I just recently Acquired a Faema Lambro and I have been experimenting with various boiler pressures and length of preinfusion. I just recently learned that I can tweak the non return valve located under the chrome nut under the group. This, supposedly, controls the flow of water into the group when the lever is locked down. Here are some questions I have and I would really appreciate the help anyone can give.

1. Does this valve control only the flow rate in which water enters the group when the lever is locked down, therefore increasing the time preinfusion must be done? Does this valve in any way affect the amount of water volume?

2. I am most interested in larger volume shots by getting as much water forced into the grinds before the lever is raised. Should I increase the flow rate (looser) or decrease the flow rate (tighter)? I want to keep the boiler pressure constant at .85-1.0 bar at this point, I know I can always increase it for accomplishing the same thing. I do not want to have to use a Fellini move.

3. Would a slower preinfusion flow rate force more water into the grinds therefore allowing the lever to engage earlier when the shot is pulled or is it the opposite? I know I would have to preinfuse slightly longer with slower flow rate.

Thanks for any help!

On mine there is a noticeable difference in shot volume from completely open to nearly closed, with completely open providing higher volume shots. You have to have faith in your shower screen though, as water flows in fast and hard at that point and could thrash the puck, so you'll have to tinker with it and find your preferred point.

Other options to get a higher volume shot would be to lower the dose of coffee you are working with or preinfusion for a loooong time.

Also to note, shot weight is a better indicator of good shot size than shot volume. A 36 gram shot on a lever will have less volume than a 36g shot from a pump, because the pump tends to create more crema, but in fact they are the same amount of coffee. So if you try to get the same amount of volume from your Lambro as a typical e61 machine you'll wind up pulling lungos.

UPDATE:
Here is a video I made to demonstrate how the valve works: As you can see the far group has the nut still in place, with the near group I have removed it to be able to adjust the valve.

Only a half turn goes from fully closed to fully open, so my suggestion is to start off at fully closed and then make small adjustments until you find your desired flow rate. I have now set my groups to a relatively slow inlet flow rate. I feel that this gentler flow rate disrupts the puck much less and plays very nicely with lighter roast / lower dose shots compared to the high prospect of thrashing with the groups fully open.

For your preference, where do you feel is best place between fully open and fully closed?

To clarify. I mean "volume" in terms of shot weight. I do weigh my shots.

Does the speed of water flowing into the group effect the temperature of the shot?. Since the water goes through a heat exchanger it seems to me that varying the flow rate should change the temp.

I have mine set at about 65% open.
The faster flowing shots definitely come out hotter. The longer the water stays in the group's "erogation chamber" the cooler it gets.

If that valve only controls the flow rate of water into group, why would shot volume be affected?? Seems you could get same shot by weight by pre infusing longer. Am I crazy?

You are not crazy, but you might have to preinfuse "a lot" longer, and that may be less desirable than a higher flow preinfusion.

What is your current pressure stat setting? How many seconds do you preinfuse usually? How many grams of coffee do you usually dose for how much out?

I tend to get a nice, large shot volume if I keep my boiler lever near mid point as indicated by the sight glass.