Some debate exists, but baristas and historians all agree that 9 bars are the right amount to make the perfect espresso shot. 9 bars, or psi (pronounced "9x" in technical terms) refers to 9 times sea level atmospheric pressure. You'll focus the pressure of hot water on a very small area (the size your espresso puck) for between 25-30 seconds. It doesn't make sense to use higher pressure, but wait for the water to warm up for 25 seconds to extract properly.