Someone tell me I'm not dreaming. I do V60 pour-over coffee. With my current coffee (from one 2 lb. bag), grind and technique, I get a get a total brew time of 3:30 +/- 5 seconds. It is very consistent...over and over. I decided to do a study of the water chemistry and it's effect on flavor. My first experiment was to maintain total hardness where it is from the tap (162 ppm as CaCO3) and just reduce alkalinity from tap level (109 ppm as CaCO3) to about 40 ppm. I did this by adding 5 grams of a 10% phosphoric acid solution to 1 gallon of water. I had input my entire water profile into software that then told me the amount of this acid to add. My brew time went from 3:30 to 2:50. This is not possible! I could actually see the drawdown occurring much faster. The coffee was weak but tasted OK. I will adjust grind and technique to optimize. Does anyone have an explanation for why and how this could happen? Thanks!

That's to be expected - slower flow with high alkalinity. I think it's been observed for decades, and the theory has to do with wetting, swelling, and foaming when bicarbonate ion is present. You can find discussion in chapters 2 and 3 of the old (2011) SCAA water quality handbook, showing old (1960's?) Coffee Brewing Center data. They were looking at pretty high levels (100 - 400 mg/L sodium bicarbonate, which would be around 60 - 240 mg/L alkalinity as CaCO3).

Adding acid to reduce the alkalinity might also have an effect because until it equilibrates with air you would have a surplus of dissolved CO2 in the water (because H⁺ + HCO₃⁻ ⇋ H₂O + CO₂ )

Many thanks for that info. I'm not (totally) crazy. Now I can proceed with my water experiments.