To perform the experiment, I pulled a whole bunch of shots at a 200F dispensing temperature. I used 14 gram doses at various brew ratios and another set of 21 gram doses at various brew ratios. Extraction time was kept fixed at 30 sec. Past experiments have shown that my modified Silvia can dispense brew water at a very consistent temperature (probably +/- 0.5F), so whatever larger differences show up can be pretty confidently ascribed to the interaction of dose and brew ratio.
After looking at the data a bit, a few things became apparent:
1. At low brewing ratios ("long" shots), the exit temperature leveled off by 30 secs; it wasn't going to get any higher. The fact that it leveled off about 8F lower than the water inlet temperature is related at least in part to the method of measuring just below the filter basket.
2. Exit temperatures measured 25 seconds showed the strongest correlation with the brewing ratio.
3. Exit temperatures at 20 sec were strongly correlated with brewing ratio, but there was more variability than at 25 sec.
4. One can say that the "average extraction temperatures," meaning the significant temperature conditions through the "heart" of the extraction, are strongly correlated with brewing ratio.5. All other things being equal, going from ristretto to normale to lungo increases the average extraction temperatures.
6. The difference in #5 above is dramatic: going from a 100% ristretto to a 50% normale raises the extraction temperature through the "heart" of the extraction by about 12F!
7. Since a few degrees F is considered to be a "tastable" difference with most coffees, the extraction temperature is a major factor in the differing flavor balance between ristrettos and normales.
8. Perhaps one's regular procedure when experimenting with longer shots should be to drop the inlet temperature, and vice versa.
9. In this experiment, it was not the dose per se but the brewing ratio that influenced the average brew temperature.
Thank you, Scott, for getting the ball rolling on this.