I know many guys replace the engine-friven fan with an electric one to gain some HP.  My assumption is that, if drag racing, they would flip a switch to disable alternator charging during the race.  If not and the alternator happened to be charging, using the theory that no new energy is created in the universe, it would seem that it would take just as much extra HP to spin the charging alternator (that has the additional demands of the fan)  as it would be to spin the mechanical fan.  

Anyone have actual facts on this?

If guys do wire in a "alternator disable" switch, can anyone provide directions? 

Also, ever heard of anyone installing such a switch so that the throttle pedal or linkage would activate the switch at near full throttle, but not at any other time?  Just curious more than anything...  

I'll start the conversation on this one chesh, although I don't know the history of using this method for racing. Since the alternator works from opposing magnetic fields, I would imagine shutting down the alternator would reduce engine load, however the longer the alternator is shut down, the lower system voltage would drop, eventually causing a weaker spark. I guess in a short (drag) race  it wouldn't matter much. As far as installing a switch, you would just have to interrupt the field wire (I believe the #1 wire on the two terminal plug) with a switch. If you didn't want to do it manually I guess you could mount a switch near the throttle somewhere, that broke the circuit only near full throttle. If our cars are already designed to do this, I am not aware of it.

When you do this, if you chose to do so, remember you will need to do more than just cut the ignition power to the alternator.  With ignition power cut, the alternator will continue to charge, once it started.  Even with no ign power from start up, at enough RPM and with residual magnatisim, the alt will start charging on it's own in many cases.

You must either change to a special regulator or disconnect battery power to the alt batt terminal.  This can apply to both the internal and external regulator cars.

Cutting the power to the batt term will always stop it from charging.  Do not use a standard switch to do this.  It will fry from the amperage.  Instead, connect a relay in the power lead to the batt/alt terminal.  Use the switch to control the relay.  You may be tempted to use a ford starter relay for this, but that relay is a momentary contact relay.  Continued use for longer periods can ruin the relay.  Use a relay such as the ones used for controling snow plow equipment.  They may look the same externally, but the insides are different.  The continous use relay won't overheat and destroy itself.

Of course, feed the relay control circuit through you switch to choose when it operates, but feed the switch from the ignition.  That way you can't forget and leave it on.  That would not harm the alt or charging system, but the relay draw would kill the battery while it sits parked.

But unless you are worried about a few, or less, hundrends of a second on a drag strip, it probably is not worth it.