Dissolved solids = muddy.
Take the portafilter out of your machine. Start the machine. Read the pressure gauge. Well, it probably says 9 Bar, but there is currently 0 psi at your head. Without resistance, there is no pressure.
The way to measure post puck pressure is to use a spoutless portafilter with a line pressure gauge screwed onto it. The density of the puck will change the dwell time of the pressure rise.
If the coffee is lightly packed, it will easily allow water flow at less than 1 Bar. The machine has 9 Bar "available" but that's not what's happening in the portafilter.
My experiance is that it tastes better when tamped. But...I've tried it and also been pleasantly surprised.
I'm still a tamper fan.
Paul Yates said:James raises another question, and I don't think we've answered it. Why does the untamped puck look so muddy and messy, as compared to the tamped puck, if the pressure placed by the group head is the same? Does tamping regulate bloom? Does it affect how dry the puck is afterwards?
Though an extraction may seem to pull in the same amount of total time no matter the tamp pressure, the first ten seconds or so are noticeably different. I believe this has to do with the variance in pressure between what the pump is putting out, what is between the group and the puck, the atmospheric pressure that the espresso is ultimately flowing into, and the effect of the puck's beginning density has on the relation between the previous three.
Hey while we are at it, lets all use old coffee,
that was roasted months ago ...and move to super autos! Oh wait the Mermaid already does that. I feel better since I let my inner shark boy out!
(snip)...But to elaborate on it, I dont mean channeling in the sense that there are huge fissures or that the puck would largely be lopsided, but more in a sense that without the tamp eliminating the gaps the water flow will change the extraction in the beginning. But by the middle to end of the extraction the same rules for a normal tamped extraction will take place because of the bloom and such...