Stress Test Before the Biturbo Conversion

Before our M57TUTOP conversion enters its final stage, we took our E46 330d back to the track one more time in its current single-turbo configuration.

The engine is still running at around 300 hp and the main goal was to validate the car's foundation under real drift conditions. Before adding significantly more power, the cooling system, steering system, and drivetrain need to prove themselves reliable.

The good news first: we've largely solved the temperature issues that used to trouble us. Engine oil is currently cooled through an external oil thermostat and a 34-row Setrab Series 9 oil cooler. Oil temperatures above 120°C at the engine outlet have become a rarity and are typically only seen after multiple consecutive laps at full load. Coolant temperatures are no longer a major concern either.

The next step will be replacing the viscous fan with an electric fan and a proper shroud. At the same time, the car is already prepared for a future electric water pump and rear-mounted radiator setup.

Another recent addition was the electric power steering pump. For power steering cooling, we're running a 16-row Setrab Series 6 cooler that currently relies solely on vehicle speed and airflow. Steering system temperatures remained completely uneventful throughout the entire event and the cooling performance exceeded our expectations.

Unfortunately, the pump itself suffered an electrical failure after roughly half a day of driving. The issue was not related to cooling but originated within the pump itself. It will now be sent back for inspection and replacement.

We also took the opportunity to test a different final drive ratio. After our 2.81 differential suffered a bearing failure, we temporarily reverted back to a 2.47 ratio. For this event, however, we decided to try something different and installed a 3.23 differential from a BMW X3.

The difference was immediately noticeable. Engine speed could be kept much more consistently within the usable power band and the turbo lag of the single-turbo setup became far less of an issue. Especially during transitions and corner exits, the car felt significantly more responsive compared to the taller gear ratios.

Another new addition was our hood-exit exhaust. Overall, the concept works exactly as intended, but we did discover some vibration-related issues under load. To address this, we'll reinforce the mounting points and add a flex section to reduce stress on the system.

While the car is still running in its single-turbo configuration, work on the M57TUTOP conversion is already well underway. The major fabrication work is complete and we're currently working through the final 5–10% before the biturbo setup fires up in the E46 for the first time.

The test day delivered exactly what we were hoping for: valuable data, a few new tasks for the workshop, and confirmation that the platform is now ready for the next step in performance.