d series is a good engine to play with nothing big straight bolt on stuff. if you want to go bigger better N/A. go a B series(this is talking within a 4g budget) if you feel that you can built your d series up to run as good if not better than a b series with mods go for it. if you cant then nothing but a few grand lost and you could start over again with a standard b series. d series are cheap man engines but still fun to have. i have a d series with a nice set of cams and cam gears and all the other bits and i find it runs well(still unsure as how it will go up against a b series, stock or at least with i/h/e)...shame its wet over here in perth so i cant take down strips but dyno results soon