The Mystery Of The E61 Group Mushroom

What the top part of the E61 group and the Mushroom really does

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In the article about servicing your E61 group, I briefly described the flow of brew water through the group, but mainly the lower part and within the cam chamber. I think it helps to clarify the flow within the upper part of the group and what the "Mushroom actually does.

The red dashed line describes the part of the group detailed in the diagrams below and the red arrow points to the "E61 group mushroom" on the picture of the internal components. (click to enlarge)

partofgroup.JPG mushroom3.JPG

The diagram below shows the group when the machine is pulling a shot and as you can clearly see the thermosyphon is moving the water all round that top chamber, keeping the rest of the group toasty warm via conduction. The brass plunger is held firmly closed by the spring, preventing any water flowing into the cam chamber and on to the group or shower screen. Simple convection currents allow the "E61 Thermosyphon" to work, as mentioned previously, this thermosyphon is either fed from the top and bottom of a single boiler, or the top and bottom portions of a heat exchanger (HX unit).

The red arrows show the hot water paths (click to enlarge)


Now we move the lever up to "pull a shot", the pump switches on and pressurises both inlets to the group. What was the thermosyphon, now becomes pressurised with 9 bar (140 psi)….we're going to make coffee. This is where the mushroom comes into it's own and it now performs a number of tasks.

  1. The O-ring prevents water bypassing the other parts of the mushroom and simply flowing rapidly into the cam chamber.
  2. Water is forced to travel through small channels in the top portion of the mushroom and through the gauze screen
  3. The "filtered" water is now passed through a small orifice or "jet" within the mushroom, typically this is 0.8 or 1.0mm diameter (it can sometimes get scaled up)
  4. The water now passes downwards through the centre of the mushroom and through the brass plunger valve (now open)

The brass plunger valve moves in the direction indicated by the green arrow and it's triangular or square section lower part, holds it central whilst still allowing hot brew water to flow past. the rest of the waters path is explained in the article E61 Group Servicing

The red arrows show the hot water paths (click to enlarge)