I assume this answer will be different for vibratory vs. rotary pumps. I find my vibratory pump takes 7-8 seconds to reach full pressure, it reaches 9.1 bar for a few seconds then slowly begins declining before finishing at 8.7 bar. (roughly 35 seconds total shot time)

Should I increase my pressure so that the average of the first and last number is 9 bar?

Oh really, so when using the lower pressure how do you adjust the brew time and/or ratio?

If you're shooting for lower extraction pressures, you would typically turn down the OPV. Then you tune with grind and dose pretty much the same way you would if you were running a nine-bar profile. Despite the Italian declaration that only nine bars is acceptable, many of the lever machines have been running at lower pressure for decades. At lower pressures, you can either choose to target traditional flow rates or faster flow rates where the entire shot might complete in under 20 seconds. The ratio will depend a lot on the beans, your grinder, and your preferences in the cup.

Also worth mentioning: I am not sure what machine you have, but, if your pressure is measured at the pump, it might not be exactly the same pressure seen at the portafilter. In many machines people will use a pressure gauge at the portafilter to calibrate the pump pressure. On my S1, the pump reads about 1 bar higher than the portafilter.

wordsum wrote:Oh really, so when using the lower pressure how do you adjust the brew time and/or ratio?

As Jeff said, adjust your PEAK pressure using a blind filter and the OPV adjustment screw. This will only limit the MAXIMUM pressure the shot will reach depending on your grind. If you grind too coarsely you won't hit that peak pressure. If you grind very fine then the shot will stay at that peak pressure for longer.

If the goal is tasty coffee then I would suggest starting by setting the OPV to 8 or 8.5 bar and then adjusting your grind so you get your output in that 25-35 second window. Once you're there the rest is done by taste. Too bitter, loosen up the grind and let the shot run faster. To bright/acidic/sour, tighten the grind to have it run longer.

Depending on your machine there are other more advanced things you can do if you have flow control, pressure control or a lever machine but those are the basics.

Vibe pump flow (and consequently, pressure) can be controlled using a lamp dimmer. It's a $10 modification that provides enormous flexibility. You then set the OPV to the max pressure you'll ever want to use, and never have to touch it again.