There's no code for setting maximum fan speed but you can set it to 100% if you want to do that very easily.
Just go to Settings > System > General, tick the Fan speed override and raise it to 100% - simple.
If you untick that but do not tick the Auto temperature control option XBMC will still use the % fan speed setting shown in the (greyed out) Fan speed setting ie. if it is showing 50% that is the fan speed being used, it does not default back to 20%.
Better, as the others were saying here years ago
, to use the Auto temperature control and allow the Xbox to set the best fan speed according to the temperature. My setting are for 50C target temperature with a minimum fan speed of 20%.
On a warm summers day (UK) with an ambient temperature in the room I'm using today of 27C those, settings on v1.6 Xbox idling on the dash are giving me a CPU temp of around 47C and GPU 36C. The only time the fan speed rises from its minimum 20% speed (to 60%) is when the screensaver kicks in after 3 minutes.
That's actually something to think about if you're into power saving/low temperatures. If so it would be better to use "Dim" in Screensaver rather than one of the animated options.
Incidentally if you use another dash like UnleashX and a v1.6 Xbox you'll find the reported temperatures are shown as higher. On average 8C - 10C higher than reported by XBMC. The EvoX dash is even more unreliable, consistently showing temperatures of 60+C (typically 64.0C whatever the ambient temperature) at any fan speed.
With EvoX fan speed apparently can not be altered. If it can be done it would have to be by editing the evox.ini. There's is no information in the EvoX Manual (still easily found) at all about this but if someone knows if it is possible please share.
I trust XBMC's temperature reporting as accurate, the other dashes - not so much.