The M20 "e2i" conversion is a popular modification for BMW enthusiasts seeking to enhance the performance of the M20B27 engine found in "eta" models by integrating components from the higher-revving M20B25 "i" engine. This conversion combines the torque-rich characteristics of the eta engine with the improved airflow and higher RPM capabilities of the "i" engine.

e2i Forum Threads

• Maytag's M20B27 'e2i' Engine Build Thread (87 528e)

Key Features

• Increased Displacement: Retains the 2693cc displacement of the M20B27.
• Higher Compression: The conversion often results in a compression ratio of approximately 9.4:1 when using the stock eta block and pistons with an "i" cylinder head.
• Power Output: Depending on tuning and supporting modifications, the e2i conversion can achieve power outputs of 170–190 hp and similar torque figures.
• Notable Identifiers: Combines the "eta" bottom end (long-stroke crank) with the "i" head, which features larger intake valves and improved flow.

Required Components

1. M20B25 '885' Cylinder Head: Includes the camshaft and intake valves from the "i" engine for better high-RPM performance.
2. "i" Intake Manifold and Throttle Body: Provides increased airflow compared to the eta components.
3. "i" ECU (Motronic 1.3): Enables proper engine management and fueling for the higher-revving setup.
4. "i" Wiring Harness: Simplifies the installation and integration of the "i" electronics.
5. Additional Items:
   • New timing belt and water pump.
   • "i" injectors for proper fueling.
   • Appropriate gaskets and seals for assembly.

Performance Gains

• Low-End Torque: Retained from the eta engine's long-stroke design.
• Improved Top-End Power: Provided by the "i" cylinder head and intake system, allowing the engine to rev freely to higher RPMs.
• Versatility: The e2i conversion offers a balance of low-end drivability and higher-end performance, making it suitable for street and light track use.

Considerations

• Tuning: Optimizing the ECU and ignition timing is essential to fully realize the performance gains.
• Compatibility: Ensure the "i" head is compatible with the eta block, especially in terms of cooling channels.
• Additional Mods: Complementary upgrades such as a freer-flowing exhaust, a lightweight flywheel, or a performance camshaft can further enhance results.