BMW N-series

Just how bad are the four-cylinder petrol engines?

It doesn’t have a glowing reputation in the trade, but what makes the N-series petrol unit tick? Andrew Everett is your guide.

Those of you who read Steven Ward’s Dealer’s Diary column may have seen his diatribe in the December 2017 issue about the N43 engine produced by BMW, used from late 2007 in four-cylinder petrol 1- and 3-Series cars until the replacement N20 and N13 units arrived around five years ago. It has a well-deserved reputation for spelling trouble.

The N43 was the final development of the engine series that began in 2001 with the N42. Back then, the N42 2.0 was a revolutionary unit and was built in a brand-new factory in Hams Hall, not a million miles away from the Rover plant at Longbridge, where it was suggested it could be used in a future range of Rover cars – that didn’t happen, of course.

The spec of the N42 was impressive for late 2001 and it would still be thoroughly modern if launched again today. It included an all-alloy construction with a ladder-type block – a regular alloy block but with the main bearing caps built into a frame for rapid assembly and extra strength – as well as Double Vanos cam timing, 16 valves with Simplex chain-driven camshafts, double cam sensors (one for each cam) and Valvetronic.

The last of these was BMW’S new concept where the lift of the inlet valves – and thus engine speed and load – was variable thanks to an eccentric shaft running the length of the head in between the valves, inlet cam and rockers. A 12V electric motor bolted to the top of the head turned the eccentric shaft via a toothed quadrant to increase or decrease valve lift, to allow the engine to breathe more easily without the restriction of a throttle body. The motor was controlled by a separate ECU that took information from the engine ECU, based on load, revs and a sensor on the front of the Valvetronic shaft. Further refinements over the old iron block/alloy head M44 that preceded it (E36 and Z3) included a plastic inlet manifold and cam cover – this helped keep the weight down, with the new engine weighing 87.5 kilos as opposed to more than 100 kilos for the old unit.

The timing chain set-up was all new and was designed for rapid assembly. This was a cassette-type chain assembly, lowered down into the engine with the head fitted as a complete unit with the chain, the plastic guide rail and pivoting tensioner blade, crank sprocket and the two Vanos units. The N-series crank had no ‘nose’ onto which the crank sprocket might slide because it was all lowered in as one unit and the big crank end bolt tightened to 300Nm. This immense torque meant there was no locating keyway, relying on the torque of the bolt to keep it together.

The Vanos units were not keyed to the camshafts and stretch bolts were fitted. The Vanos units also differed from previous BMW practice – on the pre-2005 six-cylinder engines the complete oil-pressure-fed Vanos unit was fitted to the front on the head and located onto the front of the cams. On the N42, though, separate Vanos units were built into the cam sprockets themselves. They were activated by oil pressure and regulated by the electronic solenoids in the front of the block that also controlled the oil pressure going to the units.

The alternator, power steering pump and water pump all ran off one auxiliary belt, with a spring tensioner fitted to the front of the block. To save space and weight, the water pump was combined with the power steering pump and bolted to the side of the block. Due to the Valvetronic design, there was no inlet manifold vacuum ( just like a diesel) and so a camshaft-driven brake vacuum pump produced vacuum for the brake servo.

The oil pump was in the sump, integral with the balance shaft assembly bolted to the block and driven by a second short chain from the front of the crank. An Ecu-controlled thermostat regulated engine temperature.