Understanding BMEP
Brake Mean Effective Pressure (BMEP) is a fundamental metric in engine performance analysis. It represents the average pressure exerted on the piston during the power stroke and provides a size-independent measure of engine efficiency.
Formula
Imperial Units:
$$\mathrm{BMEP\ (psi)} = \frac{150.8 \times \mathrm{Torque\ (lb{\text -}ft)}}{\mathrm{Displacement\ (in}^3)}$$
Metric Units:
$$\mathrm{BMEP\ (Pa)} = \frac{1{,}508{,}000 \times \mathrm{Torque\ (Nm)}}{\mathrm{Displacement\ (cc)}}$$
Note: The metric formula can also be expressed as BMEP (kPa) = (16 × Torque in Nm) / Displacement in Liters.
Example Calculation
Imperial Example:
An engine produces 40 lb-ft of torque with a displacement of 20 in³:
$$\mathrm{BMEP} = \frac{150.8 \times 40}{20} = \frac{6{,}032}{20} = 301.6\ \mathrm{psi}$$
Metric Example:
An engine produces 54.2 Nm of torque with a displacement of 327.7 cc:
$$\mathrm{BMEP} = \frac{1{,}508{,}000 \times 54.2}{327.7} = \frac{81{,}733{,}600}{327.7} \approx 249{,}430\ \mathrm{Pa}\ (\mathrm{or}\ 249.4\ \mathrm{kPa})$$
Applications
BMEP is valuable for:
- Comparing engines of different sizes on an equal basis
- Assessing engine efficiency and performance potential
- Evaluating the effectiveness of forced induction systems
- Determining engine stress levels and durability expectations
- Optimizing engine tuning and calibration
Higher BMEP values indicate more efficient engines that extract more work from each combustion cycle.