Bob, thanks for bringing this up and Vance, thanks for the clarification.
When we first started up my newly rebuilt motor (9.6:1, GP2 cam) Bosch 55A
alternator, 16 inch permacool fan, we noticed that the idling engine rpm
dropped several hundred rpm when the fan kicked in. Actually since I have
persistent issues with high idle it was a welcome drop but now I at least know
what is going on.
Stan
-----Original Message-----
From: 6pack-bounces+stan.foster=hp.com@autox.team.net
[mailto:6pack-bounces+stan.foster=hp.com@autox.team.net] On Behalf Of
Navarrette, Vance
Sent: Monday, September 15, 2008 4:06 PM
To: Bob Danielson; 6pack@autox.team.net
Subject: Re: [6pack] Alternator Output vs RPMs
Bob:
Yes, it makes sense.
An alternator with no load on it will only require the frictional
losses + some small amount of work to keep the windings active.
If you place a load of say, 20 amps on the alternator, you are now
demanding the original amount of work + (13v x 20a) = 260 watts. This is
roughly 1/3 HP of work (~760 W = 1 HP) that now needs to be done. This work
does not appear out of thin air, it is supplied by the motor because now it
is
much harder to spin the alternator.
The regulator is there solely to keep the voltage constant, otherwise
the faster you spin the alternator the higher the voltage would go (assuming
the load is not changing). So you should NOT see your voltmeter move very
much
at all as you add or remove loads - although the engine may speed up or slow
down if it is idling.
The other variable here is the tune of the engine. If the engine has
a
hot cam, then it does not have any torque reserve at idle so a small change
in
load can really slow down the idle speed. A stock cam delivers a robust
torque
at idle, so you may not notice the idle speed change much when a load is
applied.
An electric fan is the way to go, even though there is a small loss
of
efficiency when converting the load to an electrical one. A mechanical fan
will spin up to 5,000 RPM, and so will draw up to two or three HP. An
electric
fan will normally spin no faster than about 2,000 HP and so will draw 1/2HP
or
less while operating (0 HP when not operating).
Vance Navarrette
Cogito Ergo Zoom
I think, therefore I go fast
-----Original Message-----
I'm having a discussion with a TR6 friend who has a Delco 55 amp alternator
on his car like I do. He says that every time his Perma-Cool electric fan
comes on (he doesn't have a mechanical fan) he sees a 400 RPM drop (stock
mechanical tach) in engine speed. His reasoning is that when an accessory
calls for power the alternator has to work harder and, in turn, the engine
works harder.
<snip>
Does it
make sense that his RPMs would drop when his fan comes on?
Just curious.
Bob
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