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GXP with dual mode NPP exhaust (pics)

Old 10-12-2018, 07:50 PM
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Default GXP with dual mode NPP exhaust (pics)

I'm not new to the world of W-bodies, but this is my first project on an LS4 car; my 2008 Grand Prix GXP.

Given that the GXP is a kid hauler and "fun" car for me, I didn't want a loud-all-the-time exhaust. That being said the stock mufflers were just depressing, so I went hunting for cutouts and ended up finding a 2015 Camaro NPP exhaust system on Craigslist for $100. Here are the new mufflers in their factory configuration (right) next to the 115k mile GXP cat-back (left).

The dual-mode mufflers have a butterfly in one tailpipe that blocks a straight thru pipe, when closed all exhaust gas is routed through the muffler internal baffling. When open, exhaust is free to flow straight through the muffler with minimal baffling. The system was cheap so I took a chance and bought it on the hope that I could adapt the mufflers to my cat-back exhaust. Camaro mufflers are angled, unlike the W-body straight design, and the tips are closer to the muffler which creates some challenges with hanging the exhaust. Some mods to the Camaro mufflers were required, so I thought that I would write it down for anyone who might be interested in doing the same. I am also interested to see how other people have address the dual-mode problem!

Anyway, here we go.

The first thing that I did was to remove the exhaust system from the cat back and build a jig to locate the tail pipes relative to the mid-pipe. Once that was done I could cut the stock Y and mufflers off knowing that I had a hard point for the mid-pipe and tips:

Then it was just a matter of installing my new Y (Magnaflow 10768 2.5 single into 2.5 dual, $30 on Amazon) and mufflers in their respective positions, and sorting through my box of leftover 2.5" bends to find something that would bridge the gap:

I used an exhaust tubing expander to slip fit a few of the joints so that I could slide things in and out a little bit once the exhaust was on the car. At this point I tacked everything in place with a MIG welder and installed the system on the car. Immediately I noticed that the stock muffler hanger locations wouldn't work, so I removed them. After a little test fitting, and with the goal of modifying the mufflers and car as little as possible, I settled on reversing the inboard hangers to the inside of the crash structure and replacing the outboard hangers with a long bolt scavenged from the parts pile. Here are a few pictures, sorry that I didn't think to drop the exhaust down to record the final location of the muffler hangers in more detail:

This shot also shows one of the normally open spring return vacuum actuators that controls the butterfly valves. It is zip-tied in the "quiet" or vacuum ON position, since at this time I didn't have the vacuum system plumbed.

Overall, with a little tweaking and a lot of fussing over the exhaust tip placement, the mufflers fit just fine under a w-body without any cutting:

I finished the system off by purchasing a MAC 12V 3-port solenoid valve from e-bay (often used as a boost controller with a PWM input) for $25, and some tubing and wiring for another $25 to plumb vacuum from the EVAP line on the engine back to the two vacuum actuators. I chose to configure the valve to be normally open (vacuum on), then when 12V is applied the vacuum signal is removed and the actuators vented to open the butterflies. I think that the stock Camaro system might be the opposite, but I'm not sure. For $200 one could instead purchase an "NPP in a box" system that includes wiring and a solenoid valve designed to retrofit dual-mode mufflers to cars that were not optioned as such. That might save you a little time, but I wanted the whole project to be < $200 total. In addition to the lines and valve I added a vacuum reservoir scavenged from another car to place inline near the valve on the vacuum supply side to improve response time. Not sure if this was needed or not because I haven't tested the system without it.

Overall I'm pretty happy with the result. In "quiet" mode the exhaust is quite a bit louder than stock, and in "loud" mode it is a little bit louder yet. I was hoping for a wider range between open and closed, but for the price I can't complain. I will say that the car sounds pretty awful in 4-cylinder mode, I had to disable AFM with HPTuners before I was happy with the new exhaust note.

Thanks for letting me share,

Lysander is offline  
Old 10-12-2018, 08:04 PM
Staging Lane
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Here's a video
of the finished product.
Lysander is offline  
Old 10-13-2018, 11:31 AM
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Fantastic work, very creative fab skills.
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