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Boost Cut Strategy for the BMW N54 Engine

CM5 and CM5-LT controllers can be configured to detect various issues with the water methanol injection system and trigger one of the General Purpose Outputs (GPO1 or GPO2) to activate and cause an external boost control system to dump the turbocharger boost to safe levels in order to protect the engine.

These strategies are described HERE.

Usually this trigger is fed into an electronic boost controller (EBC) as described HERE.

However, getting the CM5/CM5-LT to dump the excess boost can also be accomplished in set-ups where the turbocharger waste gates are controller by the factory engine computer (aka the Digital Motor Electronics or DME).

The approach involves inserting a relay in-line with the signal wire between the wastegate solenoid and the DME. The relay contacts that are normally closed (NC) are used, so that when the relay is de-energized and the CM5-LT's GPO is not active (i.e. no WMI trouble is detected) the DME stays connected to the wastegate solenoid as intended.

BMW N54 Boost Cut Diagram

However, when the GPO on the CM5-LT activates the relay, it breaks the connection between the wastegate solenoid and the DME. This causes the wastegate solenoid to close and the turbocharger pressure to drop to the level determined by the mechanical rating of the wastegate spring.

BMW's N54 straight-6 DOHC has two wastegate valves. The PDF drawing below shows a detailed approach to cut boost via both valves. It includes a list of inexpensive, waterproof parts that can be used to implement this approach.

   BMW N54 Boost Cut Diagram

Posted by Torq Byter_ on 03 September, 2016
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