Master Cutoff Switch, Alternators, and the rules?

[Igor]

I think it would be best to always keep the alternator connected to the battery and have a cut-off switch behind these two items. This would prevent the dreaded load-dump which automotive electronics should be rated for, but which is scary none the less. (at least to the EE guys) I'm not sure whether the rules allow this though.
You could catch most of the load dump with a big resistor across the switch like they do in the more expensive switches. I wouldn't know what resistor value they use though.

Igor

[fsae_alum]

I heard about cutting the negative side from a very good vintage racecar mechanic. I was helping him put a Lotus Formula Junior back together and he told me that on all the cars he does (which see races regularly) he has to wire the cutoff on the negative side of the battery. He said that the sanctioning bodies for the vintage races prefer it if not all but require it.

[Big Daddy]

I have a question for you what if your battery is only for starting??? You know a magneto type where the coil is charged and fired by a passing magnet. So cutting off the battery and alt will not kill the engine but cutting the coil will.

[Igor]

Then you have to use a multi pole switch, so you can cut both circuits. Grab one of the six-pole versions; they have an extra make and an extra break switch. I bought one the other day and it also had an extra resistor which is connected to your alternator output by the make switch to catch the transient. It was a 3ohm 11Watt one.

Igor

[El Mug]

The current measured at the battery pole in an f4i is around 8 amp dc whit the starter on, also the peak of current of the coils are around 8 amps. In the alternator the current is up to 40 amps ac, that was the top of our measured lead. I dont know how up from that is the value. We use a cutoff switch of 10 amps, disconecting the battery from the ergulator. In the 4fi thats enough to stop your engine.

I hope it helps you
Andres