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Old 07-21-2007, 05:07 AM
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Default Adding an Alternator (Chevy 10si)

I worked out all this for another site I'm on about off-road racing lawnmowers. a subject came on here where I thought it would be helpful, but I'm posting it in its own thread to avoid hijacking if someone has a question. below is a pic of my 10si in place. these have built in regulators, and can be wired with just two wires.

the 10si is available at autozone/advance for under $40+core, its in MOST 70's GM cars and the replacements are 60-63 amp, if you pull one in a junkyard from a vehicle without A/C, it COULD be 30amp (assuming its never been replaced-which isn't likely)

The 10SI has three terminals (including those with a 1 wire regulator).

The large "BATT" terminal which gets connected to your battery positive. (Or Terminal Post if your vehicle is so equipped).
And a dual terminal connector. (Repair pig-tails for this connector available at any autoparts store. Or, salvage with alternator if pulling the alternator from a vehicle).
The #1 Terminal. (Marked with a "1" on the case)
This terminal is used to connect to the dash warning light.
For the warning light, a lamp is wired in series with a switched voltage source. During normal operation the lamp stays off. If the regulator is damaged, the #1 terminal provides ground, and the warning lamp will light. Usually.
This terminal is also active on 1 wire regulator equipped 10SI alternators.
The #2 Terminal. (Marked with a "2" on the case)
This terminal is used to excite the 10SI into operation. (3-wire 10SI)
It is connected to the battery positive.
For simplicity you can connect the #2 connector pigtail directly to the "batt" terminal on the alternator.
The terminal is present on 1 wire regulators. Used only for those that require the stock connector to fit snugly.
If you are converting from a 3wire 10SI to a 1 wire regulator you can hook up all your stock connectors, and run it as is. However, thats wasted money unless you plan on cleaning out some wiring under your hood.

If the 1 wire is for cleaning out wires, you only need to retain the "BAT" wire. The #1 & #2 terminal wires can be eliminated. Don't be surprised to find that the #2 wire only goes a short way into the harness and spliced into the "BAT" wire.
The 1 wire regulator comes with a dust plug for the #1 & #2 terminals.


Some other tidbits available from AC Delco for wiring up a 10SI, is wiring package 1870921 (for those 6 to 12volt conversions). This contains the terminal connector AND an extra resistance wire pigtail to connect to the ignition system (don't use a ballast resistor if you use a resistance wire). Also available is an ammeter package (1965400).

Use a voltage guage to monitor your charging system. It will definately give you signs of impending problems. (Bad regulator, failing battery, etc.)

If your looking for a high output unit, keep an eye out for your everyday rebuilt (re-stamped 63amp). In my case, my rebuilt puts out 80amps at high rpm. More than enough for most needs.

High output aside, don't expect your alternator to do anything for you at idle speeds. Alternator output increases with rpm, even a 100amp unit won't put out much more than a 63amp unit at 1000rpm. If your using underdrive pulleys, this may highlight or worsen idle output problems

use your head and ask if you need to.
below are some model numbers, first part # is the AC delco #, second is the lester #. just take both to any parts store and you'll be fine.



----------------finding one made easy--------------------------
The model 10SI Delco built alternator is the first generation, SI series alternator. It was introduced on the 1969 Corvette, and by 1973 most GM built cars and light trucks used this model alternator. It was an option with at least some 70, 71, and 72 GM cars, although it rarely showed up in those years, except for on the Corvette where it was standard equipment.

Assembled with all original GM components, the model 10SI alternator had a maximum output rating of 63amps. Output rating is determined by the windings in the alternator, and not all model 10SI alternators had the same windings.

There were 10SI alternators with 37amp, 42amp, 55amp, 61amp, and 63amp ratings. The smaller amp output rated alternators were installed on cars with no air conditioning, and few electrical accessories. The 61amp output rated 10SI was very commonly installed on 8cylinder cars, with factory air conditioning option.

For luxury cars loaded with electrical accessories, there were larger and more powerful models of alternators than the 10SI. A few of the full size Buick, Pontiac, Cadillac, or Oldsmobile could be had with model 15SI, or possibly the model 27SI alternator. The 15SI and 27SI alternators looked the same, but were scaled up in size. They were physically larger and heavier, and output ratings were often 70amp, 80amp, and 100amp.


This information is for figuring out the approximate output of your alt in situation. the information is all referring to the "chart" below.

multiply the first number by 3, and that is the alternator rpm of a 10si alternator at the given engine rpm.
so to make this chart make sense, if you have are idling at 1200 rpm engine, and you have a 4 inch motor side to a 3 inch alternator side.
(engine pulley size / alternator pulley size = ratio)

so you multiply the idle by 1.33 in this case which is about 1599 rpm divided by 3 is about 533 rpm
( (ratio x idle)/3= chart rpm )

500 rpm so you'll get 8-10 amps at idle in this case... definitely recharging, and it will be much more if you give it a little gas when your riding. keep in mind with magnetos and no major ongoing draws (except lights, still FAR less than a car) it won't take much

I were you, I'd aim for an 750+ rpm on the chart though so based on a 1200 rpm Idle (don't know what it is for most, my 17hp techumseh is around there according to the stickers and papers... you'd need around 1.875 so around a 5.5-6" motor side pulley should get you 25-30 amps in this case (giving general numbers as it will depend on what size puley you can get.

Delco 10-SI
37 Amp 61 Amp
500 12 8
550 15 12
600 17 15
650 19 19
700 21 23
750 22 26
800 23 30
850 24 32
900 25 35
950 26 38
1000 27 40
1250 30 48
1500 33 52
1750 34 55
2000 35 57
2250 36 59
2500 36 60
2750 37 61
3000 37 62
3250 37 62
3500 37 63
its not exactly 3 to 1 for the 10si spindle RPMS, they are any where from 2.3 to 2.8 in case someone wants to get literal, so don't get too hung up on finding the perfect pulley's, I left enough room in the math to make it simple and less likely for you to come short even if your alternator isn't exactly the same as the numbers...

I WOULD use 3 to 1 for all calculations though, since you will always come out pleasantly suprised that way, but if you can only get a 3.5" driver pulley, you'll probably get 10-15 amps at idle anyway... so don't stress if its all you can find locally (using the actual divider) if fact I might RECOMMEND a smaller pulley if you are not upgrading to a bigger battery. that way you aren't as likely to boil out that small lawn mower battery (like 200ish cca)
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Old 12-04-2007, 07:18 PM
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i think im going to sticky this, robert, if you want to un sticky it feel free to remove it. will leave it locked since ryf is no longer active (that i can see).
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