2015 Highlander AWD XLE 6AT
I don’t know about offering a “reasonable” explanation, but there are a few factors to consider. One is that my copy of the ’02 Forester owner's manual specifies a 75 A alternator (page 12-2).I'm wondering if anyone has any idea how Subaru determines how much charging capacity each model gets. My '02 Forester has a 70-amp alternator, while my dad's 2000 Legacy with the same 2.5 SOHC engine has a 90-amp one. I've got heated seats, fog lights, heated mirrors, and heated windshield as part of the cold weather package. My dad's Legacy has none of the above. Can anyone come up with a reasonable explanation for why the Forester gets less charging power?
Another factor is that the published values are probably the “cold” specs. Output current specs from run-of-the-mill OEM alternators drop by ~20% when they heat up. Accordingly, the 75 A and 90 A (cold) alternators are probably ~60 A and 72 A (hot) alternators, and the “gap” between their specs has narrowed considerably.
It's possible that the '00 Legacy's "big" alternator may have been a marketing gimmick or even a competitive requirement, but that's pure speculation on my part.
I gather that reduced output from hot alternators is getting some attention. My older son replaced a rather expensive water-cooled alternator on a BMW (an X5?) last year, and, IIRC, the GM Northstar V8 engine’s alternator is also water-cooled.
IMHO, auto manufacturers have struggled for a long time with optimizing alternator size. For example, I paid extra to get the optional “heavy duty” 63 A alternator in my 1986 Chevy Suburban with the 5.7L V8. The alternator in my 2000 Ford pickup with a 5.4L V8 is rated at 130 A.
Finally, if you're looking for a serious alternator, you might want to look into the turbo motor's 110 A model. Entering "110" and "alternator" (without the quotation marks) into the search window will lead you to some interesting discussions.
Jim / crewzer