Re: alternators: 63 or 94 amps?

[jlandry@domain.elided (John A. Landry)]

DORKIEST@domain.elided scribbled in the sand:

>With all this talk about alternators, mine decided to blow a fuse.   If I go
>with the 94 amps, do I have to up grade any wires in the harness.

That depends.  Assuming you have stock wiring, that it's in excellent
condition, and you don't run accessories that demand lots of current, the
stock 10 gauge wiring that runs between the starter solenoid, the amp meter
and the alternator should be fine.  If however you plan on running current
sucking devices that could utilize the full potential of that alternator, you
should give serious thought to upgrading the main battery feed connector
wiring I just described above (especially where it goes through the fire wall)
and either bypassing the amp gauge on the dash altogether, or installing a
shunt that you can close and temporarily bypass the amp gauge.  If you upgrade
the wiring, I believe the amp gauge would be the limiting factor... it's good
to 60 amps and then would be toasted above that.

>There was
>a statement about going over 100 amps and burning the wires.

I seem to recall we all agreed that the maximum *short term* amp draw through
the stock 10 gauge wiring was about 100 amps.  I'll bet if you tried to run
100 amps through that thin wiring for very long, you'd start smelling
insulation burning.  But as I said above, I don't think the amp meter can
handle much more than 60 amps.

>On page 106 of
>the Jan/Feb Summit catalog, The Powermaster 94 amps, Max output @5,600 rpm;
>Cut in @ 1,150 rpm ; 2,400 rpm: 61 amps. $96.95
>Since I rarely go over 3000 rpms, I will never get 94 amps.

The rpms given are the speed of *the alternator* not your engine!  To
calculate what the alternator speed will be at any given engine rpm, you'll
(of course) need to measure the diameter of the driving (crankshaft) pulley on
the engine and the diameter of the driven (alternator) pulley.  The difference
in size is the ratio that you can use to calculate the alternator speed.

Let's say your crank pulley measures 7" in diameter and your alternator pulley
measures 2-1/2" in diameter.  This would mean you have a 2.8 to 1 reduction
ratio.  So if your engine was idling at 700 rpm, the alternator would be
turning at 1960 rpm.  At an engine speed of 3000 rpm, the alternator would be
spinning at 8400 rpm.

So in the theoretical pulley size example I just gave above, the alternator
would in fact be spinning above the "cut in" speed even at idle and you'd
reach the maximum output at only 2000 engine rpms. 

>Also, the cut in
>seems too high.   Is this typical for 94 amp alternators,and/or will I need
>to get a smaller pulley?  The 94 amp at AutoZone is $59 plus $30 core,  my
>core is not the same. 

The specs on the Summit alternator look pretty reasonable to me.  But the only
way to know *exactly* what a particular alternator puts out (and at what rpm)
is to take it to an alternator rebuild shop... NOT a place like AutoZone.
Each alternator is going to be slightly different.  A true alternator shop can
slap the unit on a test bench and tell you not only the voltage output but the
amperage capability at various rpms.  All AutoZone can tell you is that it's
either putting out a voltage or not.  The later is usually adequate to
determine if the alternator is functioning, but doesn't tell you much about
performance (i.e. kick-in speed, max output, etc.)

Have fun and happy New Year,

John
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