I'm not sure if it's my grinder or my machine or what, but I find I can't a get a ratio of less than about 3:1 - any shorter and the shot comes out sour.
In theory I can grind finer, but the pour is slow even at the current grind level. If I go any finer, the coffee comes out in drips (even if I reduce the amount in the pf).
Could this be to do with the beans? I have been using fairly dark roasts, but not super dark.
I'm also wondering if this is affected by temperature - other things being equal, higher temp = more complete extraction?