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Discussion Starter #1
Got my battery mounted in the trunk in a make NHRA happy box, vented etc. What I was curious about before I start drilling holes for adle clamps, is there a particular routing NHRA required regarding what side of the frame rails?

Thanks
 

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Battery cable has no real rules. I would just try not to run it down driveshaft tunnel and keep it away from moving suspension. Make sure the battery has a cutoff switch, and that the trunk has a division plate from the rest of the car
 

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Every OEM trunk mounted battery I've ever seen, i.e. BMW, Volvo, GM, etc. has the cable running through the passenger compartment. It's well protected there, and it's easy to install.
 

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Every OEM trunk mounted battery I've ever seen, i.e. BMW, Volvo, GM, etc. has the cable running through the passenger compartment. It's well protected there, and it's easy to install.
Mike is 100% correct.

I'll add this, many people will say it's OK to ground the battery to the trunk floor or similar metal attachment point in the rear of the car. Granted it will work but it's not the best or most efficient method especially on a higher compression performance engine. Route the negative cable all the way to the closest mounting bolt of the starter(starter, bellhousing, etc). Use several copper ground straps from the engine to the firewall, engine to subframe, etc. Electricity flows through the path of least resistance, this will give maximum power to the starter which needs it to crank over the engine plus this will eliminate any hot start issues. It will cost more to run both cables to the front of the car but in the long run it will be worth it.
 

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Mike is 100% correct.

I'll add this, many people will say it's OK to ground the battery to the trunk floor or similar metal attachment point in the rear of the car. Granted it will work but it's not the best or most efficient method especially on a higher compression performance engine. Route the negative cable all the way to the closest mounting bolt of the starter(starter, bellhousing, etc). Use several copper ground straps from the engine to the firewall, engine to subframe, etc. Electricity flows through the path of least resistance, this will give maximum power to the starter which needs it to crank over the engine plus this will eliminate any hot start issues. It will cost more to run both cables to the front of the car but in the long run it will be worth it.
It's a big mistake not to ground the battery to the frame, even if you run a separate ground cable from the battery to the block. As was pointed out, electricity follows the path of least resistance, but not to the exclusion of other paths. The conductivity of the sheet metal is about 10% of that of a copper cable, but the cross sectional area of the sheet metal is many times the cross sectional area of the copper cable. The frame is already there, it's a good conductor, so use it.

Every OEM grounds theirs with a short cable directly to the frame.
 

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I grounded mine to the frame in my chevelle. Engine was frame grounded up front, body to engine, battery to frame in the rear.
 

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Discussion Starter #10 (Edited)
Every OEM trunk mounted battery I've ever seen, i.e. BMW, Volvo, GM, etc. has the cable running through the passenger compartment. It's well protected there, and it's easy to install.
This is what I really want to do, however the previous owner installed custom piece by piece carpet that's glued down. Do I really want to peel that up to run cable? I got to think about that.
 

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Discussion Starter #11
Mike is 100% correct.

I'll add this, many people will say it's OK to ground the battery to the trunk floor or similar metal attachment point in the rear of the car. Granted it will work but it's not the best or most efficient method especially on a higher compression performance engine. Route the negative cable all the way to the closest mounting bolt of the starter(starter, bellhousing, etc). Use several copper ground straps from the engine to the firewall, engine to subframe, etc. Electricity flows through the path of least resistance, this will give maximum power to the starter which needs it to crank over the engine plus this will eliminate any hot start issues. It will cost more to run both cables to the front of the car but in the long run it will be worth it.
Thanks Mike
 

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The previous owner installed custom piece by piece carpet that's glued down. Do I really want to peel that up to run cable? I got to think about that.
Those *******s !!!!!

I would pull of the passenger door sill plate and see what could be done. Luckily on the passenger side there is a trough that is a mirror image of the driver's side that has wiring in it and may be available to run your new wire in if it's open and unused.

Just be careful as I have tried pulling up carpet that has been glued down and the fibers get pulled out of the backing. You might be able to work underneath it with a warmed up putty knife to cut through the glue but it depends on how things were done.

Jim
 

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Discussion Starter #13
Those *******s !!!!!

I would pull of the passenger door sill plate and see what could be done. Luckily on the passenger side there is a trough that is a mirror image of the driver's side that has wiring in it and may be available to run your new wire in if it's open and unused.

Just be careful as I have tried pulling up carpet that has been glued down and the fibers get pulled out of the backing. You might be able to work underneath it with a warmed up putty knife to cut through the glue but it depends on how things were done.

Jim
Thanks, I'll take a peek.
 

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Every OEM grounds theirs with a short cable directly to the frame.
The OEM would use the shortest cable possible to save a penny. You know as well as I do the OEM way isn't always the best way, it's the cheapest way.

Mike, I mentioned grounding to the frame and the firewall, if the ground cable is routed from the battery to the engine several ground straps or cables from the engine to the frame is sufficient to ground the frame.
 

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Discussion Starter #15
Those *******s !!!!!

I would pull of the passenger door sill plate and see what could be done. Luckily on the passenger side there is a trough that is a mirror image of the driver's side that has wiring in it and may be available to run your new wire in if it's open and unused.

Just be careful as I have tried pulling up carpet that has been glued down and the fibers get pulled out of the backing. You might be able to work underneath it with a warmed up putty knife to cut through the glue but it depends on how things were done.

Jim
Just routed the cables today. As you suggested, I was able to pull up the door sills and run the cables through the trough. The positive on one side, the negative on the other; where they run to their respective bulkhead connectors through the firewall.
 

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Discussion Starter #16
The OEM would use the shortest cable possible to save a penny. You know as well as I do the OEM way isn't always the best way, it's the cheapest way.

Mike, I mentioned grounding to the frame and the firewall, if the ground cable is routed from the battery to the engine several ground straps or cables from the engine to the frame is sufficient to ground the frame.
Yep. Battery is grounded to engine along with battery to frame. Engine grounded to frame and body, and it's a solid motor mount car. Kind of redundant, but hopefully any grounding issues are off the table.
 

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The OEM would use the shortest cable possible to save a penny. You know as well as I do the OEM way isn't always the best way, it's the cheapest way.

Mike, I mentioned grounding to the frame and the firewall, if the ground cable is routed from the battery to the engine several ground straps or cables from the engine to the frame is sufficient to ground the frame.
Unnecessary heavy cable adds weight to a drag car.
 

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The OEM would use the shortest cable possible to save a penny. You know as well as I do the OEM way isn't always the best way, it's the cheapest way.

Mike, I mentioned grounding to the frame and the firewall, if the ground cable is routed from the battery to the engine several ground straps or cables from the engine to the frame is sufficient to ground the frame.
Any electrical engineer will tell you that the best wiring practice is to use the shortest route possible for the wire you are running, hence the ground wire runs to the nearest place on the frame.

There are two distinct grounds in your car. When the engine is not running, the main ground is the negative terminal of the battery as all current used in the car returns to this point. Once the car has started and the alternator is functioning, the case of the alternator becomes the main ground where all current returns.

You seem to have overlooked the main points of using the frame as a return path ( or a parallel return path ) for your starting current.

It's already there. It's solid metal. It's a good conductor or you wouldn't use it as a return path for the rest of your circuitry. If it's free and works good - use it, even if you run a separate cable.

It has less resistance than most any cable you would use in your car.

A few simple calculations:

2/0 cable has a diameter of 0.3648", and a area of 0.1045 in², and if it's copper it has a resistance of 0.0777 ohms/kft. Your 15' return cable will have a resistance of 0.0012 ohms and drop 0.12 volts when you are running 100 amps through it to start your car.

The steel of your car has about 8.3 times less conductivity than copper but let's look at the area. Let's assume 18 gauge ( .048" thick ) sheet metal for the body, 16 gauge ( .062" thick ) on the floors.
The area of a 72" wide piece of 16 gauge metal ( your floor pan ) is 4.5 square inches, about 45 times the area of the cable.
The cross sectional area of the body sheet metal ( roof, pillars, sills, etc. ) will vary, so I'll estimate it at 4' average, for a cross sectional area of 2.3 in².

We'll discount the actual frame and frame connectors or any other paths for electric current and use just the sheet metal.

The steel return path has an area of 6.8 square inches, about 68 times the cross-sectional area of the cable with 12% of the conductivity.

68 x .12 = 8.2 times more current carrying capability in the steel than in the copper wire.

Even if these calculations are off by a factor of 8, adding the frame in parallel to the cable will cut the voltage drop in the return path by about 50%.

If you put these two paths in parallel, the vast majority of the current will return via the steel. There is no reason whatsoever not to use this path.
 

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^ Mike, great response! This is exactly the calculations I was imagining in my head.. the cross sectional area of the frame greatly out-ways that of the cable, even given the conductivity.
 
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