Outside air temp affecting running, what gives

The hotter the day gets the better my 77 Z runs, the engine temp does not rise according to the temp gauge, but when the outside temp is in the upper 80's to the mid to upper 90's the car just gets better and better , the engine smoother and smoother..can someone tell me why this is happening..

Again the overall engine temp is great , could it be a air fuel mix that as the air becomes hotter it allows for the engine to run smoother or what...

Could I have possible when looking at the inside of the air flow meter ( a lttle more than looking) accidently without knowing it made a change that would make for someting like this to happen) ...

77 Z