when i change gears, why does a puff of black smoke come out?

This is nothing serious, in fact with todays lean fuels it could be a slight plus in your favor to prevent a burned piston. What is happening is that the fuel is loading up a little in the cylinders causing an over rich situation during the lag time between the shifts. Then after your in the selected gear and throttling up it burns the excess fuel out. On most street engines there is absolutly nothing to worry about. Try shifting faster. Its called power shifting.

However, this is a slight power robbing situation. You did mention racing. Any race engine for maximum power out put needs to be dead on in the fuel delivery department for the absolute maximum horse power your engine is capable of putting out. Any over rich condition will rob a slight amount of horse power. To check my fuel delivery I make a hot pass on the track and at the endo f the pass I immediately kill the engine and stop the car with my brakes only. I then pull every spark plug on both banks and read the plugs. The spark plus will let you know how rich or lean your engine is running during racing events. What I like to see in my plugs are a very light tan and nearly white coloration. This telles me I`m getting the most out of the fuel burn. If your plugs come out sooty black then your fuel is overly rich and you are loosing both horse power and mileage. Ask around town and find an old racer that actually has trophies in a trophy case that builds and tunes his own engines. That feller will have the capapilitys of helping and teaching you a bunch. Good luck

I wouldn't worry to much about it.