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2002 Forester S Premium 4EAT
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Discussion Starter #1
While shopping for a new alternator today I found out that there are a few "different" alternators that will fit my 2.5 SOHC- the difference being the charging capacity, 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? Doesn't make much sense to me.
 

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Ah crap.. I'm back with my 05 XS
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Actually I think it was 05 Bobby, but yeah we NAs have 90 amps too from 05 up.
 

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Discussion Starter #4
After measuring the Legacy's alternator and pulley to make sure it will fit, I ordered that. 20 more amps for all my heated stuff and subwoofer! (That's probaby why my 70-amp alternator started going bad in the first place!)
 

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Discussion Starter #7
Rockauto lists it as a 90-amp too. Looks like 2002 is the end of the wimpy allternator years for the Forester. Good thing for me the 90-amp is a direct swap.
 

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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?
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).

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.

HTH,
Jim / crewzer
 

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Discussion Starter #9 (Edited)
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).

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.

HTH,
Jim / crewzer
Yes, you're right- 75-amp. Rockauto lists a 70-amp option. Interesting info! Thanks, Jim!
 

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Ah crap.. I'm back with my 05 XS
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My '03-'04 N/A shop manual says 90A. I looked on the '03 alternator, but there's no amp rating. :confused:

If it's 70A, I'll have to consider upgrading it. :wink:

Bobby...

['07 FSXT MODding Journal] ['03 X MODding Journal]
Weird! According to opposedforces, the 03's PN shows the older alt, which was reported as 75 amp! 05 is where the change to the new PN alt (supposedly, the 90 amp, same as STI up to 06) is shown. Eh I should know not to trust PNs I guess.. I found my PNs don't match what I have on my brake system.

So.. we all really have 90A alts from 03 up? Huh... The one Silke took off her car was supposedly the 75 amp on an 04, but her PN matches yours... I am so confuzzled!

http://www.subaruforester.org/vbulletin/f113/alternator-upgrade-2792/

You're right.. the mitsubishi PN for the xx370 alternator is A2TB2891 = 90 amp OE alternator.
 
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