Kill Switch Plus or Minus?

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Kill Switch Plus or Minus?

Postby ol55 » Mon Mar 30, 2015 10:30 pm

I have always put my kill switch on the negative because I figured without a ground, no chance of spark.

Reading the NHRA rules, It says to have the kill switch off the positive. I don't get this.

I am not planning on racing my car but I would like to add a switch for safety and theft reasons.

So, Which is better?

Thanks,
Larry
'73 Vega GT Hatchback hot rod.
'76 Cosworth
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Re: Kill Switch Plus or Minus?

Postby SeniorSavage » Tue Mar 31, 2015 4:29 am

hey Larry, I used to drag race a few years back and the consensus was that in case of some sort of collision that "new" electrical grounds would be created at impact points and that switching off the main power kills any chance of that being an issue where electricals would still run if the main ground wire was disconnected but the power was still live.

Spencer will know for sure ..

cheers.
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Re: Kill Switch Plus or Minus?

Postby 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"-
77 Vega Estate wagon- project(someday)will have TPI305/T-5, S-10 spindles/axles
72 Vega HB Drag Car -383/'Glide/9"(9.35@146.19)(5.94@117.28 1/8th)
76 Vega GT- 400/4spd/9" retired from active duty(rusty)
03 Silverado 2WD ex.cab (work truck/parts hauler)
04 Silverado 2WD ex.cab (retired)
06 Haulin' 20ft enclosed car transporter
93 Suburban- 350/4L60(retired)
00 Saturn SW2- wife's car
01 Saturn SC2- younger son's project
00 Saturn SL2- daughter's car
98 Saturn SW2- spare
and 3 more Saturn "parts cars"
Note- the very act of listing all of these has made me realize I have some kind of problem.....

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Re: Kill Switch Plus or Minus?

Postby chevyart » Tue Mar 31, 2015 6:18 pm

spence is right about the negative switch being the right way to do the disconnect. when i built my race car my engine builders father, who started hot rodding rodding in the 50s and drove pro stock in early 70s told me that using the positive to shut off the iggy was wrong, but thats the way NHRA wanted it. i distinctly remember him telling me that NASCAR did it the right way(negative shutoff), didnt know about the new type shutoff switch. thanks spence art
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Re: Kill Switch Plus or Minus?

Postby spencerforhire » Tue Mar 31, 2015 8:39 pm

Just to be clear, I'm NOT a fan of having the master shut off on the negative side, nor ever have I wired one that way. I just started my explanation with why it was once so popular. I'm with IHRA/NHRA on this one.
I also do a lot of Marine Electronics work, and boats all are positive switched.
The "fleet"-
77 Vega Estate wagon- project(someday)will have TPI305/T-5, S-10 spindles/axles
72 Vega HB Drag Car -383/'Glide/9"(9.35@146.19)(5.94@117.28 1/8th)
76 Vega GT- 400/4spd/9" retired from active duty(rusty)
03 Silverado 2WD ex.cab (work truck/parts hauler)
04 Silverado 2WD ex.cab (retired)
06 Haulin' 20ft enclosed car transporter
93 Suburban- 350/4L60(retired)
00 Saturn SW2- wife's car
01 Saturn SC2- younger son's project
00 Saturn SL2- daughter's car
98 Saturn SW2- spare
and 3 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
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Re: Kill Switch Plus or Minus?

Postby chevyart » Wed Apr 01, 2015 10:50 am

Spence: the only thing i know about electricity is:dont stick your finger in an electrical outlet. thanks for a good explanation on this matter and other electrical matters you have spoken about in the past art
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Re: Kill Switch Plus or Minus?

Postby Gator » Thu Oct 15, 2015 1:03 am

I have both the neg is on the back of the car and a pos is on my consol.
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Re: Kill Switch Plus or Minus?

Postby EVL VEGA » Thu Oct 15, 2015 12:07 pm

In its previous life my Vega was a certified NHRA Super Gas car. It has an exterior positive disconnect and continues to run after the switch is flipped.
I know this because one time my "friend" shut it off while I was idling near him and nothing happened.
What really sucks is climbing in through the cage, getting belted in, trying to start it, and realizing you forgot to turn the switch back on. :bang:
EVL VEGA
'73 Vega Hatchback (former Super Gas car)
406 SBC, AFR 210 Eliminator Heads, 9:1 comp, pump gas, 328RWHP/412RWTQ (old heads, no N2O 1/17/04), 6.86 @ 100.8 in 1/8th (No N20 9/4/16), Mike's Trans TH400 (reverse manual), 5000 stall, 12 bolt, 35 spline strange axles, 4.33 gears, spool, hipster transbrake, linelock, backhalved, Alston 3 link, 100 shot NX Hitman+ kit, LC1 Wideband sensor, 31x18.5 MT Sportsman Pros, "Plum Ugly"™ Purple paint.
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Re: Kill Switch Plus or Minus?

Postby spencerforhire » Thu Oct 15, 2015 5:27 pm

The reason your car still runs after the switch is shut off is due to the way the alternator is wired. A 4 pole switch like I mentioned further back in this thread would fix that problem. But it can be made to shut off by wiring the alternator to the other side of the master awitch.
The "fleet"-
77 Vega Estate wagon- project(someday)will have TPI305/T-5, S-10 spindles/axles
72 Vega HB Drag Car -383/'Glide/9"(9.35@146.19)(5.94@117.28 1/8th)
76 Vega GT- 400/4spd/9" retired from active duty(rusty)
03 Silverado 2WD ex.cab (work truck/parts hauler)
04 Silverado 2WD ex.cab (retired)
06 Haulin' 20ft enclosed car transporter
93 Suburban- 350/4L60(retired)
00 Saturn SW2- wife's car
01 Saturn SC2- younger son's project
00 Saturn SL2- daughter's car
98 Saturn SW2- spare
and 3 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
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Re: Kill Switch Plus or Minus?

Postby EVL VEGA » Thu Oct 15, 2015 5:43 pm

spencerforhire wrote:The reason your car still runs after the switch is shut off is due to the way the alternator is wired. A 4 pole switch like I mentioned further back in this thread would fix that problem. But it can be made to shut off by wiring the alternator to the other side of the master awitch.


Yes i saw the explanation and the alternate switch info. Makes perfect sense.
My car hasn't seen the track in at least 5 years.
I just turn my kill switch off when i park it since the key only locks the steering column and i want it to start.
I also keep a Battery Tender Jr on it.
I'll most likely keep it the way it is.
EVL VEGA
'73 Vega Hatchback (former Super Gas car)
406 SBC, AFR 210 Eliminator Heads, 9:1 comp, pump gas, 328RWHP/412RWTQ (old heads, no N2O 1/17/04), 6.86 @ 100.8 in 1/8th (No N20 9/4/16), Mike's Trans TH400 (reverse manual), 5000 stall, 12 bolt, 35 spline strange axles, 4.33 gears, spool, hipster transbrake, linelock, backhalved, Alston 3 link, 100 shot NX Hitman+ kit, LC1 Wideband sensor, 31x18.5 MT Sportsman Pros, "Plum Ugly"™ Purple paint.
"Pound to fit..paint to match"
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