Dyno time is billed by the hour, and a meaningful fraction of every session we run is spent fixing things that should have been sorted in the customer's garage the night before. A car that shows up properly prepped gets more pulls, better data, and a tune that actually reflects how the car will behave on the road.
Fluids first. Fresh oil at the correct level, at the viscosity you intend to run afterwards. Coolant topped up and no weeping from any hose. A dyno pull loads the engine harder and for longer than almost anything you do on the street, with far less airflow across the radiator, so a cooling system that is merely adequate on the road will show itself on the rollers.
Fuel matters more than people expect. Arrive with roughly three-quarters of a tank of the exact fuel you plan to run, filled at a station you trust. Half a tank of unknown 93 mixed with whatever was left from last month gives us an octane number we cannot verify, and the safe response to an unknown fuel is a conservative tune.
Mechanical health: no active check-engine codes, no vacuum or boost leaks, healthy spark plugs gapped for your power level, and a clean air filter. Tires matter too, because they are what transmits torque to the rollers. Mismatched wear, low pressure or a tire near the end of its life will slip on the drum and put a scatter in your numbers that has nothing to do with the engine.
Finally, come with a goal and a data logger, and plan to be there for the session. The most useful dyno days are the ones where the owner can say what the car does badly on their actual commute. Numbers on a sheet are the easy part; a tune that fixes the flat spot you feel every morning is the reason to be there.
