Inlet manifold - W124 E300D there are two butterfly valves, what do they do?

[maxypriest]

On the inlet manifold on my W124 E300D there are two butterfly valves directing the air between chambers. These are vacuum powered. I cleaned them up and got them operating correctly again.

What do they do?!?

Also it’s very sooty in the inlet manifold – I take it’s as a result of the re-circulation system from the exhaust gasses. Is this system to reduce the emissions?

 

[SilverSaloon]

Also it’s very sooty in the inlet manifold – I take it’s as a result of the re-circulation system from the exhaust gasses. Is this system to reduce the emissions?

i believe so. I've blocked mine off

 

[Number_Cruncher]

>>i believe so. I've blocked mine off

Not a shrewd move.

They are resonance flaps which alter the effective inlet manifold length as the engine speed varies, maximising the amount of air the engine draws in. They are a "Good Thing", and shouldn't be blocked off.

At low engine speeds, the inlet manifold pulses are at low frequency, and, to obtain a ram air effect, the manifold needs to be as long as possible, so, the valves are both closed.

At intermediate speeds, the manifold needs to be mid-sized, and so, the valve in the crossover pipe opens.

At high engine speeds, a short manifold is best, and the valve in the main plenum opens.

They aren't emissions strnagling devices, this is serious performance engine technology, as fitted to many modern highly specified 6 cylinder engines, and is an important part of making the best of these multivalve engines.

It's worth checking that the valves do actually work like this, it's not unknown for them to be wired/plumbed in incorrectly.

The EGR valve on the other hand.....