Could be from running too rich. My friend had his cats cut out and when he was running rich he was shootin some smoke out the back. As for me, I have a bassani X with no cats so its basicall straight pipe and I have zero smoke coming out, and my plugs say my fuel is right on....so check there. Also not so scare you could be oil or coolant. Your friend say what color the smoke was?
i dont think its coolant, i havent touched the coolant in ages (should prob flush it soon) and my oil level seems to be fine. im probably running extremely rich. for 6 gallons of gas, i get about 70-80 mpg.... but my odometer is running fast due to 3.73s in the rear. so im probably getting 8-10 mpg.