There has been recent references to varying the pressure on the lever to achieve the desired shot time. I am not sure if this is what we should be aiming for. The grind dictates how much flow there is and if feels that the lever wants to go or it feels comfortable at a certain pressure. Applying more pressure does no doubt increase the flow somewhat but not in proportion to the pressure applied. It is like a displacement hull on a boat. It will only go so fast through the water no matter how much horsepower the engine delivers. To achieve a certain shot time but just varying lever pressure does not, in my opinion, achieve the optimum parameters of making good espresso.

I'm interested, I just received my Pavoni on Monday and I have pulled 3 or 4 dozen shots so far. I have shot for the 25-30 second shots. To do that I definitely have to maintain a constant speed, and that involves more pressure at the beginning of the stroke until the puck is saturated, then the rest of the shot is typically much easier.

To get the cool single, and narrow, stream coming from the center of the filter basket I have to have a slow steady pull. If I speak up the pull to maintain a harder pressure then the espresso stream creates a bubble like stream.

So what is most important to good espresso? High pressure? Longer extraction time? My guess is both would be optimal, but I have yet figured out a way to do both with the lever.

Something to understand is that when I aim for a certain rate of flow with my lever, it isn't to make up for a wildly off grind. It's for a grind that might otherwise be just slightly off. With the control over the flow, the shot comes out better than it would have. And I can still adjust grind for the next one.

But even some pretty large differences, like getting a new bean, can still be made drinkable, and even enjoyable, instead of in the sink when you can be sure the flow rate is kept where it should be. Again, for the best shots I'll still change the grind after, but it's one less wasted shot in the name of dialing.

In a shop, you can pull a couple shots to check timing every half hour and just roll it in to the prices, it'd be incredibly wasteful at home to do the same.

Hers an opinion and explanation on why decreasing pressure produces a more evenly extracted shot.

http://www.jimseven.com/2011/02/08/cont ... profiling/

Basically it is a faster flow or slower flow but the same volume is going past the puck. Maybe a blind taste test to settle the matter.

Varying the lever pressure alone (at the later part of the brew) does not affect the flow rate much. To actually manipulate the flow rate via pressure (aka pressure profiling), you need the other puzzle pieces. Namely the preinfusion length, pressure ramp up rate, and max brew pressure. To do any of these repeatably and delibrately, a pressure gauge is key and it is usually not feasible on lever machines without a gauge.

In my experience, the importance of flow rate & pressure is a 80:20 relationship. Get the flow rate right and everything else wrong, you can still get a 80% good cup. Get the pressure wrong, you'll only affect the shot by 20%(provided you're at 4 bar+). This is a gross simplification but the main point is to describe the importance of pressure vs flow rate.

Although grind setting determines the major part of the flow rate outcome, the pressure profile/strategy you apply can transform a choked shot to a gusher.

Manual lever can manipulate the flow rate to a certain extent as mentioned, but the most useful part is in a gusher scenario where you can delibrately reduce the flow rate to a proper one. Getting the flow rate right itself can salvage the shot from being a totally unpalatable drain waste to a 80% good shot. Logically, the next step you'd want to tighten up the grind to bring it closer to 100%. When used this way, the dial in process is much more forgiving and less sacrificial shots are needed. This is less wasteful and extremely beneficial for home barista/roasters that constantly change beans.

Above are based on my own experience with Portaspresso, EspressoForge, and multiple home & commercial spring levers (vs pumps for completion sake).

I agree - pressure vs flow isn't linear. At some point, additional pressure further compacts the grinds and further restricts flow. This is not a case of fixed resistance.

IMO, flow profile (and correct grind and puck depth) is critical and pressure is pretty much irrelevant. The only value to pressure profiling is its (poor) correlation to flow profiling.

Agreed, flow control is highly beneficial to consistency.