Flow rate primarily affects two things:

1. During pre-infusion of very finely ground coffee, a low flow rate for a long enough length of time will fully saturate the puck and open up the spaces between particles enough to permit the shot to flow without choking the machine. You also get extra contact time during the long preinfusion, allowing the water to extract more. This technique, however, is primarily applicable to very light, light and perhaps medium-light roasts, which are hard to extract using "standard" espresso extraction techniques. The low flow rate during preinfusion allows two things that can extract more from the coffee: finer grind and longer contact time.

2. Low flow rate after preinfusion increases contact time, allowing the water to extract more. By reducing the flow rate gradually after maximum pressure has been reached, you can prevent the flow from accelerating as the puck becomes more and more permeable. Some of us use this technique to maintain a constant flow rate after max pressure has been reached. Again, the benefit is longer contact time, which in turn provides greater extraction for lighter roasts that need it.

As for pressure, bear in mind that when pulling a shot the pressure you may be seeing on the boiler gauge isn't necessarily the same as the pressure at the puck. If there's a flow restrictor or needle valve between the boiler and the puck, the boiler gauge shows the pressure before the restriction. As long as the puck is permeable (i.e., you don't have a blind basket in place), there will be a pressure drop after the restriction. This is similar to what happens with electric voltage (pressure), current (flow rate) and resistance (gicleur or needle valve.) On the upstream side of the resistor, you might have 12 VDC. on the downstream side of, say, a 1000 ohm resistor, where it's connected to the negative side of the power source, the voltage drops to zero (equivalent to the zero pressure you measure at the flow coming out of the puck.) By Ohm's law, the current in all parts of the circuit is 83mA. Remember that the water path during a pull is open at one end, albeit with several sources of resistance along the way, including the puck. Like voltage and current, pressure isn't constant throughout the circuit, but flow rate is.

Also note that pressure plays another role after the basket fills. It compresses the puck, which will impede the flow. The longer it takes to preinfuse the puck, the more time water has to completely saturate the puck before pressure reaches maximum and the puck is fully compressed. If the flow rate is too fast, pressure will build too quickly and the water may not have a chance to fully saturate the puck. That can result in channeling, uneven extraction, etc.

What flow rate are you using for preinfusion? Slayer recommends a flow rate of 1.6 - 2 ml/sec. The Decent folks believe this is *too* slow, causing the puck to open up way too much and too soon, resulting in uneven saturation and flow. They recommend a flow rate of 3 - 4 per second. I switched from the Slayer timing to the Decent timing and feel that it does make a difference in terms of even extraction.

The stock flow rate of 11g/sec you cite must be at free flow (no puck in place.) That would be roughly 660ml/min, which is about what an LMLM does at free flow, I think. My GS/3 does around 450ml/min because it has a quite small flow restrictor (gicleur) of 0.6mm.

What's important is the flow rate with a puck in place. 6ml/sec is roughly the flow rate my machine has with a puck ground for normal espresso technique (no flow control) is in place. This produces 9 BAR at max pressure. For light roasts, I use a needle valve to reduce the flow rate to about 3 ml/sec with the gear pump running at the speed that will produce 9 BAR when max pressure is reached. When the first drops fall into the cup, I bypass the needle valve, which raises the flow rate to about 6 ml/sec. There's no change in pressure because it's already at the max (this is the way Slayer machines work, the theory being that running the pump at 9 BAR throughout the shot ensures there's no shock from a sudden pressure change when the needle valve is bypassed.) After max pressure is reached, I reduce the gear pump speed as needed to keep the flow rate between 20-30 ml/min. I could do that by reducing the flow rate with the needle valve, but adjusting via the gear pump is more precise and reproducible.

With extraction profiling, skipping over the intricacies of basket-fill and soak, the two "bulk" numbers that I track are peak pressure and flow rate are the end of the shot. My profiles decrease in pressure with time to keep the flow rate during high-pressure extraction roughly constant or modestly increasing. I don't know of any proof that is "optimal". It's how I cut infinite possibilities down to something I can wrap my head around.

I think there may be some relationship between peak pressure in the 4-8 bar (in the basket) range and flavor profile, but not enough to say anything definitive. I've definitely have found that the light roasts I pull can go flat or even cardboard if the pressure is too high.

For a given roast level and my selection of profiles, I find flow-rate to be a better metric for me than time. When the fill/hold phases can vary by several seconds and I might be changing ratio as well, "2.5 g/s" measured by a scale is more meaningful to me than "37 seconds". Unfortunately this requires a scale with built-in flow estimation or a connected scale with an app that can do the estimation. (Or a slick setup with a metering pump)

This measured flow rate is very different than the position of an E61-style flow-modulation valve. As Dick pointed out above, the flow in a system like that is a function of the pressure at the pump, the position of the valve, and the condition of the puck. They aren't flow-metering valves, but an adjustable resistance to flow. If, say, 4 o' clock is 2 mL/s with no basket or puck in place, it will be less when there is pressure built up by the puck. In some cases, like a ristretto-style shot, the puck resistance is so high that it dominates and you can open the flow valve a lot and have very little change in flow.

Wow. Dick and Jeff just unloaded a lifetime's worth of great advice, knowledge and experience on you. This should be pinned to the list of must read posts. I wish that I'd have read this when I started. Then again I probably wouldn't have understood or appreciated it back then.

All I can add is how my own use of the e61 flow control device has evolved along with my understanding of extraction theory and best practice. I plan to do a video at some point but below is my current "profile". I only use this for medium light to light roasts. Anything medium or darker I just run at a 7.5ml/sec position and use the in built e61 preinfusion as I have the stock spring installed.

- Start with the flow control in the position that gives me 7.5ml/sec water debit
- Let that run for 3 seconds to fill the headspace and begin to saturate the puck
- Move the control to the 3ml/sec position as the pressure begins to rise
- Wait until the pressure rises just above 2 bar and then close off the control so there is no water flow and pressure holds at ~2bar
- Wait until I get 1-2ml of drips in the cup
- Slowly open the flow control back to the 3ml/sec position
- When the pressure reaches 4 bar open the flow control up until I hit my peek pressure of ~8bar
- Taper off the flow (optional)