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Switching off alternator during hard acceleration

  1. Mar 16, 2008 #1
    Some time ago I was wondering about disconnecting my alternator from my race car just during a quarter-mile drag run. The lesser friction on the motor's drive belt system would be ideal. Then I realized that spinning the alternator itself is not that much of a problem, it spins pretty freely. Spinning it while it is charging a load though, that is what's in question. I'm sure nobody would really know, but I was wondering what kind of force it would take to spin the alternator while charging the car's electrical system. Not the car's system when the lights and accesories are on, just when the battery is pretty much charged up and there is no other significant loads on the engine.

    The idea then came to just put a switch on the alternator, and turn it off during a track use. It would be easier to flip a switch than to dismount the alternator, and even if the gains are very minimal, we are talking about one relay and some wiring here.

    A buddy of mine who worked at a tuning shop was going to test the idea for me by dyno'ing a stock acura integra and then dyno'ing it again with the alternator electrically but not mechanically disconnected.

    Just wondering if anyone else has had this idea or anyone with dynonameter access could try this out for me. My buddy never got around to doing it.
  2. jcsd
  3. Mar 16, 2008 #2
    I dont believe just switching it off would help, the alternator would still be in the belt drive and cause friction. If you could put a clutch controlled by switch on it to let the alternator spin free when not wanted (maybe thats what your saying). I don't know?

    There are a lot of choices for alternators out there. For drag racing you would want one with an overdrive pulley ratio, that way you charge while in staging and on the return slip. Then the battery will be fully charged for optimum ignition when your at the line. A pulley ratio of 1.75:1 or more is what most drag alt's run.

    It has been a long time since I ran the 1320. I had a 55 chevy, no alternator we used only battery power. (bring a few) Also did not run a fan in this car. Strictly quarter mile. If your not driving it on the road you can remove a lot.
  4. Mar 16, 2008 #3


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    Installing a switch to interrupt current in the field winding (actually the rotor) used to be a common thing. Not sure if it still is. It will definately gain you some horsepower. DO NOT simply install a high current switch to interrupt the main charing current.
  5. Mar 16, 2008 #4


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    Yeah, an alternator cut-out was always pretty standard. The EM drag during charging sucks up a lot of power.
  6. Mar 17, 2008 #5
  7. Mar 17, 2008 #6
    if you get to the dyno, let us know what the result are.
  8. Mar 17, 2008 #7


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    It's a common trick for short-term use; particularly drag racing. You can work out the gains. Just have a look at what your ancilliaries consume, plus whatever is required for your ECM and ignition system. Then add some on (say 20%) for losses.
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