I've noticed the water pressure from espresso machines varies from about 12 psi to 22 psi. What effect does this have on flavor?
Working on a Synesso (prototype), I was lucky enough to be able to play with controlled pre-infusion. They state that their pre-infusion has a pressure of 3-4 bars and " bring[s] an extra textural element to your brew in the form of a greater volume of crema. It can also lead to a more syrupy mouth-feel overall." I find this to be mostly true if the pre-infusion was used for about 5 seconds.
What I found through personal experience was that, if I kept the pressure consistent, not switching to the full 9 bars, the shot pulled at the same extraction rate, finishing at my desired time. So Keeping dose, time, and extraction the same, the only variable was pressure. I found that the most notable change was a reduction in brightness. The more pronounced acidity and citric flavors were dulled. There was however a silver-lining that gave the pre-infusion a redeeming quality: Because of the reduction in brightness, I found that when using really fresh coffee ( < 4 days old), the pre-infusions reduction in brightness took away from the character metallic taste found with fresh coffee. For that reason, when I am dealing with fresh coffee, I may increase the pre-infusion time (from 5 to 9 seconds for example) and this helps make the espresso palatable.
i have found that pressure will give you a better solubles yield. i mean, the more pressure you apply to your extraction the more solubles you will get in your drink. this doesnt mean its a better extraction cause it also depends on the roasting profile you have, and remember that in espresso, the more solubles you get, the more defects will show up too. i mean, if you use more pressure, you may extraction a more syrupy espresso but also flavor that you may not want. so i go tend to go back to the roasting profile. the darker the roast the less defects you will find but also the less flavour you get. so if you are working on a low pressure machine you may want to use darker roasts to keep flavour of espresso even tho you sacrifice some flavor (still in low pressure extractions you may not get all the flavour you want) you may may find better flavour in lighter roasts with more pressure... still this is a matter of debate cause in brewing you obtain great flavor in light roasts without adding pressure.
i have a reneka 2 group machine that has 8 bars pressure for extraction and also a simonelli appia 2 with 9.1 bars pressure. the same coffee in medium roast tastes more sweet and syrupy in the simonelli, so i had to darken my espresso beans for the reneka and obtained a richer extraction.