive cars. In contrast, roller dynos may show varying levels of drivetrain loss, making it essential to measure results on the same dyno to avoid discrepancies.

Understanding Power Figures: RWHP vs. FWHP

While auto manufacturers report horsepower and torque numbers in a flywheel format, aftermarket tuners may provide rear wheel power numbers (RWHP) from a dyno or flywheel horsepower numbers (FWHP) for marketing purposes. It’s crucial to compare apples to apples and consider factors such as drivetrain losses and real-world conditions when evaluating performance upgrades.

Conclusion: Making Informed Decisions in BMW Tuning

In the realm of BMW tuning, understanding the difference between rear wheel and flywheel power is essential for making informed decisions about modifications. By grasping the nuances of power measurement methods and drivetrain losses, enthusiasts can navigate the world of aftermarket tuning with confidence, ensuring that they achieve the desired performance gains without falling prey to marketing gimmicks. So, whether you’re looking to unleash the full potential of your BMW or simply seeking to optimize its performance, remember to read the fine print, compare apples to apples, and choose wisely when it comes to tuning.