Pre-infusion is the low-pressure wetting phase before full extraction pressure. To isolate its effect, keep dose, grind, and target yield fixed across tests, then change only pre-infusion time.
Run a controlled timing ladder
Start at 2 seconds pre-infusion, then test 4 seconds, then 6 seconds. Pull one shot per step with the same dose and target ratio.
If your machine supports fixed pre-infusion only, use that fixed value as your baseline and tune grind and ratio around it instead of guessing.
Keep key variables locked during tests
Do not change grind, dose, or target yield while comparing pre-infusion durations. Otherwise you cannot tell which variable changed the cup.
If shot time drifts outside your target band while testing pre-infusion, reset grind first, then repeat the same timing ladder.
Use flavor failure modes to choose timing
If the cup is sharp and underdeveloped with uneven starts, test the next longer pre-infusion step. If it turns muddy and dull, shorten pre-infusion by one step.
When taste and flow look similar at two durations, choose the shorter one for repeatability and easier daily dialing.