Here's my situation. I have a Lelit PL41EM (i.e. the one with the pressure gauge). I also own a Vario grinder. I use fresh beans (2-14 days from roasting).

My shots have been on the bitter side, and I think it's related to two challenges with this machine.

1/ Boiler temp is very hot. I've never measured it, but I get a lot of steam coming from the group head when I run water through.
2/ OPV on this machine is set around 11bar, and it not adjustable.

Regarding #1, I think I can work around this with a 2-3 second cooling flush. It's #2 which I am more concerned about, and here is why.

Let's consider the perfect double shot to be somewhere in the neighborhood of 2-2.5 ounces in 25 seconds, with 9 bars of pressure. If I dial in my grind and tamp to get 9 bar (which I read off the built in pressure gauge), I get a shot in under 20 seconds. If I grind finer and/or tamp harder to get that time up to 25 seconds or so, suddenly my pressure also goes up to around 11 bar.

The problem with 11 bar? Well, I think it's making it much harder to avoid channeling. Also, I get bitterness in my shots which I can't seem to work out. Some of this is due to issue #1 above, and the cooling flushing is helping.

So I guess my question is, is it possible to pull a 25 second 2-2.5 ounce shot out of this machine WITHOUT using 11 bar of pressure? Is it my machine that is limited, or is it my technique? I'm hoping it's the latter in which case this thread can be moved to Tips and Techniques:)

Thanks.

(P.S. I did post the same question on CG here, though it's not getting much love there )

h3yn0w wrote:...is it possible to pull a 25 second 2-2.5 ounce shot out of this machine WITHOUT using 11 bar of pressure?

If the brew pressure is indeed 11 bar, then the pulls will be less forgiving of errors in barista technique. You may also be forced to grind more finely and dose more than you would otherwise.

While I've never tried it, I've read about owners of OPV-less single boilers "cheating" by opening the steam wand valve a teenie bit while pulling a shot (e.g., Rancilio Silvia Preinfusion Technique). It's not as repeatable as an over-pressure valve, but it will demonstrate whether lowering the pressure makes for better espresso. To be sure that the difference is indisputable, crack the valve such that the brew pressure drops a lot, e.g., to 7 bar. You'll need to move the grind setting back a couple detents coarser; if you're updosing, you'll also need to reduce the dose a couple grams.

I don't know your machine so feel free to disregard this post but I believe that you should still be able to pull a decent shot.

Longer water flushes should bring the temperature down and probably improve your shots. Channeling may also be contributing bitterness to your shots. 9 bar would be better, but a lot of machines that are factory set to 11 bar still pull good shots. It's not as big a difference as you might imagine. It does make your shots more prone to channeling however and demands that your distribution and tamp are very good. I recommend the nutating tamp method. With a little practice, it is all but immune to channeling. You may also find that manual preinfusion may reduce channeling.

Vibe pump? If your machine has an over pressure valve (OPV), you can usually adjust the pressure. Some OPV's have adjustment screws and others require that you change internal shims or springs. If your machine doesn't have an OPV, then you're back to needing to optimize your distribution & tamp. Or if you are a DIY-er, you could possibly add an OPV to your machine.
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The machine does have an OPV, but it's factory set at 11 bar and loctite is applied to prevent any changes.

I'd consider adding a better/adjustable OPV myself, but would it be machine specific? Or would any OPV work with any machine?

Loctite can be removed with heat. Assuming you have the right plumbing bits, any OPV should work since they're nothing more than a spring pressing against a piston/seal. The main difference between the various OPVs is how touchy they are to adjustment and pressure variation; generally speaking, the smaller ones are more finicky than the larger ones. If you're interested in their inner workings, see Over-Pressure Valve (OPV) / Expansion Valve Basics.