# Pontiac G8 GT owners...



## therapture (Jan 31, 2013)

I posted this in a PM to another member, thought it might go well here for Pontiac G8 owners and n00bs to SQ installs like me.



Well, I read a test somewhere on one of my G8 forums about the OEM head unit, it's signal output is fairly clean to about 45 on the level, and actually does very well when you feed the output to a processor that flattens the built in level based EQ. I wanted stock looks and HVAC functionality so I use the OEM HU, and will not change so I make the most of what I have.

Originally I used an MTX REQ5 to accept the speaker level inputs and output to RCA's for the amps and it did well but was terribly limited in scope and tenability.

I wanted more control so I went with a DSP and here is what I ended up with:

OEM HU feeds my Rockford 360.3 DSP with just the 2 front channels of input. The 360.3 creates up to 8 channels of output, I use only 3 right now (front left, right, and sub). I spliced into the stock HU speaker wires that are located on the right side of the unit, by removing the glovebox so you can see the right side harness where it plugs into the HU. Ran the wiring back to the 360.3 located in the trunk., and new speaker wire back from the amp to the doors.

The 360.3 feeds my Alpine PDX-V9 5-channel amplifier, fully active.

Doors have Hybrid Audio Imagine 6.5" drivers run in co-ax mode, looks stock from inside. However, I have extensively sealed and deadened the doors, created a custom 3/4" thick MDF baffle to hold the driver and space it away from the power window. The plastic door trim has had mat added everywhere inside it, as well as silicone in several places to stop the damn thing from resonating and vibrating on heavy midbass. I spent more time on the doors than anything else in the system! This has resulted in very strong midbass output that can vibrate the mirrors a bit and actually sounds very good even without the sub turned on.









The roof on the G8 is a giant metal tin can that resonates, rings, shakes, vibrates, and acts like it's own speaker! I dropped the headliner and went to town on it, covering all flat surfaces with mat (GTMat Onyx) to control it. I also added small amounts of expanding foam between the support rails that run across the roof, between them and the outer panel, to stop vibration there. While you are at it, take the map lamps in the headliner and silicone the metal clips that attach it because they rattle like crazy, add matting to the back of it. It matters!

Before roof deadening









AFTER rood deadening, see how much lower the resonance is?









The deck lid needs some attention! Add matting to as much metal as you can by removing the carpet trim, reach into the holes and cover as much flat as you can. I also did the expanding foam into the cavities as well, but be CAREFUL, fill a MAX of 50%, it will expand too far and warp the metal if you fill the entire gap! Ask me how I know...lol. Better to under fill and touch up a second pass if need be. This resulted in a DEAD knock on the deck lid, no vibrations from it.

The Infinity Kappa 120.9 subwoofer is mounted into a 1.44ft³ net sealed enclosure that I made into the spare tire well. Being that the tub is made of plastic, it needs some matting added to it to stop resonance. Cover all of it, pretty easy. I cut the trim that covers the trunk floor to match the opening so the sub can be seen, and I added a grill to stop trunk cargo from contacting the sub. Cover as much of the trunk metal as you can to stop resonance, including the metal floor and the 1/4 panels inside.









If you go with sub(s) that are not in the spare well, the best response I had was with the sub at the front of the trunk, firing backwards. I did not want to give up trunk space (family of 4, we drive the G8 out of town on trips) so I went into the spare well.

In the trunk, the stock 8" holes in the rear deck are empty, allowing sound to pass into the cabin. Very noticeable vs. having them covered, since the rear seat baffle is metal and blocks ALOT of sound. Speaking of that, go to town on the top AND bottom of the rear deck AND the rear seat baffle. Those two large metal pieces add considerable vibration with a subwoofer cooking them! The rear deck trim comes out easily so you can access the top, cover as much as you can. The various onstar boxes, etc., rattle as well, tighten them up, mat them, damp them down.

The G8 is notoriously bad on alternator whine, however, it can be fixed easily. Amp is mounted on MDF on the back seat baffle in the trunk so it is only connected to the chassis via it's power and ground. I grounded it to the rear seat belt mount, behind the seat, keep it as short as you can. I grounded the DSP to the same place as the amp, no ground loops! Keep signal wire as separated from power as possible, cross them if you have to at 90 degree paths, never run alongside each other (you prob know this already). At the battery, make sure you have the cleanest connection possible, and keep the mounts tight, if they get loose, you get whine. I also run a fuse block right off the battery. In your engine compartment, make sure the ground straps are clean and tight, as well as the hot wires that connect up there. The hot wire runs up the left side, so keep speaker wires to the front away from that side (mine are all on the right, front to back). I did not do the "big three" yet, I just made sure everything is tight and clean, and I suffer from zero induced whine.

With some tuning help from here, this system sounds fantastic!


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## pjc (May 3, 2009)

I'll pass this on to my brother. He's a very proud G8 GT owner. Good info.


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## Instaburn (Aug 22, 2013)

Thank you for the reply. 
The G8 is due up for a nice system now that im driving so much more for work.
Here is the plan.

Soundstream 500.4 for the interior.
PPI 2350 for the subs.

I want to go with components up front and midbass drivers in the rear doors. So its basically a 3 way stereo plus subs.

I have the amps Only at this point and am doing as much research on DSP units, subs, midbass drivers and components as possible.

Leaning towards the 3sixty.3 DSP and the IDMAX12 v4 12's at this point. 
Undecided on the interior speaker components.
I like my systems loud and clean.

I previously had kappa perfects and loved their bright sound.... but wonder if there is something else I should consider.

Advise is greatly appreciated. 

I also plan to swap in the Caprice PPV alternator for the massive amounts of current my system will draw.
PPV alt = 170Amps
Stock alt = 100amps

Sent from my SPH-L720 using Tapatalk now Free


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## bigbegum (Mar 30, 2008)

Thanks for this info. I was wondering what kind of output the oem G8 headunit sent to the factory amp. I plan on running a bit10 in my g8. This helped a lot. Thanks!


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## Instaburn (Aug 22, 2013)

Hey.. looky here... way back in 2013 I was still contemplating my system..
Funny how things change...


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