Originally Posted by
Gizmatron
Not very much unless there is a huge change but generally richen the hsn a fraction if it gets hotter and lean it if it gets colder.However in 20 years racing I don't think I've ever had the conditions change so drastically in one day that I've had to really retune..Methanol burning engines do like cool early mornings with plenty of humidity though so a day that begins cool and damp then dries out and gets very hot in the afternoon might certainly mean running a tad richer later in the day.
Not trying to be funny Giz, but I always believed that to be the other way around.
On a cold day the air is denser, so you get more air (O2) in the cylinder per stroke, requiring more fuel (a richer mixture) to run normally with better power output. Conversly on a hot dry day the air is generally less dense, so you get less air (O2) in the cylinder per stroke requiring less fuel (leaner mixture) and not quite as much power output ?
If I start racing in the early morning on a cold autumn day, I generally end up leaning a tad between the heats and the finals as the air temperature heats up quite noticibly during the day.
Same principal as drag racing, where they are down on power in the heat, or at high altitudes, but can end up taking power out of the engines or tweaking the clutch packs as the cooler evening air comes on for the finals to stop lighting the tyres up of the line because of the extra power the engines make in the cooler conditions.