9 bar isn't a law of nature. It's an industry standard Achille Gaggia established in 1947 with his lever machine.
The history
Before Gaggia, espresso machines brewed at 1-2 bar. Gaggia's spring lever generated ca. 8-10 bar — revolutionary: crema, depth of flavor, consistency.
What 9 bar does to coffee
At 9 bar, hot water pushes through the puck and extracts the right balance of acids, sweetness, and bitterness in 25-30 seconds. More pressure — over-extracted. Less — watery.
Why Slayer runs at 6 bar
Modern pressure-profiling machines like Slayer or Decent vary pressure during the brew. Pre-infusion at 2 bar, peak at 6 bar, slow decline. This opens new taste worlds — but 9 bar remains the industry baseline.
How we check it in the workshop
Every machine leaving us is calibrated to 9 bar (±0.2). We measure with a calibrated gauge at the group outlet, not the pump.