actually its hard to see. ive shown a kinda progression. in pic 2 you can make out a 2" white stack with a foil cap. I started with the 3-3/4 vents but quickly realized it wasnt enough. so i added the 2" vent as advised from guys on this forum. the lighting makes it hard to see. I left the elbows on there. I might actually add 5 more 3/4 vents and cap the vent to see if theres any noticable difference. The 2" stack made a world of difference as did using quality lump fuel and the makeshift heatshield I made out of perforated aluminum pans. I dunno which improved the build the most but the thing holds them temps alot better now.