alternator cut off switch

mrbobby · Post by **mrbobby** » Wed Jun 24, 2009 11:14 am

what do you guys think about a cut off switch on your alt. when you make 1/4 mile pass?

Post by **Dave Koehler** » Wed Jun 24, 2009 11:29 am

Cutting off the flow of power when you need it the most is counter productive.

mrbobby · Post by **mrbobby** » Wed Jun 24, 2009 11:38 am

i have a hot battery and use msd ignition system. will a 10 second pass really drain the battery?

Post by **Dave Koehler** » Wed Jun 24, 2009 3:57 pm

You will not gain 1 iota of HP or ET but you can lose if that MSD doesn't see enough voltage.
Spend your energy elsewhere.

You probably still don't believe it so try this on.
Let's say you have a minimum of things running and maximum amp load will be 30 amps. The HP required to spin that alternator producing 30 amps at 14.3 volts is 0.72 HP.

14.3 volts makes electric fuel pumps and MSD ignitions hum right along. Lower voltage slows the pumps down changing your adjustments run to run. Despite the factory claims the MSDs get strange below 12.5 volts.

The alternator will help keep you consistent.

BCLS776 · Post by **BCLS776** » Wed Jun 24, 2009 4:23 pm

Dave is right. You'll lose more power from a 1-2 degree change in air temperature than that alternator will take from your crankshaft. I've seen cars lose 3-4 tenths in ET simply because the MSD box didn't get enough voltage. What a frustrating way to lose a race!

Use an alternator or a 16V system and keep life a little simpler.

cjperformance · Post by **cjperformance** » Thu Jun 25, 2009 10:25 am

It is definetly not worth switching the alternator off. You will see more gain maybe by slowing the alternator speed to a minimum but still allowing a good charge rate.

On a friends drag car we fitted an alternator to avoid battery probs.
After fitting it we had no probs with keeping the battery good for a whole meeting without having to hookup a battery charger, swap batterys or jump start it.
Fitting the alternator made no difference to track times, so the extra weight and what little drag it gave were being out weighed by perhaps a slight improvement in voltage resulting in a little more hp.!!

It used to run the 1/4 with 12Vmax available, BUT with the alternator there is always 13.7V+ available.

I would always suggest an alternator where possible.

I recently fitted one to the front of the diff housing on a 30 Chev Hotrod due to the BB Ford leaving no room up front to fit it neatly. It runs a pulley on the diff yoke and is geared 1:2.3 by the pulley sizing. It is charging 13.9V at 1100rpm in top gear. So with 4.11 gears it charges almost the whole time the car is moving. This thing runs MSD 7AL, Mallory pump, Elec Water pump, Cyclone fan, and full lighting and has no battery probs now.,,,

Definetly use the alternator.

Cheers,

mrbobby · Post by **mrbobby** » Thu Jun 25, 2009 12:17 pm

I surrender , i will not unhook alt.

RCJ · Post by **RCJ** » Sat Jul 04, 2009 12:08 pm

I installed an on /off switch in the wire going from the alt. to the battery.Didn't make any differents in et,but it did make differents when I hooked my timing light to the back of the alt. with the switch off.With the switch off there is around 30 or 40 volts at the back of the alt., timing light don't like that high of a voltage.

af2 · Post by **af2** » Sat Jul 04, 2009 6:26 pm

RCJ wrote:I installed an on /off switch in the wire going from the alt. to the battery.Didn't make any differents in et,but it did make differents when I hooked my timing light to the back of the alt. with the switch off.With the switch off there is around 30 or 40 volts at the back of the alt., timing light don't like that high of a voltage.

You have to disarm the alternator ( switch the regulator off ) Disarming the wire to the battery is not correct as you found out.

As stated leave it alone.
I have run fractional HP(green) belts and never toasted one belt. Kinda tells you there is very little HP involved

dwilliams · Post by **dwilliams** » Mon Aug 24, 2009 4:38 pm

Switching off the alternator when the engine is running is very hard on the diodes in the alternator. They usually won't blow, but they'll start to "leak" and eventually they won't control backflow enough and the car won't charge any more.

$3 for some GM SI diodes is no big deal, but some modern-design alternators use welded-in diodes that are a lot more expensive and difficult to replace.

For you guys who have your kill switch tested when you pass tech, this means you, too.

The alternator should be wired directly to the battery. The kill switch should be between the battery and the rest of the electrical system. For a rear-mount battery, this means running the charge wire all the way back.

Downside is the charge wire is hot even when the kill switch is open. If your sanctioning body objects, you'll have to run a relay between the alternator and battery, dumping the alternator to a big resistor and then ground.

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