Join Date: May 2004
Location: Toronto, Canada
What's "ideal" depends on what you are looking for.
If you are simply looking for the biggest possible uncorrected horsepower number to brag about, then the colder, drier, and higher barometric pressure (lower altitude), the better.
If you are tuning the engine to give best real world performance, then the "average" temperature, humidity, and barometric pressure of the conditions that the engine will be used in would be a good choice, doing that keeps the corrections that the ECU will have to do to a minimum.
If you are tuning the engine to make sure it doesn't blow up under extreme conditions then you need to test it during those extremes of hot or cold or whatever.
If you are really splitting hairs to tune it for a race weekend, and you don't want to rely on the ECU's built-in corrections (e.g. if you are in a racing class that has specified horsepower maximums!), then you need to do the testing under the same conditions as foreseen for the race.
Helibars, MRA screen, Ohlins damper, reversed shift pattern, sorted suspension, braided lines, Michelin Pilot Power, all else stock 'coz it's fast enough!