There’s a practical reason: spring-loaded OPVs and early E61 groups stabilized around 9.0 bar under flow, so manufacturers standardized calibration; I still set OPVs with a portafilter gauge to 9.0±0.2 and recheck every 6 months. Anyone seeing a different “sweet spot” when measured with a Scace at about [redacted]/s?
On my E61 I see a sweet spot around 8–8.5 bar for light roasts with a Scace — “lower pressure can tame channeling,” though darker roasts still seem happiest near 9. Try dropping the OPV to about 8.5 while holding your Scace flow and shot time constant, then nudge ±0.2 to confirm. What flow are you targeting on the Scace?
Set 9.5 bar static on gauge to hit about 9 on Scace, @OP; vibe pumps sag under flow — still doing “9.0±0.2” checks?