I bought a new espresso machine with the main intention to hack it and modify various parts while enjoying really good coffee along the way. This machine is a DeLonghi EC155 and cost $90 at Fry's Electronics with $20 mail-in-rebate. I normally don't buy anything that includes a rebate, but I made an exception in this case since $90 is already a good price for this machine.
The EC155 has a stainless steel boiler, "15-bar" pump, large water reservoir, separate baskets for single and double shots, and a pressurized portafilter. Espresso purists will tell you that a pressurized portafilter is just a kludge that is meant to help newbies get the right extraction time despite improper grind size or tamping, but I might disagree -- more on that later. In any case, this machine is a really great value, and a HUGE upgrade from steam-powered machines.
I previously hacked a steam-powered machine and added an air pump and temperature regulator to have more control over the brew process: http://benkrasnow.blogspot.com/2009/12/using-my-hacked-espresso-machine.html. The machine worked pretty well, but it was time to retire it because of a few nagging problems including a leaky portafilter, overall junky basket design, and other very difficult-to-fix items.
The DeLonghi EC155 with added pressure gauge.
I could not believe the unit would ever reach its stated "15-bar" pressure, and I was also curious what the brew pressure really was for an average shot. I started by removing all of the machine's guts from the case. It's built surprisingly well. The grouphead is made of very thick cast metal (probably an aluminum alloy), and the boiler is all-stainless with brass fittings and silicone seals.The pump is a ULKA brand pump. There is no obvious model number, though.
The boiler contains a large heating element and two separate thermal switches -- one for espresso, the other for steam generation. The boiler empties into the grouphead via the brass tube sticking up through the boiler's bottom. This way, the water level must rise above the brass tube's top for water to enter the grouphead. When the machine is off, there is no chance of water leaking out.
The brass tube also contains a spring-loaded valve that keeps the grouphead closed until the pressure in the boiler reaches 75 psi (5 bar). I suspect this is to prevent the grouphead from leaking while using the steam wand.
I removed the original 4mm plastic tube that ran from the pressure regulator to the boiler and replaced it with 1/8" PFA tubing. I added a tee in the line and attached the pressure gauge with more pipe fittings and 1/8" compression adapter. The machine will reach 13 bar when the portafilter is completely blocked (no flow), but is usually 7-8 bar during a double shot. Thus, the pressure regulator doesn't do anything in normal operation since the pump cannot maintain more than 8 bar during an average brew flow rate. I may be hacking this part in the future to have more control over brew pressure. However, I think the espresso experts agree that the brew process should have constant flow, not constant pressure.
The machine is designed to use a pressurized portafilter, and produces LOTS of crema with the pressure valve in place. I removed the valve and tried a few shots, but they did not taste as good or have enough crema, and it was very difficult to make the shot last more than 15 seconds. I used the finest grind that I could muster and tamped very hard, but the shot was still underextracted. I suspect that the machine's designers didn't worry about the portafilter geometry, or depth of the coffee puck since they knew the valve would regulate the brew pressure and extraction time. It may also be true that the machine's pump flow rate is not high enough to produce enough pressure without the portafilter valve. I did some tests:
10.4 seconds per ounce at 7.5 bar (170 ml/min)
9.1 seconds per ounce at 5 bar (192 ml/min)
3.3 seconds per ounce at 0 bar (530 ml/min)
I am guessing that the pump is specifically sized so that the brew pressure will be about 7.5 bar and a double shot will take exactly 25 seconds.