Please excuse me if this is posted in the wrong place: I'm new.
As far as I can tell if you want get more oomph from your engine, you can either reprogram the ECU directly, or you can add some kind of dongle to intercept the ECU's outward instructions.
Worried about BMW catching me out, I thought I would go for the dongle option. These two are big on ebay.
http://www.chipexpress.com/ (at over £300)
Does anyone have any experience of these, or anything similar. Their claims sound so appealing.
Or maybe I should get the ECU reprogrammed? - But wouldn't it be detectable in the service? And unless I brought the guy round again, irreversible.