While the Fox body Mustang remains a popular platform for drag racing builds, weight distribution has never been its strong suit. One of the oldest tricks to getting better weight distribution has been moving the battery to the rear of the car. For help with relocating the battery in Project Rehab to the trunk we went to Accel, Mr Gasket, Ron Francis Wiring, and Optima.

Why Move It

Weight is the primary reason for relocating any vehicle battery to the trunk. In the case of our Fox body Mustang with a cast iron V8 sitting over the front wheels and very little sitting over the rear axle, there are numerous advantages.

One of those advantages is traction due to better weight transfer. Car batteries typically weigh between 30 and 40 pounds and in the case of the Fox body, the battery is mounted on the front frame rail in front of the front wheel centerline. This affects how fast the front suspension reacts, how fast the car moves in terms of weight transfer, and even the car’s braking ability. “Moving the battery to the rear will generally give you a one or two percent improvement in the car’s weight distribution,” says Accel’s Rick Carrier.


In 1979-2004 Mustangs, the battery sits in front of the front axle centerline putting at least 30 pounds of weight nearly on the front bumper of the car.

Moving the battery to the rear of the car also allows the weight of the battery and its associated mounting hardware to act as a ballast. This improves the car’s launch and acceleration characteristics. Moving weight off the nose of the car and to the rear also improves weight transfer on deceleration, thus reducing brake dive.


Moving the battery to the rear will generally give you a one or two percent improvement in the car’s weight distribution. -Rick Carrier, Accel Performance

To relocate our battery we went to the Accel Performance Group for one of their Mr Gasket battery relocation kits, part number 6279. Included in this kit is a new polypropylene battery box which is NHRA legal for non-vented batteries, or for mounting the battery in the trunk of vehicles with a separate trunk compartment.

Mr Gasket also includes the necessary battery hold down hardware and strap to keep the box securely closed. Additionally, 20 feet of battery cable is included with all of the necessary battery terminal ends and lugs to do the job.

We’re using a battery relocation kit from Mr Gasket, along with a new Optima REDTOP 35 battery and Accel’s light weight Lightning cable for our installation.

Harnessing Lightning

Accel’s Lightning cable uses copper cad aluminum in place of traditional copper strands to reduce weight without sacrificing electrical current carrying capability.

Since we’re on a mission to make Project Rehab as light weight as possible, we also opted for a 20 foot spool of Accel’s Lightning Cable, part number 1846. This lightweight battery cable uses copper-clad aluminum in place of stranded copper. This results in a cable that is approximately 50 percent lighter in weight without sacrificing any electrical current carrying capability or performance. “The Lightning Cable is able to utilize the weight savings of aluminum while maintaining the conductive properties of copper,” says Carrier. “Obviously the more weight you can take out of a vehicle the better it accelerates, turns, and performs.”


Big Power, Durable Package

Since we’re using the Mr Gasket battery box, we needed a battery that is a vent-free style and for that we turned to Optima. Not only will this battery allow us to meet NHRA safety requirements for its placement in the trunk, but it will also supply us with reliable starting power.

With 720 cold cranking amps on board this battery will easily start our stock 5.0 engine. As we increase power with bolt on parts and eventually build a new engine, the Optima REDTOP 35 won’t let us down when we hit the key. At this time, we also don’t need a deep cycle battery considering that Project Rehab is a street/strip car still running an alternator, computer, and a handful of other components.

Left: After carefully selecting a location in our trunk area we drilled the floors. Right: We test fit our battery and hardware to make sure everything will fit securely.

Optima designs their batteries with several technical innovations that make them ideal for starting everything from daily drivers to hardcore race cars. “Regardless of how much you modify your engine whether you increase cubic inches, compression, or add a supercharger, you’re going to require more energy to start the vehicle,” says Optima’s Scott Parkhurst. “Street and strip cars produce additional vibration and g-force loads making a battery that can withstand those types of scenarios necessary.”

Optima uses heavy lead castings to transfer battery energy between each cell. According to Parkhurst, this is advantageous over the more commonly used welded plates used in some flooded battery designs. He says this allows for greater energy flow into and out of the battery.


Battery Service Life

Field Disruption

NHRA requires that the alternator trigger wire also be tripped when the master disconnect switch is thrown. To do this we ran our alternator signal wire from the ignition switch source to our master disconnect switch. We installed a Master Disconnect switch from Ron Francis Wiring, part number MS-56, which includes terminals specifically for the alternator field circuit. Doing this will ensure that when the master disconnect is switched to off, the alternator will stop charging, preventing the engine from continuing to run on alternator power alone.

Parkhurst says there’s no way to throw a number on the life of a battery. “With any 12 volt battery if you keep it fully charged at all times it will maximize it’s life. Our tests have shown that batteries that are kept fully charged last up to twice as long,” says Parkhurst.

Keeping the battery fully charged reduces sulfation, which is a crystalline corrosion that builds up on the lead plates. The buildup of sulfation on the lead castings reduces the amount of surface area available to carry current.

We could write an entire article on the battery and it’s technology alone, and we’ve recently delved deep into Optima’s battery offerings in a tech feature you can find here.

Selecting A Location

Relocating a battery is a fairly straightforward venture. In a Fox body, the gas tank must be removed from the car to gain access so that the battery hold downs can be bolted in place and to prevent damage to the tank when drilling through the trunk floor. We also had to determine where the rear subframe of the car meets the trunk floor in order to drill appropriately.{ad:BLOCK}

Regardless of how much you modify your engine whether you increase cubic inches, compression, or add a supercharger, you’re going to require more energy to start the vehicle. -Scott Parkhurst, Optima

Following the included instructions, we drilled two holes in our trunk floor for the battery hold down bolts. We then placed the securing straps in position followed by the battery box. “It’s very important when you relocate the battery to get it properly secured. The last thing you want is the battery to move around on hard acceleration or deceleration,” says Carrier.


We chose to run our battery cables inside the car. This will protect the cables from any debris under the car as well as weather elements. This also allowed us to easily route the cables. We had recently relocated our starter solenoid to the passenger side inner-fender of the car and this is where we connected our positive cable end. The other end of the positive cable is connected to our Ron Francis Wiring master cut-off switch located on the rear of the car.{ad:BLOCK}

Left: We checked the trunk area and found the lower portion of the license plate mount to be the best area for installing our master disconnect switch. Center: Using a stepped drill bit, we carefully drilled through our license plate and the rear panel of the car. Right: Our switch installed as viewed from outside the car with the On/Off label installed.

Good Ground

Unibody cars require some special consideration for electrical grouding. A full frame vehicle in many cases can have the negative battery terminals simply grounded to the frame. In a unibody car like a Mustang, establishing a reliable ground is vital to the life of electrical components and to proper operation of onboard electronics. We used the Mr Gasket battery cable included in our battery relocation kit to run a battery cable from the negative battery terminal all the way to the engine block, specifically grounding the cable to one of the bolts for the starter. As an additional measure we also grounded the block to the vehicle’s frame using a braided ground strap.

We ran our negative battery cable from the negative terminal on the battery to one of the mounting bolts for our starter.

Shut It Down

NHRA rules mandate that if the battery is relocated to the trunk of the vehicle a master disconnect must be installed at the rear of the car. This allows a safety official to quickly shut down the car in an emergency situation. To handle this we’re using a master disconnect switch from Ron Francis Wiring. There are a variety of ways this switch can be mounted but it must be easily viewed by an official and the off position must be clearly labeled. {ad:BLOCK}

Left: We routed the positive cable to the starter solenoid. Center Left: The positive cable is then connected to the starter solenoid. Center Right: Another section of cable was run from the other side of the starter solenoid to the connection on the starter. Right: Once all of our grounds and positive connections were complete on the car we connected the battery terminals and tested 0ur work.

We drilled the driver side lower left corner of the license plate and license plate panel with a 3/4-inch stepped drill bit to allow the switch to pass through the body. This doesn’t interfere with our license plate legibility, and allows us to install the switch and label it in a conspicuous location.

The end result, our battery box installed, cables routed, our battery secured and connected.

With everything connected, we tested the switch verifying that the battery current is completely shut down when it’s in the off position.

With a substantial amount of weight removed from the front of the car and now acting as ballast at the rear we’ve further simplified and upgraded Project Rehab’s weight distribution and electrical system. We’ve also given the car better weight transfer characteristics for the track and street.