Battery Disconnect Wiring - Road Course and Drag Racing Compliant

[AU Doc]

As much as I would like to build my car to spend all its days on a road course, the reality for me is there are several drag strips that are a lot more convenient. That, and most of my friends like drag racing. So, I'd putting my car together for road course and autoX, but I'm trying to make sure I don't do anything that would prevent me from going to the drag strip when I get the chance.

As I understand the rules, the road course sanctioning bodies want a battery disconnect within reach of the driver, but NHRA wants the disconnect outside the car in the rear for trunk mounted batteries. If that's the case, it looks like I will need to run two #8 wires to the trunk along with my 1/0 for the starter. I've got two options in mind:

Here's the first option where I would need a heavy duty switch for the main cable from the battery. Mounting the "NHRA Disconnect" will be tough because my battery is above my axle, and I want to limit the length of unprotected cable that is always energized. I think the switch in the driver's compartment can be lighter duty as it will only need to carry the operational current of the car, but not the starting current.
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This next option I think is my preferred approach because it will be easier to route a fused #8 around in my trunk to a convenient place for my disconnect to mount. As far as I can tell, both options kill the power to the car. Will this satisfy both sanctioning bodies?

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Does anyone have, or have you seen, any other clever options that would let me run just one #8 to the trunk? Could I use a relay to switch a disconnect in the engine compartment, or is that not allowed?

[andrewb70]

FWIW, I have my battery in the trunk, a big lead going to the starter + and another big lead (-)going directly to the engine block. Nothing else. I've raced at multiple drag strips and have done numerous road racing event and nobody ever said anything about the way mine is set-up. Your mileage might vary...

Andrew

[AU Doc]

So no disconnect switch at all?

[andrewb70]

So no disconnect switch at all?

No switch at all. To disconnect the battery I loosen a nut.

Andrew

[AU Doc]

No switch at all. To disconnect the battery I loosen a nut.

Andrew

Haha! I like that. I'll take a 1/2" wrench with me and hand it to the tech inspector if he asks about a disconnect :)

The times I've been, I've never had a car fast enough that the tech inspector even bothered to look at my car.

We need to meet up sometime. I'm hoping to have my car complete enough to start looking for some autoX or track days in the next couple months.

[andrewb70]

Haha! I like that. I'll take a 1/2" wrench with me and hand it to the tech inspector if he asks about a disconnect :)

The times I've been, I've never had a car fast enough that the tech inspector even bothered to look at my car.

We need to meet up sometime. I'm hoping to have my car complete enough to start looking for some autoX or track days in the next couple months.

Get in touch any time. I think you have my number...

Andrew

[gator68428]

For security and convenience, one could put a disconnect switch in the trunk. And for this scenario it would be better to put it on the negative terminal. Also, one could add a fused 14 gauge wire in parallel, bypassing the ground disconnect so that way you don't lose all power to the car when you park it at the mall or at the hotel on a road trip--so you don't have to constantly re-prog your radio presets (or wipe ECU memory). If starter is engaged it will blow fuse and then all power will be lost. One could also put a quick disconnect connection in the bypass wire. Then when you're working on the car, disconnecting the batt is a cinch, just hit the kill switch and unplug bypass wire.

[blitzer454]

In both of your drawings you're really not removing battery power from the vehicle since you still have a wire going from the battery to the alternator. The way you have it wired the kill switches would only kill the engine but there would still be a hot lead running from the rear to the front of the vehicle which is not safe.

I think you would be better off putting a bumper mounted kill switch on the negative lead of the battery. I would also connect the alternator directly to the distribution point and then a wire from the distribution point back to the battery. Keep the trunk mounted starter solenoid the way you have it. Then you could wire the driver's kill switch to the wire that runs from the distribution point to the battery.

Just don't get into the habit of using these kill switches as another way to turn off the vehicle as disconnecting the alternator from the battery while the engine is running can do damage to the alternator or other electrical components.

[andrewb70]

Just as a note, I believe the NHRA rules call for the disconnect switch to be on the positive wire.

Andrew

[AU Doc]

In both of your drawings you're really not removing battery power from the vehicle since you still have a wire going from the battery to the alternator. The way you have it wired the kill switches would only kill the engine but there would still be a hot lead running from the rear to the front of the vehicle which is not safe.
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I'm not sure what you mean here. Opening the switch will isolate the battery and alternator from the car's electrical, so the engine dies and the alternator stops providing power to the battery. The wire to the battery will still have 12 V, but it will have fusible links on both ends to stop any sort of cable damage from starting a fire.

NHRA does require the disconnect on the positive side. Per teh Googles:


8:4 MASTER CUTOFF
Mandatory when battery is relocated, or as outlined in Class Requirements. An electrical power cutoff switch (one only) must be installed on the rearmost part of each vehicle and be easily accessible from outside the car body. This cutoff switch must be connected to the positive side of the electrical system and must stop all electrical functions including magneto ignition. The off position must be clearly indicated with the word “OFF.” If switch is “push/pull” type, “push” must be the action for shutting off the
electrical system, “pull” to turn it on. Any rods or cables used to activate the switch must be minimum 1/8-inch diameter. Plastic or keyed switches prohibited. Switches and/or controls must be located behind rear wheels on rear-engine dragsters.

EDIT: also, isn't the alternator grounded through its case? So disconnecting the battery ground wouldn't stop the engine if it's already running since it would be fed from the alternator, right?

[TheJDMan]

Unless you plan to compete in NHRA sportsman classes and unless you plan to run NASA Lightning or Thunder races or SCCA club racing, NO ONE will ever care that you don't have a cut off switch either driver accessible or externally accessible. I have a master cut off switch mounted inside the trunk but I have to open the deck lid to access it. Don't over think this you are not likely to ever compete at a high enough level for it to be an issue.

[AU Doc]

That seems to be the general consensus. Unless I start competing, no one is going to care.

[gray86hach]

Switch must be on the positive side. Use a remote handle that you can remove unless it is required to race. I have a rod thru the rear body pannel to the switch and you can use a morris cable to have a remote handle for the driver. Is not a bad thing to have our local dragstrips check this if the battery is not up front.