Bear with me...
 

Alright so I had one of those lightbulb ideas that may be great or may be just stupid.

Recently major car companies have come up with a way to make alternators work less or even turn off during hard acceleration in order to reduce the resistance it has. This would improve acceleration and fuel economy during hard acceleration.

What if we wired a little something up to do the same on our simple cars. Just trick the alternator into thinking it doesn't need to generate so much power during hard acceleration.

How hard would it be?
Worth it? (keep in mind just how cool it would be to tell people you replicated cutting edge automotive technology in your 25-or-so year old car)
Could it be done effectively without draining the battery too much?

Note: I'm not much of an electrical engineer... so hopefully I'm not completely overlooking something.

to be honest it would have little impact and probaly not worth it
when spending that effort into tuning or repaing mehcanicl parts on the car will improve your fuel ecomney
just my 2c

i dont think the electrical load is anything near the drag the pully and belt make on the system.

Well I know BMW posted results of 3% on their tests.
That is on cars that have already been stretched for efficiency though.

LOL, I think saving fuel and hard acceleration cant be put together in a sentence relating to the rotary,

However I thought about this idea myself quite a while back. A simple switch that either works a relay to disconnect the battery charging wire, or depending on how our alternators work ( Im not terribly familiar with the internal regulator ) but maybe just doing something with the other wires to the alt.

After a while I just really forgot about it all, and decided to tune up my car instead. Thinking back I doubt it would make much of a difference, but then again there are people here who swear their e-fan helps power/revs. IMO its not worth it, but then again you may feel differently!

Alright so its probably not worth it, but could it be done?
As far as I understand there is a line to the alternator from the battery letting it know how much current there is. Alot of current=alternator doesn't work so hard. It couldn't be too hard to fake that under hard acceleration right?
Again I don't know a whole lot about the electrical system.

Yah I believe your right, picks up usually from somewhere near the main fuse or I would assume fusible links in our cars,I really dont know however I can tell you is that its definately possible!

cendi, not a useless idea at all. Make a list of items in the charging system that suck hp. I don't know the hp used to rotate the alternator system & make electricity but I do know that MANY road race cars have no alternator. Some road racers have a switch in their charging system attached to the right pedal that eliminates the charging when desired. There will be a gain in hp. 3 to 5 hp gain would be 3 to 5 % gain over the OEM hp.

Its not the generation of the power so much as the drag from the pully, belt and magnetic field inside the alternator. I think what you guys are describing would just reduce the juice coming from the alt. I think what you would need is some sort of clutch for the alt so at a desired time you could free-wheel the alt pully and not spin the armature inside the alt thru the magnetic field.

Does anyone know how BMW does it? Am I close?

The little article I read said it was just a change in the software, since I'm assuming the ECU controls the alternator.
While a little clutch would be ideal it would be way too complicated.

The drag created by an alternator is directly affected by the amount of charging it does. If the battery doesnt require any charge the magnetic feild wont put much stress on the engine, however if charging is required drag will increase with amperage the alt. is putting out, which is what creates the magnetic feild in the alternator.

This theory is why I personally think E-fans are fairly useless.
Sure, the engine wont directly be turning the stock fan, but as soon as the e-fan kicks on, it loads up the alternator putting drag back on the engine as the stock fan would have.
Maybe the e-fan goes easy on the alt. but I highly doubt it would lessen the parasitic loss, just overwork the alternator.

A combination of an e-fan and the above alternator switch idea may give you a bit of extra jam, but Id put my pennies toward a more surefire mod.

Thanks for clarifying that, its how I thought it worked but wasn't sure.
I think the same about e-fans. Because really... converting physical energy to electricity (alternator) and then back again (e-fan) can't be that efficient.

I never expected such a mod to be the best bang for the buck, just an interesting idea.
What I'm really looking for above all is someone who knows a little about electrical engineering to come up with an idea on HOW to do it.

Imho, it would not be worth the extra effort. Forgetting to re-engage the alt and ending up with a dead battery isn't worth the effort. You also have a risk of not providing enough juice for the ignition to fire the spark plugs if the battery goes low.

yeah, my idea wasnt for a switch but a little automated controller that would tell the alternator to lay off during heavy acceleration only, but could crank it back up if the battery voltage drops too low.

To give a rough idea on power gain, you can figure it out like this:

electrical power (W) = I x V

I =current in amps
V = voltage
power in Watts

For an FB, the max alt output is about 55-60 amps depending on model. For the voltage, just say 14v as a rough estimate.

This means that the max alt draw (in Watts) is 60x14 = 840 W. 1 HP = 746W, so you are looking at 1.13 HP. There is some loss converting mechanical power to electrical in the alt, but alts are pretty efficent. I would say that the max gain from this project would be 1.3 HP,

This is assuming max electrical load (headlights, blower, wipers, etc.). During normal daytime driving, the gains would be much less. Some newer cars can see a bigger gain by doing this because they simply have a higher electrical demand (and much bigger alts).

I just think it is good to get an idea of the potential gains (or lack thereof) before doing any project like this. It gives you a good idea if it is worth doing or not. It is not worth it in my opinion.

Good solid science... exactly what I was looking for.

Alright, my grand idea sucks. Back to the drawing board.

solenoid
 

Seems like an adjustable solenoid could do this quite simply.
The solenoid would open the field wire when manifold vacuum dropped below your set level. Six inches of vacuum would be an indicator of heavy load for example.