my alternator has recently failed and i was looking to buy a new one, a more powerful one (because i am running 2 12" subs) and i was wondering if a more powerful alternator would rob any horse power from the engine??

 

Simple answer: yes

Detailed answer: 1HP = 0.75 kW ..... 1kW = 1.33 HP

Every kW used will cost 1.33 HP. The alternator will only make the power that is needed, but will consume that plus it's losses (on integrated average, they are about 80% efficient). The larger alternator will have a little more losses than the OEM, so it will cost you a bit of power, but it shouldn't be significant.

 

so would i feel the difference in horsepower if i went from a 90 amp alternator to a 105 amp alternator

 

15A * 12V = 180 W
15A * 14.4V = 216 W
15A * 15.5V = 232.5 W


Is this little power boost worth the price of the upgrades alt? To answer your question, the 15.5V case would yield less than a third HP loss (ideally). At 80% efficiency, this becomes about 0.4HP ..... not noticable on a butt dyno.