by spencerforhire » Tue Mar 31, 2015 6:46 am
I've had this conversation many, many times; the "old school" (and oval track) way was to switch the negative. Mainly done so the vehicle will actually shut off when you turn the switch; if there is an alternator in the vehicle, (especially a 1-wire style) switching the positive will often NOT shut off the engine, as the alternator is still making current even if the battery is disconnected.
As Senior Savage mentioned, in a post-crash situation, the negative switched shut off could come in contact with the metal body of the vehicle and re-establish the ground.
There is a newer,4-post type of battery switch available that has a second, smaller pair of terminals that allow you to shut off the ignition+ as well as the battery+. These work well with all types of alternators and are what I reccommend. On my drag car, the second pair of terminals are used to shut off the small 18 gauge "ignition sense" wire to my MSD box. It doesn't really carry any amount of current, so extending it to the switch and back causes no problems. As an added bonus, switching the positive is much easier on sensitive electronics; apparently it's pretty easy to blow up an ECM on a fuel injected vehicle if you disconnect the ground.
All the tracks I race at check the operation of the master shut off during tech inspection; it's one of the most common reasons that freshly built cars fail tech.
The "fleet"-
72 Vega HB Drag Car -383/'Glide/9"(9.35@146.19)(5.94@117.28 1/8th)
77 Vega Estate wagon- project(someday)will have TPI305/T-5, S-10 spindles/axles
76 Vega GT- 400/4spd/9" retired from active duty(rusty)
06 Silverado 2WD ex.cab daily
03 Silverado 2WD ex.cab (retired)
06 Haulin' 20ft enclosed car transporter
06 GMC Canyon Shop truck
07 Colorado project( 5.3 4L60e swap)
99 Saturn SL1- wife's car
01 Saturn SC2- son's project
07 Saturn Ion Redline project
and 4 more Saturn "parts cars"
Note- the very act of listing all of these has made me realize I have some kind of problem.....
Visit http://www.spencerforhire.ca