2005 Pontiac GTO - Long Overdue Refresh

So, time to break out REW, as I have experience with it from tuning my home theater as well as live venues, since as a musician, I've often times had to setup live sound .... but, my rig for HT and live sound is a Tascam interface with a phantom power microphone, with all the signal and power requirements that come along with that ... and man, that's just a lot to manage for tuning the car.

Then I find this cool Dayton Audio iMM-6 calibration mic, that'll work with my phone and the AudioTools app ... sure, that'll be enough

So I pick one up, along with a lightening to 1/8" TRRS adapter, and an extension cable ... NOW I can get that clarity I'm looking for, right?



Armed with the measurement setup and the 80PRS capabilities, I get to tuning ... and man, the system takes another leap forward in terms of clarity.



But am I now completely satisfied?? Naa, of course not! It must be the cheap Toro amp, right? I mean, it sounds better than the setup when I had the old PPI and RF amps in play ... but that must be due to the Hertz speakers, the sound deadening, and the power upgrades, right?

So I start thinking way down the road ... What if i were to replace the 80PRS with a nice modern touch screen, with backup cam, navigation, wireless carplay, etc, etc ... But then I'd need to find a HU with all those features, which can also match the SQ capabilities of the 80PRS.

And then I ran into the world of DSPs ... Nice! With one of these, I can offload all the SQ processing out of the headunit, freeing up my HU selection to focus more on the usability features and less on the SQ capabilities.

That leads me into more reading, more research .. and wouldn't you know it, there are DSPs which also have integrated amps! Whoah, ok ... so now I can not only address the future scalability of the HU, but I can also get the cheaper Toro amp out of there at the same time ... and, now that I'm thinking long term, it sure would be nice to reclaim my trunk space so I can actually fit my guitar and gear in there when I need to drive the GTO to an event...

Does the rabbit hole even have a bottom??

So all of this thought process leads me to a Helix V-Eight DSP MK2 ... nice, this thing not only has 10 channels of DSP capability from one of the leaders in the space ... but also has an integrated 8 channel amplifier built in. And ... it'll fit in the same place where the GTO's OEM amp was mounted for the stock 6" subs which came in the rear deck.

But, I need a way to control it from the driver's seat ... Hmm, the URC, the Helix Director, the Helix Conductor, or WiFi control ... more reading, more research ... I decide the Conductor is the way to go for me. and I rip the system apart yet again ...

To mount it, I attach two strips of wood to the bottom, with 4 philips head screws exposed for mounting to the OEM location




Yep, perfect fit! Everything wired up, cables ran, and it's a nice clean install in an out-of-the-way location, perfect!




So the conductor, hmmm ... There's this perfect place next to my window switches, but it's a self contained module. Ahh, but that doesn't stop me ... I pull the window switch module out, and hack it open and start modifying. It really is a perfect fit!





So now I need a way to tune the system easily, and opening the trunk, pulling back the panel, and plugging a USB cable in isn't really easy.

To solve this, I ran a USB extension cable into the center console, and installed a USB jack for connecting my laptop.




For the power, I ended up relocating everything to be nicely hidden behind the trunk panels.



The 2ga power from the battery comes through the hole at the top left of this picture, then there's a 3-way distribution box hidden by the fuel filler hose, which splits the power out to the two circuit breakers. There is a very conveniently located channel across the entire back of the car, and you can see where I ran the cables for the Helix DSP through it.

Just a quick look at the underhood changes as well ... The blue line shows where I installed additional 2-awg tinned OFC ground from the battery to the chassis and the engine block ... and you can see the single fuse block now, with 2-awg tinned OFC power wire as well.




So now I need to tun this thing, but the little iMM-6 mic with AudioTools seems inadequate now LOL ... it probably would have been fine, but I really wanted a way to use REW ... which leads me to picking up a dayton UMM-6 USB mic.

Here is what I end up with after this phase ...







... and wow, after tuning, the system takes yet another leap forward in clarity!

But I'm not at the bottom of the rabbit hole yet, so why would I stop? Heck, I still can't even see the bottom from here!

After all, I have a 10 channel DSP now, with 8 amplified channels and 2 preouts .... and I still have 4 of the amplified channels going unused.

Hmm .... rear fill sound like a really good idea now ... and heck, this thing has the "Real Center" processing on it ... sure, that's what I need next ... some rear fills and a center.

Rear fill? Easy enough... I pick up the matching 6.5" coax Hertz Centos, and mount them up in the rear side panels.






So now for the center .... which is not so easy. The GTO doesn't have a center, and no options exists for a center ... but it does have this nifty top trim plate which seems like the perfect place to install one.






So I picked up an AudioFrog GS25, and got to modifying ...






And then I modified a parts express speaker bezel, and molded it to fit into the hole ... came out pretty good I think




then finally, a little but of routing setup ....



... and real center enabled



And I arrive at this:




... all I can say is, WOW! The system sounds really, really good after I went through a full tuning session.

So am I done? Nope ... I would still like to recover my trunk space, and have some plans to try and accomplish that. But that'll be an update for this thread at some point in the future.

for now, the system is sounding really, really great!

dmb1650 said:

Nice progression and pictures! I've always been a fan of the GTOs. Almost ended up with one on a trade deal 2 years ago but the owner backed out of the deal.

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Thanks, me too! A nice clean GTO is getting harder and harder to find these days. For a while, they got pretty cheap (like under $10k) and a lot were bought up and heavily modified. I've tried to keep mine "collectable" through the years, even when it was dedicated to track duty. I only did mods which were reversible so I could go back to OEM.

few35t said:

Nice thread, nice work

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Thanks, it's been a fun experience

mumbles said:

You had me at GTO! Wasn't '05 the first year for the LS2 or am I remembering wrong?

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LOL ... ya, 2004 had the LS1, and 2005/06 had the LS2. For me, the lack of hood scoops and dual exit exhaust made the 2004 a no-go when I was looking for one back in 2010. The 50 extra horses of the LS2 was just a bonus

The last dyno was 386 ft/lbs @ 4292rpm and 366hp @ 5931rpm at the wheels with all stock internals. Just a different air intake to make room for a laminova engine cooler (for road course use), some dyno tuning, and a removed catback ... since who needs the extra weight when it was a track car being trailered everywhere LOL!

It was definitely fun times, but the risk of body damage was too much, considering how much parts for these cars are going for now, if you can even find them. I lost a side mirror to some track debris one day on track, and it was over $300 for the mirror. I ended upprincing out different parts ($600 for a headlight!) and decided it was too costly to continue to run it.

AudioBob said:

Very nice work! I love your “Rabbit Hole” comments...are we ever done?

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Thanks! I'd say it's probably never done, although there does come a time when it's "done enough" LOL! I'm pretty satisfied with everything right now, except the loss of my trunk space and the dated head unit. I plan to replace the HU when these supply chain issues resolve and they become reasonably priced again ... and for the trunk space, I'm going to try IB 8's in the rear deck to see how they do, and then long term, put 10's behind the rear seat panels and move the rear fills to the rear deck.

kevski- said:

Nice work!! Can you comment on those Tang Bands you had on the rear deck?

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Thanks, and sure ... anything particular you'd like to know? The model # on them is Tang Band w6 1139sif. The specs are pretty impressive for a cheap 6.5" sub, and they were one of the very few which dropped right in to the rear deck location.

The issue with the GTO is the fuel tank is in the trunk, and is located right under the rear deck; so there's not a lot of room to work with. When these were in there, and being fed by the RF punch 250a.2, they worked well for the mid bass region. Of course, I didn't get much below 50hz or so with them installed like I had them. Not I didn't do any sealing of the rear deck or wall between the trunk and the cabin when I installed these either, so I wouldn't say it was a "good" IB install. At the time, I was just looking for something better than the OEM, which would drop right in.

Here's a pic from the trunk side, when they were installed. It's tough to tell, but I had to trim the fuel tank strap with a 4" grinder just to get these to fit in there.



To fit 8's in there, a fellow DIYMA member with a GTO designed some 3D printed adapters to replace the OEM 6.5" adapters.







The added height from the adapters provided the necessary clearance for these Cerwin Vega 8" subs to fit nicely



I also went through and sealed up the holes in the deck, rear wall, and other places where the trunk and cabin were venting. I'm waiting on the amp I chose to be delivered now, and will give them a go when it is. My hope is they'll be good enough to keep me satisfied with the 12" sealed box removed from the trunk, but we'll see.

Like I said, long term plan is to get custom boxes built to allow installation of 10" subs behind the rear seat side panels, as there's plenty of room in those locations.

Here's a pic I took before any sound deadening or rear speaker work, showing how much room is available .... I don't see any reason why a couple of 10's (one on each side) wouldn't work perfectly here, so long as I can build (or have built) the right enclosures for them.



I was thinking maybe ported enclosures, with the port exit lining up with the 6" hole where the OEM speaker grill is located? Or, sealed enclosures with the 10" woofer face overlapping with that grill ... not sure yet, as I'm still thinking through it.

My Jaguar has the gas tank behind the rear seat like your GTO and I never could get decent sounding bass in to the cabin from the trunk area so I gave up on that. It all started when the factory 6x9 "sub woofer" gave up the ghost as the foam surround crumbled. I ended up removing the stock sub, fabricated a new mounting ring and replaced it with a PowerBass 6x9 sub that does pretty decent for what it is. My car also has solid steel between the rear seat and the gas tank with a lot of sound insulation and I don't know if the GTO is like that or not?

I hope that in the end two 8" woofers IB works as I can't live without a trunk, need space for golf clubs. I think that if my Jag had two 6x9 subs it would be decent enough as I don't want to modify it to that degree either due to the nature of them getting harder to find in good condition.

AudioBob said:

My Jaguar has the gas tank behind the rear seat like your GTO and I never could get decent sounding bass in to the cabin from the trunk area so I gave up on that. It all started when the factory 6x9 "sub woofer" gave up the ghost as the foam surround crumbled. I ended up removing the stock sub, fabricated a new mounting ring and replaced it with a PowerBass 6x9 sub that does pretty decent for what it is. My car also has solid steel between the rear seat nd the gas tank with a lot of sound insulation and I don't know if the GTO is like that or not?

I hope that in the end two 8" woofers IB works as I can't live without a trunk, need space for golf clubs. I think that if my Jag had two 6x9 subs it would be decent enough as I don't want to modify it to that degree either due to the nature of them getting harder to find in good condition.

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Ahh, so you understand the dilemma

The 12" Rockford sealed sub does really well in the trunk, especially when I had the rear deck 6.5" holes open. Well, almost open ... I ended up making two circles out of open cell foam, with a thin layer of CCF on top and vinyl tape on the bottom. These were just to block the fuel pump noise from the cabin, and still allow the bass to pass through.



With the 8" subs in there now, I'm still getting good bass into the cabin. I do run it pretty maxed out, and crossed over at 63Hz, but it's very well balanced. I've been really impressed from this Rockford Fosgate P300-12 all-in-one setup

The GTO also has a steel wall between the trunk and cabin, but there's quite a lot of holes in it. The rear seat is very thick though, and does a decent job of insulating the cabin.




When I treated the interior, I 1st put Second Skin CLD on that wall, sealing up the holes in the process




Then I added a layer of mass loaded vinyl over the top




Then with the rear seats in place on top, it worked out really well.




This next pic gives you a sense of how thick the rear seats really are ... they're solid foam, covered with softer foam, then the leather.




All in all, the sound treatment completely transformed the driving experience. Maybe my age is showing, but out of everything I've done to this car, I enjoy the result of the sound treatment the most

The GTO system continues to progress ....

As stated earlier, I really want to reclaim my trunk space, and with hopes of doing so, had installed 8" subs in the rear deck but have been waiting on the amp to be delivered to power them. Well, the amp was finally delivered a few weeks back, and I had some time this past weekend to work on it.

The amp is an NVX XAD21, which is one of NVX's brand new X-Series line of amplifiers. These are 1-ohm stable amps, and the specs on it are impressive.

RMS Power Ratings:

• 4 ohms: 250 watts x 2 channels
• 2 ohms: 550 watts x 2 channels
• 1 ohm: 1000 watts x 2 channels
• Bridged, 4 ohms: 900 watts x 1 channels
• Bridged, 2 ohms: 1600 x 1 channel

The 8" subs are Cerwin Vega V82DV2 "Vega" series:

• 8" Dual 2 ohm Vega Series Subwoofer
• Switchable between 1 or 4 Ohm
• Power Handling:
  • Peak: 750 watts
  • RMS: 250 watts

Finding a place to mount the amp was kind of a pain, as I want everything hidden, and there's not a lot of spaces behind the trunk panels for mounting. I finally settled on this location, on the driver's side of the trunk. You can see the Helix V8 on the far left, and then the NVX amp as well.



I was able to fish the power cables through a channel which spans across the trunk area, which made it very convenient to keep the power cables separated from the signal cables.

And here's a look at the other side of the trunk, where the power cabling is located. You can see the power and ground wires feeding up, and to the left, into the channel I mentioned above ... heading over to the NVX amp. I decided to leave the wiring for the RF 12" sealed sub in place as well, so I can do some A/B testing between the two 8's and the single 12.



For connecting to the Helix, I decided to hook up both the 8's and the 12, and did so by taking advantage of the unused powered channel (Virtual Channel 'F') of the Helix. I moved the RCA preouts of the Helix from the RF 12" enclosure over to the NVX amp, and then connected the unused powered output of the helix to the RF 12" sub, and set it for High Level input.

So here is the end result of the connections:



And the power ....




I should note that prior to moving the RCAs, I took some RTA readings of the 12" sub performance 1st. Then when I connected the high level input to the 12" sub, I then adjusted the channel config to match the same performance. I did this so I could do the A/B comparison later.

To set the gain on the NVX amp, I disconnected the speaker leads from it, turned the HU up to 52, and played a 40Hz tone at -3dB ... then I used a DMM to set the gain to output 31.6v. So, 250w @ 4-ohm.

Then I reconnected the speaker leads, and setup two configs in the Helix ... one with the RF 12" sub active and the 8s muted ... and a second one with the 8s active and the 12" sub muted. Now I can easily change between the two configs using the Conductor remote to do some critical listening comparison.

I was running out of time by now but was able to take some initial readings.

Here is the REW RTA result with the 12" sub activated and the 8's muted ... playing pink noise through the system ... i.e. My baseline:



Here is the REW RTA result with the two 8's activated, and the 12" muted:



And here are the two overlaid on each other, for easy comparison:



I have to say, I am quite impressed with the result. I did not expect these Vega 8's to be able to match the performance of the sealed 12, but it's pretty darn close. Keep in mind, I did not do any EQ work after moving the RCA cables over to the NVX amp. It was late, and I was out of time to do any more ... so the EQ in the Helix on the sub channel, which I set up for the 12" sub, is what is feeding the 8's right now.

Seeing as the 8s are in the cabin, instead of in the trunk, I believe a proper EQ session will greatly improve the curve even further.

Before I finished for the night, I did listen to a bit of "Boston's Greatest Hits" CD, and it sounds really good! Like better than I expected. The 8s are much more musical than the 12 (which was completely expected), but they still dig down really low (the unexpected part).

I'll say I'm really pleased with the result thus far, and have no doubts I can remove the 12" sealed enclosure from the trunk now and be satisfied with the balance of the system.

That said, seeing how easy it is to create configs in the Helix, and recall them with the conductor ... I think I may try running both the 12" and the 8's at the same time, by configuring the 12" sub to handle say 30Hz and below and the 8's to handle 30Hz - 80Hz? It might be interesting to see how this does, just as an experiment. But in the end, I really want my trunk space back!

AudioBob said:

Thanks for sharing and I'm glad that you are happy and it is working out. I recently started building the system in my 2021 VW Atlas Cross Sport and am in the process of testing sub configurations and drivers. I ran the power, ground, signal from the head unit to my DSP and wires to both doors to either run active or passive depending on which route I end up going. I am currently running and 8" Image Dynamics ID8 v.3 in a small sealed box and it sounds really nice. It does not go as low as I would like, but I have a Memphis Mojo Mini 8 that I am going to try next in both sealed and ported boxes next and I know it will play much louder and lower.

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Good luck with your project. The Vega 8" subs I'm using seem to work really well in an IB config, especially in the GTO. I'm sure the cabin mode of this car has a lot to do with how low they're playing. Since you're using a box, and deciding between ported or sealed, I'd think your experience will be quite different than mine.

Up to this point, all of my tuning has been with my UMM-6 mic in a stationary position, at the driver's headrest, and using the Helix V8's general EQ bands.

The next thing I want to do is take measurements using the moving mic method, and then use REW to define PEQ settings, so I can use the parametric EQ in the Helix instead. I think this will greatly improve my overall system response, especially considering all the sound treatment I've done to the vehicle.

I just wish there were more hours in the day!

Another update to the evolving system .... I've noticed the Hertz Cento tweeters created some odd artifacts under certain conditions, and thought maybe it's some distortion by having them crossed at 2500Hz. I don't want to raise the crossover any higher though, as my front stage is really good at 2500Hz. When I raise the crossover point, the artifacts I'm hearing do go away, but the stage drops too low for my liking.

So I picked up a pair of Hertz Mille Legend 280.3 tweeters from a fellow member in the classified's section, to replace the Hertz Cento C26 with. These have an fs of 900Hz, compared to 1500Hz for the C26's. So at 2500Hz, the C26's are really struggling, but the 280.3's should be good, even down to 2000Hz I think.

1st off, these ML280.3's make the C26's look like cheap toys!





When I first opened the box, I got a little concerned they may not fit in the factory locations ... but they slid right in using the OEM mounting brackets. I still have the OEM tweeters tucked away, so out of curiosity, I pulled them out and measured them. They're right at 2", and the ML280.3's are 2.1". For reference, the C26's are 1.73"






I didn't change anything in the tune yet, but I did go for a 40 min drive after swapping them out ... and wow, what a difference! The artifacts I was hearing are gone, and the highs are much more crisp and clear. It's night and day from the Centos; Not that the Centos were bad, but in comparison ... well ... there just is no comparison. Listening to tracks I know well, I'm hearing things in the highs I didn't notice before. And vocals ... I don't know how to describe it ... it's like I can hear the breath of the singer now.

The difference is so great, it's making me wonder if I should consider swapping out the Cento C165 6.5" mids in the doors as well, but I don't think I could bring myself to pay what a set of ML-1650.3's go for.

Who knows, maybe another good deal will pop up?