A PCIII power commander is a small box of electronics which plugs into the fuel injection system, in between the ECU and the injectors. The box is only the size of a fag packet so fits under the seat easily. They can be fine tuned to alter the signal sent by the ECU to the injectors so that the amount of fuel delivery can be precisely set,they also have a wider range of adjustment when compared to a standard ECU which relies on an O2 sensor to set the mix at certain revs. You have to disable the O2 sensor or it would just override the PCIII map at certain points defeating the whole point of having a PCIII in the first place.
Engines work best at a particular air to fuel ratio (from memory around 13 parts air to 1 part fuel). Usually, manufacturers alter this ratio at certain points in the rev band to get around emission controls, typically by making the mixture lean. A power commander works by allowing this ratio to be 'tweaked' to suit the particular engine concerned. For this reason they are best when properly set up on a dyno so the exhaust gases can be measured at the various points of engine range when under load, although they come with a standard 'map' which works okay. Different maps can be quickly loaded into PCIII by using a lap top and USB cable.
When installed they can result in a smoother running engine which may make a little more power than standard, although many people use them in conjunction with a more freely breathing exhaust system as the standard ECU can't always alter the fuelling enough to make it run properly with an increased air flow. The ECU's on later V Stroms (certainly the Vee's) were refined somewhat so fitting a PCIII may not improve things too much.
If your bike is running nice and smooth I don't see there being much of a need to spend around £400 having one fitting and set up. If however you do get one you will notice a subtle improvement. I've found that the fuel consumption increased slightly after mine was done, presumably because because it doesn't quite as lean as before, but my bike now runs really smoothly, driving cleanly, even from low revs, with no popping or back firing on the over run and it makes a little more power. As an alternative it is possible to have the standard ECU altered at a dealer using a 'Teka' box or similar, a slightly cheaper way of doing the job but not quite as 'fine tunable' as a PCIII. another good thing about the PCIII is that it doesn't change any ECU settings, if you unplug it the bike will run as it did on the standard ECU.
