What are swirl flaps?

Swirl flaps are one of BMW’s greatest design mistakes, causing 1000’s of owners throughout the world either very expensive repairs or even vehicles being written off.

The flaps live in the inlet manifold of BMW diesel M47TU & M57TU engines (either 4 or 6 flaps depending on the amount of cylinders).

These values are designed to create a swirl (hence the name) in the inlet manifold, this aids towards better combustion under light load. The flaps are fully open during higher load conditions to maximise volumetric efficiency and power output. In reality this system looks better on paper than it does in the real world.

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The above picture is of a standard BMW swirl flap removed for a inlet manifold.

Sadly swirl flaps are poor in design and are prone to failure. The below picture is of a used swirl flap removed from a vehicle with 80,000 miles on the clock, as you can see the shaft is fractured (where the closest bolt meets the base).

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When a swirl flap fails, the debris drops down into the engine, either sticking in the valve port and jamming it open, causing the piston to collide with the valve, or falls into the cylinder itself causing the piston, valves and head to become damaged.

The other issue we regularly see with these swirl flap units is when the small screws holding the flap to the shaft work loose and find their way into the combustion chamber causing considerable damage. They cause further damage on their way out of the engine when passing through the turbo and CAT.

Below is a picture of a new piston and swirl flap (on the left) with a damaged piston and swirl flap (on the right). When a component collides with a piston at 3000 rpm you can see the damage is quite extensive.

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BMW introduced a modified inlet manifold (with swirl flaps still fitted) trying to address this problem, the spindle of the valve is thicker and stronger, sadly this design is also prone to failure. BMW finally gave up on swirl flaps in late 2008.

If you are unlucky enough for your vehicle to suffer from swirl flap failure BMW only offer one option. A new engine (circa £8,500 to £13.500 fitted).

At O.G.S. Mechanics , we can repair swirl flap damaged engines for around the £2,750 mark (depending on the extent of the damage).

But… There is a solution! As they say, prevention is better than cure.

We always look to solve a problem and prevent it from ever happening rather than picking up the pieces after the inevitable happens.

We have had swirl flap blanks custom engineered and developed for us from aircraft grade aluminum with viton seals (modification plate pictured below). We recommend this modification on any engine over 60,000 miles or 4 years to avoid you as a customer, becoming one of the unlucky numbers.

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How we do it in O.G.S. Mechanics.

  • We remove your inlet manifold which is then steam cleaned.
  • The EGR valve is then cleaned.
  • The badly designed swirl flaps are removed and replaced with our billet modified blanks.
  • The vacuum rail (that drives the swirl flaps) is then removed and blanked off.
  • We refit the manifold using brand new inlet manifold gaskets (these are often over looked elsewhere).
  • Remove and replace engine block breather.
  • The car will function the same without the flaps, the flaps are there for emissions purposes, however the car will remain within guidelines and you will not notice a performance change!EGR valve removed, ready to be cleaned – Pictured below.

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When the inlet manifold is removed, this is what we are quite often left with. The carbon build up really restricts airflow through the inlet manifold.

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The top breather was removed from a car with 60,000 miles with a new unit pictured below. This sums up how important it is to change this unit as the old breather has quite literally swelled up due to being so contaminated with oil.

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Finally, Inlet manifold gaskets as replaced during reassembly – Pictured below.

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