Calling our Pontiac G8 GXP a “grocery getter” would be a gross underestimation. Rolling off the factory floor with the Dodge Charger SRT8-devouring LS3, a six-speed manual gearbox and – least we forget – the bevy of aftermarket go-fast goodies we’ve installed over the course of two years, this silver four-door Poncho is definitely no slouch. We’ve already warmed over the engine with a Rotofab intake, and Corsa’s dual rear exit exhaust with twin 3-inch Pro Series tips and we ramped up the sedan’s handling prowess with Whiteline suspension gear, Forgeline rollers and Nitto rubber. While we were liking our Pontiac’s cat-back system, we weren’t loving the cast iron exhaust manifolds and air-restricting catalytic converters.

We’re just going to say it: stock exhaust manifolds are bulky, restrictive cast iron boat anchors. Even in the rare-instance that your particular ride came with “factory headers,” chances are those are rough-cast finished manifolds that can’t flow the air compared to a decent set of aftermarket long tube headers. Cast manifolds (and most factory headers) simply don’t feature the maximum-performance mandrel-bent 304 stainless steel tubing and longitudinal geometry that will allow the engine exhaust to flow quickly and freely. More importantly, that higher flow rating will unlock far more horsepower and better throttle response all while providing a cleaner look and a crisper exhaust sound.

Notice the difference between the factory manifold and the new headers. Our AMR kit (PT# G8GT78WC) will clearly flow much better.

Given the benefits of an upgraded exhaust system, we chose to install American Racing Headers’ long tube header kit (G8GT78WC) which includes a set of high-flow 200-cell metallic substrate cats on our LS3-powered Pontiac G8. The kit comes with everything required to plumb a 3-inch stainless steel system from the engine clear back past the rear axle to the existing mufflers without the need for cutting or welding. Given the fact that the back half of our exhaust system has already been upgraded, we’ll be meeting the existing system in front of the rear axle.

American Racing Headers
2008-2009 Pontiac G8 GXP System

• PT# G8GT78WC
• Header: 1-7/8-inch x 3-inch tubing, 3/8-inch thick stainless steel flanges with hand ported inlets
• Tubing: 3-inch X-Pipe and connecting pipes with high-flow 200-cell metallic substrate catalytic converters

We talked to Nick Filippides at American Racing Headers who explained, “The ARH G8 system was designed to do two things: first, provide a system you’ll never have to replace and second, to eliminate what’s arguably the most restricted factory exhaust [manifolds] we’ve ever seen. Combine that with all the great features the ARH system has, and you have power increases that exceed 30 horsepower!” Even though Nick knew our particular GXP already had a Corsa cat-back system, suggested those with stock exhaust, “run the system through the factory mufflers first. The system sounds great through the factory mufflers and unlike the rest of the factory system, they are not restrictive.”

American Racing Header’s G8 long tube header and high-flow cat kit includes everything needed for hassle-free installation.

Out With the Old…

Before we could remove and replace the factory manifolds with the new headers, we began with the removal of the old system. First, we started by disconnecting all of the sensors from the tubing bungs. Next, we removed the rubber support straps from the system to ensure no hindrances during removal. We loosened the exhaust flanges at both ends of the X-pipe and dropped the mid-pipe from the vehicle.

The factory oxygen sensors were first to be disconnected from the existing exhaust system before removal.

The rubber support straps help support the weight of the exhaust system and are easily removed. We suggest using a spray lubricant to help slide them out.

After the exhaust flange bolts are loosened and free, the X-pipe (or crossover tube) can be removed. A rubber mallet can be your best friend here.

The engineering of the G8 shines in the removal of the stock manifolds. Needing to loosen the exhaust manifold bolts from the cylinder heads, we typically run into all sorts of clearance issues. Such was not the case with the G8. Rather, we dropped the old cats and manifolds through the bottom of the engine bay – completing the disassembly process – with little to no problem.

After the manifold bolts are free from the cylinder head, the exhaust manifolds and old CATs can be removed.

Installation Preparation

To fit the new AMR long-tube headers into the engine bay, some minor adjustments had to be made. AMR requires that the battery cables be rerouted between the driver’s side front wheel well and the aluminum A/C condenser lines. Additionally, the steering shaft was temporarily removed from the steering box in order to ease header installation. Not nearly as easy a job as removing the manifolds…

A simple modification, the battery cables must be rerouted behind the A/C condenser lines.

The steering shaft needs to be removed from the steering box to install the headers. Remember to reconnect this after installation.

It’s important to clean the gasket surface on the cylinder heads from any debris. We suggest some brake cleaner (that stuff cleans anything and maybe a wire brush if necessary). Once the preparation is completed, the vehicle is ready to accept the installation of the new long-tube headers.

Our new set of AMR headers are ready to go in our G8.

…In With the New

With the proper clearances made, the headers were clear to slide upwards into position. Keeping them in place, we snugged them up the cylinder heads loosely with the outermost flange bolts. Before tightening anything down, we “trial fit” the entire kit into the vehicle. This might sound like extra work, but pre-assembly is a key step in properly modifying your car for nearly every facet. Following suit, the X-pipe assembly and high-flow cats slid onto the ends of the headers with a little motivation from a rubber hammer (See, what did we tell you? – Ed.). We connected the rubber support straps to the assembly and fit the remaining tubing into the car.

The headers will fit nicely in the empty spaces once filled by the old manifolds.

With the headers still loose, the mid-pipe assembly can be hung in place with the assistance of the rubber support straps. Keeping the system together with only “snug” bolts allows you to tweak and adjust your exhaust system for the best fit imaginable.

As stated previously, we were planning to butt our new AMR system to our previously-installed Corsa cat-back system. Meeting just in front of the rear tires, the two aftermarket system required a slight modification to the extension tubes to complete the joint. With the exhaust assembly hanging in the car loosely, we were able to fit the modified piece of tubing into place.

The entire exhaust kit can be assembled without any cutting or welding.

Returning back to our cylinder heads, we slipped the header gaskets between the two outermost header bolts and proceeded to tighten the header bolts evenly across the flanges until they torqued to the factory 18 ft-lbs with a small amount of anti-seize on the threads.

After the gaskets are in place, the headers can be torqued to the heads at 18 ft-lbs.

Once the headers were tight, we began tightening the exhaust flanges and clamps. The O2 sensors were connected to the appropriate bosses on the headers before doing a once-over to ensure that everything is connected properly, tightened correctly, and out of danger of the extreme heat. Once everything passed inspection, we started the car and allowed the engine to get up to temperature. Once there was heat in the engine, we shut it off and readjusted the header bolts.

After all of the exhaust components are in place, the C-clamps can be tightened down.

The oxygen sensors can be plugged in after assembly.

Carrying a Tune

Even with the smallest upgrade – such as aftermarket exhaust – your car can benefit from a custom tune for optimum performance. In our case, the G8 already had a tune prior to the header install, but after our upgrade, we needed to rewind the clock and reset the computer back to the factory defaults so we could custom tune the vehicle properly.

For that, we contacted HP Tuners. Known for their lightning-fast VCM Suite packages for late model GM and Ford vehicles, the VCM Suite is one of the most comprehensive program editors. And since its designed to be easy enough for the home enthusiast and as powerful and inclusive as possible for the professional tuner shop, it was a no-brainer. So as Ryne Cunningham – the owner of Cunningham Motorsports – arrived at our shop, he immediately plugged into the car’s OBDII port entering its computer system and downloaded the stock tune file to his laptop computer.

Ryne’s HP Tuner-equipped laptop was plugged into a car’s diagnostic port.

In addition to modifying the Pontiac’s automatic cooling fan timing, Ryne also turned off the CAGS. Otherwise known as Computer Aided Gear Selection, CAGS was initially designed for the EPA to give vehicles the ability to be more fuel efficient. Although fuel efficiency is great, performance is often sacrificed. However, when a tuner disables the CAGS, the computer allows our car better acceleration.

A closeup of the HP Tuner software shows the different edit options within the vehicle computer.

Street tuning is critical to the overall tune. This technique puts the car in its most natural state, making frequent stops and maintaining a cruising speed between 30-50 mph. After a ten minute drive, Cunningham pulled to the side of the road, revised the fuel delivery tables, and within five minutes of down time, he was back on the road. After another ten minute drive, the G8 pulled into the shop where it was strapped to the dyno for the final tuning stages.

Street driving in typical city conditions is a great way to develop a good base tune.

Street tuning allows you to set up the computer for typical city conditions, but the best way to adjust the fuel table for freeway speeds and a track environment is to make pulls on a chassis dyno. By the time the vehicle is ready for the dyno, most of the setup is already configured. While the previous SCT Tune improved our G8’s rich factory tune, Ryne was able to massage the current AFR even more than before thanks to our free-flowing AMR headers. Even before we rolled the Pontiac up on the dyno, Ryne pretty had the sedan dialed. With just a handful of pulls, he was able to tune the car to perfection, giving our new American Racing Header-equipped G8 a 29 horsepower and 19 ft. lbs. of torque gain. Not too bad for a couple hours worth of work, huh?

Our final dyno run showed a final rear wheel horsepower rating of 409; a 29 horse improvement over the previous 380 and an impressive 19 foot pounds of torque increase from 383.

Get In, Strap Down and Shut Up

Originally, our slightly modified LS3 engine pumped out 380rwhp with a somewhat stock exhaust system (factory manifolds, tubing, catalytic converters and Corsa’s cat-back system). After our upgrade to American Racing Headers’ long tube headers and high-flow cat kit, we inherited a final horsepower rating of 409 and 402 ft. lbs. of torque at the rear wheels. During our road test, we experienced a crisper throttle response, higher torque curve, and a more pronounced exhaust tone that had our signal light neighbors quivering in their sneakers. Is this Pontiac a “grocery getter?” Yeah, maybe. But we suppose it all depends on how big of a rush you’re in.