I think there is alot of confusion about EK43S espresso grind settings due to us not defining the espresso parameters. Here is my take on the current confusion.
I currently have an EK43S, SSP red speed burr, high uniformity, Titus latest burr carrier, and sanded chamber to 0.01 mm tolerence.
My espresso or EKpresso taste best around 3-4 on the EK43S dial using light-to medium light coffee, around 20s, 18g in 18g strada basket, out 50-55ml. DE1 flow rate is set to 7ml/s. Pressure at this grind setting builds up minimally at around 2-3 bar max, before dropping to 0 bar. Most of my typical shots, Ext taste best around 22-24%, depending on the roast level, roasting skill, beans. Good quality filter roast with minimal roasting defects (smooth declining ROR), well developed thru out (inner + outer), i am able to "push" for more extreme extractions with out over extractions notes via dialing slightly finer, however total shot time still remains easily below 30 secs, starting with a few drops and quickly increasing in flow rate.
The ek43 is not your typical grinder, meaning its does not conform well to the 1:2 30 secs shot time norm and trying to pull shots by grinding finer while maintaining the typical brew parameters 1:2, 30 secs (short preinfusion, direct ramp up to pressure, mantaining pressure) typically leads to extremely over extracted notes of extreme sourness and bitterness at the same time.
For me personally this dial zone is around 2-3, which i personally term the "dead man's zone" for the Ek43, at this zone, nothing taste good, while shot timing and naked portafilter extraction looks good, everything taste extremely over extracted, the puck in this zone still behaves like the typical fast increasing flow rate, but with a higher peak pressure. I am usually getting 25% extraction at this zone.
Going deeper in to the dial zone, 1 - 2 on the EK43S, the puck starts to behave abnormally, at the zone, the grinds are so so fine that they clump together and resist water under pressure leading to massive micro channeling and under extraction can occur. The grinds and puck at this level acquire super hydration properties, meaning its able to hold and absorb water much more, but with a caveat that which it needs extended time to absorb the water and its does not like to rushed, meaning using methods like blooming, slayer style preinfusion, 1 bar preinfusion maintaining for 30 secs, etc etc, shot times are typically are around the 1 min mark, the barista here is rewarded with this strange looking reddish thin crema and espresso which is extremely high TDS and extraction cup. Not many roast can survive such high level of extraction and still taste good. I typically avoid preparing such espresso because dialing in can be a real pain, wasting tons of good coffee, requiring puck dissection, naked portafilter observations, graphs and everything in between.
From the 0-1 dial zone, i have not had the chance to reach this point yet with any of my coffee i experienced so far, i am guessing its like the black hole for me