James Davis jld at wk.net
Thu Jul 29 15:37:09 EDT 2010

Let's look at this comparatively.  We know from the GM dyno test the fan 
and belt adsorb 14 hp @ 4,800 rpm on a late model.  Let's consider that 
to be a 180 degree belt wrap on a 4" dia. sheave.  The alternator has a 
2.5" dia. sheave with a 85 degree belt wrap.  The power transmission 
factor of less than a 180 degree wrap is calculated at 0.59 for 85 
degrees.  The sheave ratio is 0.62:1 ---  Thus 14 x 0.59 x 0.62 = 5.2 
hp.   Conversion of HP to watts is 746 watts = 1 hp.  Now consider the 
alternator at 60% efficient and the belt drive at 80% efficient so 5.2 x 
0.60 x 0.80 = 2.50 hp.  Converted to electrical terns is watts or in 
terms of amps 1862 watts /13.5 volts = 140 amps.
Will belt life suffer?  Yes, but no more than operating the engine at 
4,800 rpm. 

One thing to remember --  HP transmitted by a belt is always a function 
of belt speed.  The same Corvair fan belt that can deliver 100 amps at 
6,000 f/s will slip if it tries to deliver 50 amps at 1,500 f/s.   
Fortunately alternator output is proportional to engine speed until the 
alternator reaches 6,000 rpm; then its output is constant until its 
rotor explosion speed of around 18,000 rpm.
Jim Davis

corvairs at pacifier.com wrote:
> Tim - My youngest brother was a commercial custom electrical component
> rebuilder for nearly 20 years and he said that anything over 75 amps in a
> Corvair alternator was a shaky proposition.
> The reason is the small contact area on the pulley made by the Corvair fan
> belt. You get much above 75 amps and start getting slippage and other
> problems.
> As usual, his standards were based on a safe margin, and like many things,
> your experiences may vary.
> Lon
> www.corvairunderground.com
> Only 2 days left on our motor mount sale!
>> i am running 100 amp  "3 wire?" or more alternators on my rampy and 64
>> A/C
>> coupe with no issues. the rampy has aircraft landing high beams , 100 watt
>> ham radio , gas heater , and heated seats. yes i upgraded a few other
>> things  along the way to cope with increased loads .
>> i had 2 140 amp  (real one wire units with the little module plugged  in )
>> units made at the local auto electrical shop that were the same housings
>> for my boat. they also worked well , but we had to go to double pulley/
>> belts
>> on  those.
>> i bet the rampy draw gets right up there at times , but so far it works
>> fine. dimmer switch was my first failure , so added a relay. that was
>> after
>> running a new larger wire for battery feed up front , isolator for extra
>> battery  to run a marine refrigerator freezer on trips .
>> i think there were photos posted on the old website
>> _http://www.corvair.de/corvair_ham/corvair_ham.html_
>> (http://www.corvair.de/corvair_ham/corvair_ham.html) .
>> i run a voltmeter in my spyder dashes to monitor .
>> so far , i am very glad i did this upgrade.
>> got the old 70's 80's gm alternators for $5 each cores at the junkyard ,
>> then put new parts and corvair front and fan on them. i think they ended
>> up
>> costing @$50 each by the time i was done. that was new aftermarket rotor ,
>> field  , regulator , and brushes .
>> regards, tim colson

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