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One hp is generally converted to about 750 watts. (746 actually). Using 0.00134 may be more precise but that's also a lot more numbers to carry in your head.
The voltage does matter--big time--because you are dealing with watts (actually volt-amps not just watts, and there's more loss in the conversion) so anytime either the voltage or amperage changes, so will the total wattage. An alternator power system is generally set at 14.4 V not 12V when the system is nominally rated at 12V. A nominal "12V" battery is 12.6 volts or 12.7 volts depending on the chemistry when it is charged, and the charging system will be running at 13.8-14.4 volts in order to overcome charging losses, so if your alternator is running because you've decided to charge the battery and apply power other systems, it will probably be running at 14.4 volts. Anything less is a bonus, you can't presume it will have cut back to a lower voltage level "yet".
From 12V to 14.4V there's a difference of some 20%, so running "the same" 100A output at 14.4V will produce some 20% more wattage, and in turn consume some 20% more horsepower form the engine.
120A @ 14.4V is indeed 1728VA, not 1728 watts. I've no idea what the conversion factor is for an alternator--it can vary widely. But "easy math" says at 750 watts per hp, 750 x 2 = 1500 watts for two hp, remainder 228 watts, that's another one-third hp, roughly.
2.3"ish"HP at full output, ignoring some decimal points AND more important, ignoring the power factor in converting from VoltAmps to Watts. (That can range 40-90% in various applications, it is not ignorable.) And again, ignoring friction losses, which will be real with something that big belted up tightly. I'd just round that up to 2.5, if I couldn't spare 2.5hp to run it--I wouldn't worry about whether it was really going to be 2.2 or 2.3 or maybe 2.7 if the power factor and friction losses were higher than expected. (Unless the engine "as installed" has been on a dyno, do we really have such a precise faith in the hp ratings for the engine anyway?[g])
Rick, if you go to cogged belts, don't you need to get "more stuff" in order to add a tension adjuster since you can't just swing the alternator anymore, and the layout dimensions become more critical in order to get the cogs and new cogged pulleys to all fit?
Gates may supply matched belts, but in point of practice, it is still harder to match and maintain two belts than one. I'd rather use one ribbed belt. (And even plain v-belts are getting damned hard to find!)
Last edited by hellosailor; 08-31-2008 at 12:15 PM.