cut-off switch
 

On our old car when we go to the track,we install a cut-off switch on the alternator,because at hight rpm the alternator rob horsepower.We make the 1/4 on the battery,can we make thet on a ssei 1994???On a old chevy 350 the rpm at idle raise 200 rpm .If we can make it on a ssei 1994,which wire we install the switch??just for the run at the track.

No, most fuel injected GMs from like '86+ rely highly on the alternator, and the car will most likely stall out or die when you cut the alternator off, as the PCM needs a signal from the alternator.

the signal is not from the battery?but from alternator??if the alternator fail the car stall immediatly???we can't drive on the battery for few distance??

Pretty much from experience, when I disconnected the positive cable from the alternator on my '94, the car stalled with a known, good alternator.

This is why a little battery corrosion on the cables can kill your car.

You don't want to anyway. Your ignition coils need the highest input possible to produce the hottest spark possible. With a battery only, you'll have a weak spark.

I just ask this because we can make that on our old car ...we installed a 1000 amp bat and we make the run without alternator (I think the car needed a litlle voltage because we don't have pcm) it was just for 1/4 ......sorry for my english

Amps will get you nowhere for spark. You step up the voltage through a coil (transformer) and the current drops proportionally to the voltage gain.

Voltage is what allows you to jump a spark gap, not current (amps).

Let's put this another way. Based on a known coil:

The MSD 8224 coil has a turns ratio of 80:1

At 12VDC that equates to a spark voltage of 960 V.
At 14VDC (typical alternator output) that voltage is 1,120.


15-20% hotter spark. I'll keep my alternator.

Let's look at this.

200A * 14v = 2800W = 3.75Hp

So even a 200A alternator at full load is pulling less than 4Hp.
Even the hottest MSD ignitions are pulling less than 10A at 6000rpm.
During a run at the track (lights, AC, stereo all off) the alternator has less than 50A load.

50A * 14v = 700W = 0.94Hp

This is less that the run to run variation you will see on a dyno.

The extra weight of the bigger battery costs as much as the gain from the alternator shut off.

The ICM boosts the voltage going to the coils, so spark output is a lot more than 1,000 volts. Somewhere around 30,000 volts IIRC. So at 15% you'd be more like 30,000 vs 34,500 volts.