I was going to upgrade all the factory ground connections under the hood on my Accord to a bigger guage. My buddy was telling me when doing so I should leave all the factory cables attached and just add bigger wires along with them as opposed to just removing them and just adding bigger wire in place of them. That doesnt make any sense to me why I would need to leave the factory wires on if I'm upgrading to a bigger wire. I dont see any advantage and it would just be ugly, IMO. So is there any reason why I would want to do that?

Also, I'm wondering what guage would be appropiate? I ordered a 150 amp H.O. alternator, bought a yellowtop Optima battery and have a stereo that consist of about 1200 watts. What guage of wire should I upgrade everything to? I'm running 0 guage all the way from the battery to a fused distribution block in the trunk with 4 guage going from that to each of my amps. Same goes for the grounds, two 4 guage wires running to a distribution block with 0 guage running to the chassis ground. Should I use 0 guage for all my ground upgrades or will a smaller guage work? I dont mind using all o guage if that will work better, I just want to do whatever will give me the best results.

 

In my opinon it's better to remove the old ground wires when you upgrade them, they're not helping any being there. I've done it both ways and it doesn't seem to matter much......but it cleans the engine bay up and keeps the clutter down.

On the wire size's I'd say 4 gauge for the grounds and run a fuse 1/0 gauge from the alt to the positive on the battery, this will flow alot more power to the battery and since you've got 1/0 running to the back it keeps everything on an even level. If you wanted to you could also connect your amp power wire dirrectly to the alt, this minimizes the dimmage (if any) as well as give the amps a more dirrect power but this is only while the car is on.......

 

There is no advantage in removing the stock ground cables, [other then for looks].

As it is the sound system that is going to need the extra current and the amp(s) are grounded to chassis your most important upgrade is the batt. to chassis ground, then the chassis to motor, [alt.] ground.
You do not need a 1/0 gauge cable from the alt., unless you have a 350A alt. a 4ga is enough for up to 150A, a 4ga can also be used for the chassis to motor ground.
Gauge of cable used for amp power and batt. to chassis ground will depend on system wattage, 1200W RMS will require a 2ga cable if it is longer then 4 feet, up to about 20 feet.
It can't hurt anything if you use thicker gauge cable, but it gains you nothing but costs you more.

And please, do not connect the amps power cable directly to the alt., there is no advantage. 94

 

10 gauge is fine for all the under-hood grounds except of course the battery ground which should be 4. 8 gauge works good even though it's slight overkill. 4 gauge or anything bigger is stupid overkill. There's no benefit to it.

Don't just replace the existing grounds. Run a ground to the thermostat housing (where many sensors are grounded), the headlight grounds, the IM, head, block, tranny, valve cover, starter housing, and alternator bracket.