whats the difference between the stock fan and this aftermarket one and how much is it?
Lets nip this one right now.
the 27 HP claim is an outright lie. The reality is, it probably saves you 27 HP compared to the stock fan running at full tilt. The only problem with that, is that you can NOT tow anything that would otherwise survive the load that requires that fan. Remember, that is 27 HP consumed ONLY while the clutch is engaged.
These are not straight across replacements. The moment you put electrics on the truck, you lose 6000-8000 lbs of tow capacity from the cooling system. The manufacturers are not shy about telling you this, but they are completely dishonest about the power benefit.
Also deceptive, is the "5000 cfm" ratings they attach to them. Pure BS. What they forgot to mention is that rating is with fans not mounted to anything. All electrics are rated at zero pressure drop, in free air, which is stupid, unless you are going to hang them from the mirror to cool your face. Once mounted on the radiator stack, the enormous amount of resistance they encounter, reduces output to less than 1/3rd of this value. So again, pure BS.
Nothing is free. The 40 amps they consume is NOT free. The alternator gets very hot as it nears the limit of its rated output. It is going to have a shorter life. But while it is huffing down 40 amps, that is a 5-10 HP parasitic loss of power. Nothing is free, that is except the advice you are getting here.
AVOID! Complete waste of money.