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Old 04-18-2012, 10:33 AM   #1
monkeyracer
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Default 2012 Impreza Big Three Upgrade

Alright, I am looking to do the Big Three Upgrade when I put my Amp in, and I am sure there are others on this forum that have done this or want to do this for their car.

I'll be using 4 gauge wire and soldered terminals all around and they'll be just zip tied to the wire they are upgrading.

What are the Big Three Cables? (covered in numerous posts on this and other forums)

Alternator - Battery Positive
Battery Negative - Chassis Ground
Transmission/Engine - Chassis Ground

I might have it all mapped out, but can't find one of the big three cables. In this car though, the Battery Negative goes to the transmission, and the cable I can't find is the Transmission - Chassis Ground.

Also, the Alternator - Battery cable goes through the fuse panel. I should be fine going straight from the alternator output to the battery positive post right? Or will this need to be fused? I've seen some set-ups with and without fuses. The pic below shows the cable I am talking about:



So, has anyone else done the Big Three on the 2012? If so, where is the 3rd wire? Thanks for your help in advance!
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Last edited by monkeyracer; 04-18-2012 at 11:03 AM.
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Old 04-18-2012, 02:20 PM   #2
Lateralis
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The ground for the tranny is underneath the intercooler so you need to take it out to do this. You are fine running an extra cable from the alternator to the battery post. If you check my thread I got pictures of how I did it on my 1st page.
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Old 04-18-2012, 02:34 PM   #3
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if your going to dig in... get extra wire and terminals... do the various ground straps and ground wires ... just open your hood and look around there are several ground straps in various places... do them all.

Rob
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Old 04-18-2012, 04:40 PM   #4
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I don't think you want to add a cable from the battery post to the alternator directly. The 2012 Impreza has a special charging control circuit, which measures the current flowing in/out of the battery (this is why the main battery cable goes through the fuse box). There is a 120A fuse for the battery, plus the battery cable goes through a current sensor.

From the 2012 Impreza New Car Info:

"The new Impreza sees the first deployment of a new charging control system comprising a current sensor, a battery temperature sensor, and the alternator. When the vehicle's electrical load is small, it reduces the recharging voltage; when the electrical load is large or during deceleration, it raises the voltage. As a result, the overall engine load is reduced, boosting fuel efficiency."

So, pick either the battery or the alternator as your power cable connection point. I used the battery post, and have had no problems.

Also, without getting in to a religious argument about wire gauge, current draw, etc., the actual typical current draw (unless you have some massive sub system with 2000W, and 3 ea. 12" woofers), is MUCH less than the peak power rating of either the amps or the speakers.

I have 120W RMS sub (B-8PT), and a 300W amp (Alpine MRX-F30), and my base current is 1.3A (amps running, no audio). With everything up LOUD, I measured current in the 2A to 3A level; that was all! I'm running 8GA power/ground wire; I brought the power wire back to the battery post, and the ground wire back to chassis ground inside the center console, and didn't upgrade any of the OEM power wiring.

Just my 2c...

Last edited by Rymar; 04-18-2012 at 05:15 PM.
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Old 04-18-2012, 05:05 PM   #5
Lateralis
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This is may be different from the Impreza to the wrx/sti. They are not same line of vehicles any more from 2012 up. On my sti the alternator line goes straight to the battery with a post where it then branches off to the main fuse box.
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Old 04-18-2012, 05:07 PM   #6
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Rymar... 2-3A draw doesn't add up and is beyond the laws of physics.. how were you measuring the current draw? You can't just put +/- to the battery terminals and see what the "drop is" that's not DRAW. You have to put the VOM INLINE with the + battery cable and change the setting to amperage to see true draw.

The "active battery" set us is news to me, but still can be improved. I don't know anything about the '08-up cars beyond what I read. What is the alt rating stock?

what gauge or better yet MEASURE the OE Bat to alt wire? You could easily upgrade the wire to the 120a fused circuit, then upgrade from the 120a fuse to the alt.

Its not always about wire gauge or size, as much as type of wire and design.

I have talked about it 100's of times, I took the 0/2 ga welding cables from my arch weldrer. Bought brand new, heavy strand welding cables and new clamps/ends. I did the same with fine strand 0/2ga "car audio" style wire, the exact same length and another pair of clamps/ends. The welder ran cooler, laid a smoother bead, and had better penetration with the fine strand wire. Transfer of current/amperage was clearly better with better wire. this goes up with wire length.

Just an example that, not specifically improving the SIZE but the QUALITY of the wire can help.

its the grounds and the quality of wire overall that the improvement is found.

Rob
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Old 04-18-2012, 05:24 PM   #7
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Quote:
Rymar... 2-3A draw doesn't add up and is beyond the laws of physics.. how were you measuring the current draw? You can't just put +/- to the battery terminals and see what the "drop is" that's not DRAW. You have to put the VOM INLINE with the + battery cable and change the setting to amperage to see true draw.
Yes, I know that measuring current requires in-line measurement, and that's how I did it: I disconnected the audio power cable, and put my ammeter in-line. What this means is simply that the actual average power draw for my system is much less than the peak rating, and that I'm only using 20-30W for loud listening levels. Of course, audio is also about accurate peaks, and maybe my ammeter wasn't seeing these. To a certain extent, the amps themselves filter the power peaks, due to their internal capacitors.

I agree that the quality of the cable is very important; both resistance and inductance can have a big effect on the minimum voltage seen by the amps.
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Old 04-18-2012, 05:31 PM   #8
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Rymar as an experiment you should totally put a 5amp fuse inline of your main power run to your amps... seeings as they only draw 2 or 3 amps you should be fine.. let me know how that goes...
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Old 04-18-2012, 06:01 PM   #9
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Thanks for the info about the 2012 having a different type of charging system than the previous years. I thought it was kind of odd that the alternator didn't go directly to the battery. I wasn't looking at where to tap the amp's wire into the system, since that is always on the battery post.
The point of a big three upgrade is to upgrade the entire path of electricity. So if I put bigger wire from the amp to the battery, and from the amp to ground, I'm missing the link between the ground and the battery (which is the three links between the alternator, chassis ground, and engine/transmission, AKA the big three.)
2-3A is way too small for any amp, so I think your Ammeter is not seeing the spikes or it is not calibrated correctly.

I have a Pioneer GM-5400T amp running 1x 4ohm 12" sub at RMS 380W, Peak of up to 760W. According to Ohms law, W=V*I, or I=W/V. W is watts, V is volts, and I is current in amperes. Watts divided by Volts will give you the current draw.

At my peak 760W on about 14 volts, I should see a draw of about 30+ish amps, at 380W RMS, I should see about 16 amps. This isn't about religion, it's about science and law (ohms law).

Regardless, in my previous car, my lights would dim with the same amp and same power output. I figure if I am putting all this wire in the car for the new amp, I might as well do it right. I don't want the car trying to suck a milkshake through a coffee straw and passing out.

I guess I am going to dig around the engine bay and under the car to see if I can find out where the engine/trans to chassis ground is, and if the charging system is different, I want to see if I just need to upgrade the wire from the alternator to fuse box, and the wire from the fuse box to the battery.

I've got 30ft of high-strand count audio power 4 ga wire coming, so hopefully that's enough to run to the amp and the Big 3.

Also, to attempt for this to be a resource for other 2012+ 2.0i owners, lets try to keep this about how to do the Big Three, rather than why.
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Old 04-19-2012, 01:24 AM   #10
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After some time on the12volt.com on their Hot-Topic Thread: "How to upgrade the Big Three" it seems the expert consensus is that it is ok to add a wire straight from the alternator to the battery positive but recommended to fuse the cable (to protect the cable, rather than anything else.)

Also, the expert opinion was that the ground cable should run from alternator case to chassis ground, and alternator case to negative battery post since the alternator case is the final ground on the vehicle.

Here's the specific post from a PM where someone asked about a Subaru (in 2006):

Quote:
Originally Posted by the12volt.com
Sent by : DYohn
Sent : December 26, 2006 at 6:07 PM
Sure. Glad to help.

Is it OK to just piggyback an addt'l 4ga from the alt to the battery+ (skipping the relay box)?

Yes, this is a good idea. Make it as short as possible but be sure to leave slack for engine movement.

Next, there's an existing 8ga battery ground to engine, but the engine connection is buried under an intercooler. OK to ground to another engine location?

Yes, it's OK. In fact, if you can tap into one of the alternator mounting bolts this would be best.

Last - can't find any definitive engine/battery to frame ground line. OK to start a new one? Am I creating ground loop points?

Take the same connection at the alternator bolt mentioned above and run it to the car's frame. No worries about ground loops.

Good luck!
I was thinking I would do what it says in the post above; add a wire from the battery negative post to the alternator case, and add a wire from the alternator case to the chassis to complete the circuit. I just have to look to see if there is a good place to mount the ground cable to the alternator (maybe just the bolt holding it to the engine) and if there is a good place to connect to chassis for the ground without having to drill a new hole somewhere.

I will add pics when I get it all done, but need to get a fuse for the alternator wire first.
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Old 04-21-2012, 11:30 AM   #11
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I also did the big three. I used zero gauge, one wire from the alternator to the positive terminal (fused). One from the negative terminal to the strut tower, and one from the engine block to the negative terminal. Instantly solved my dimming issues.





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Old 04-21-2012, 01:17 PM   #12
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I love it. I'll take pictures of mine and post em up. Nice pictures RSXNTN

How do you like your Kinetik battery? I'm currently running a Stinger and it fairs ok. Sorry off topic

Last edited by RSXtoWRX; 04-21-2012 at 01:25 PM.
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Old 04-21-2012, 03:41 PM   #13
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Love it! If I had to do it again I would have gone with the 1600 rather than the 1800 though, its a little big for the space available.
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Old 04-22-2012, 09:16 PM   #14
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Quote:
Originally Posted by RSXNTN View Post
I also did the big three. I used zero gauge, one wire from the alternator to the positive terminal (fused). One from the negative terminal to the strut tower, and one from the engine block to the negative terminal. Instantly solved my dimming issues.





What size fuze did you use from alternator to battery (+) ??
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Old Yesterday, 08:46 AM   #15
Zabsolvo
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Definitely beating a dead post on this one... but is there better information on this anywhere else? (Really curious about where the OEM battery sensor(s) would go in a 2015+ WRX.)
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