A couple of things. It does not matter how long the starter is drawing the current for. The fact that it draws a lot of current is the reason you need a larger gauge wire. Also, you are talking at least twice the distance to run from the solenoid in the rear to the starter as opposed to the solenoid mounted in the factory location. Longer distances require a larger gauge wire due to resistance in the wire itself. Why are you worried about the cost of good quality 1/0 welding cable. Get 20 ft of it. It will be enough for your ground and starter cable. At $1.50 you'll have a whopping $30 invested. That couple with a few extra bucks to purchase the properly sized termianls is still peanuts. You don't want to do all this and find out your wire is undersized. It may do fine now, but what about the future? Put a higher compression motor in that is harder to start and you need a little more help. Wire close to the header will reduce the wires capability to carry current to some extent also.
Cost should not be an issue. Do it right the first time, or skimp and regret it.
I installed 1/0 welding cable from the start to rear mounted solenoid. I grounded the battery at the bottom front of the spare tire well (it's thin material, but makes for a great ground). I also have dual engine grounds up front. I'm using 2 4gauge wires for those. I have absolutely no starting problems or current flow problems. You may not need 1/0 gauge, but why not be rest assured you have plenty to handle any job.
If you are planning on making your install NHRA acceptable, you better do some serious reading and understand how the various switches, wires, alternator, battery, etc, etc, all interact and make for a properly designed system. If you don't know the difference between the current flow of a starter and that of the alternator with the mere load of the ignition and several accessories, I'm assuming you don't have the knowledge to properly install a rear mounted battery. If you are not worried about NHRA legality, that is one thing, jury rig it and cross your fingers. If you are doing an NHRA legal install, call [email protected]
MADelectrical.com and tell him your plans. He will take the time to give you all the info you need. Beware however, it involves running a few accessories off of relays and isolating them from the alternator. If the alternator is not isolated, your car will stay running even when the safety switch is flipped. If it is not isolated properly, then you risk damaging your alternator, ignition system, computer, etc when the tech officials shut your power off to make sure it kills power to the whole vehicle.
Don't take anything I said as a flame, I just hate seeing so many people install rear mounted batteries and not finish the job correctly. It's not easy, but the benefits will be great if you do it correctly. Mark can also explain to you the benefits of running different accessories off of relays instead of Fords method of routing the current for headlights, foglights, etc, through the wimpy switches. There's an easy fix for flickering headlights, assuming you have upraged your alternator previously.
Need any specific info for your application, pm me, I'd be glad to discuss it.
Red 89' 25th Anniversary LX
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ET's --> [email protected]