Installing a Borla Cat-Back Exhaust on Your GTO: A Detailed Guide for a 15+ Horsepower Increase

Upgrading the exhaust system on your Pontiac GTO is one of the most effective bolt-on modifications you can make. A cat-back system like Borla’s not only frees up engine breathing but also adds an aggressive, deep note that matches the car’s muscle-car heritage. This guide covers every step of the installation process, explains why a Borla system delivers a genuine 15+ horsepower gain, and includes tips to ensure a leak-free, lasting fit. Whether you’re a seasoned DIYer or a first-time modifier, careful attention to detail will let you enjoy both the sound and the performance boost.

Why Choose a Borla Cat-Back for Your GTO?

Borla’s cat-back exhaust systems are known for their T-304 stainless steel construction, mandrel-bent tubing, and patented Multi-Core™ muffler technology. On the 2004–2006 GTO (LS1 and LS2 models), the restrictive factory exhaust creates a bottleneck. Swapping to a Borla reduces backpressure, improves exhaust flow, and can add 15–20 horsepower at the wheels with no other changes. The sound profile is deep at idle, aggressive under throttle, and drone-free on the highway when paired with Borla’s resonator options.

For reference, independent dyno tests on the 2005 GTO (LS2) show an average gain of 16 horsepower and 18 lb-ft of torque from a cat-back system alone. These gains are consistent across Borla’s Touring and S-Type cat-back kits. The Borla website provides specific part numbers for your GTO model year.

Tools and Materials Needed

Gather everything before lifting the car. A complete installation can be done in 2–4 hours with the right tools.

  • Borla cat-back exhaust kit (includes muffler, tailpipe sections, clamps, and hangers)
  • 1/2-inch drive socket set (10mm, 13mm, 15mm, 18mm)
  • 1/2-inch drive torque wrench (capable of 30–80 ft-lbs)
  • Floor jack and two jack stands (at least 3-ton capacity)
  • Penetrating lubricant spray (e.g., WD-40 or PB Blaster)
  • Flat-blade pry bar or exhaust hanger removal tool
  • Safety glasses and heavy-duty work gloves
  • Rubber mallet (for aligning stubborn hangers)
  • Wire brush or abrasive pad (for cleaning corroded bolt threads)
  • Floor jack and wheel chocks (front wheels)

Optional but recommended: new OEM-style exhaust gaskets for the catalytic converter outlet flange and a tube of anti-seize compound for the threads.

Preparation Steps

Park the GTO on a flat, level concrete surface. If working on asphalt, use a thick plywood board under the jack to prevent sinking. Disconnect the negative battery terminal to avoid accidental shorts while working near the oxygen sensors. Let the exhaust system cool completely if the car has been run recently – hot exhaust parts can cause severe burns.

Lay the Borla kit on the ground and confirm all pieces are present: muffler section, intermediate pipe, tailpipe(s), clamps (U-bolt or band style), and rubber isolators. Compare hardware to the instruction sheet. Some later GTO models (late 2005–2006) have different flange configurations, so double-check the included parts.

Step 1: Raise the Vehicle and Provide Access

Use the floor jack at the rear differential or the factory rear jacking point to lift the car. Position jack stands under the rear frame rails, just behind the rear axle. Ensure both stands are at the same height and the car is stable. Chock the front wheels. The exhaust system should be high enough off the ground for you to comfortably work underneath (approximately 18 inches of clearance).

Step 2: Remove the Stock Exhaust System

Start by spraying penetrating lubricant on all nuts and bolts you will loosen: the two bolts at the catalytic converter flange, the two nuts at the rear muffler hanger brackets, and the bolts securing the rubber hangers to the chassis (if equipped). Allow the lubricant to soak in for 5 minutes.

Important: The factory exhaust is heavy and bulky. Have a helper or a transmission jack ready to support the rear section once unbolted.

2a. Disconnect the Muffler Section and Tailpipe

Using a 15mm socket, loosen the two bolts on the flange connecting the intermediate pipe to the catalytic converter. Do not remove them fully yet – leave one turn of thread engaged to prevent the pipe from falling. Next, locate the rubber isolators that hold the muffler hangers. Use the pry bar to gently push each hanger out of its rubber mount. If they are tight, spray more lubricant. Once all hangers are free, remove the final two bolts at the converter flange. Slide the stock muffler and tailpipe assembly out from under the car. This assembly can weigh about 35–40 lbs, so be careful not to strain your back.

2b. Remove the Intermediate Pipe (if separate)

On some GTOs, the cat-back system includes a separate intermediate pipe between the converter and muffler. If your kit requires it, unbolt the clamp at the rear axle and remove the intermediate pipe. Clean the converter outlet flange with a wire brush to remove any rust or old gasket material.

Step 3: Install the Borla Cat-Back System

Borla systems are designed to be a direct fit with minimal adjustment. But because aftermarket parts have small tolerances, you’ll need to align everything loosely before tightening.

3a. Test Fit the Tailpipe and Muffler Sections

Start by positioning the new tailpipe section along the factory route, sliding the hanger rods into the rubber isolators. Do not fully seat them yet – just enough to hold the pipe in place. Then lift the muffler section into position and attach it to the intermediate pipe (if included) or directly to the converter flange. Use the supplied gasket between the flange surfaces. Borla includes a new gasket, but if the kit uses a flat flange, you may reuse the stock metal gasket if it’s in good condition.

Tip: Apply a small amount of anti-seize to the flange bolts to prevent future corrosion.

3b. Secure the Connections

Hand-tighten all bolts and clamps. For a slip-joint connection (common on Borla’s Touring systems), slide the pipes together until they are fully seated, then tighten the band clamp. For flanged connections, tighten the two bolts evenly to avoid warping the flange. Do not final-torque anything yet. You want the system to be movable so you can adjust clearances to the underbody, axle, and heat shields.

Step 4: Align and Final-Torque All Fasteners

With all sections loosely connected, check the following clearances:

  • The tailpipe should not touch the rear bumper cover or the spare tire well.
  • There should be at least 1/2 inch clearance between the exhaust and the rear axle half-shafts.
  • The muffler case should not contact the fuel tank straps or the lower control arm.
  • All hanger rods should be vertical, not twisted, and fully engaged into the rubber isolators.

Once you’re satisfied, use the torque wrench to tighten the flange bolts to the manufacturer’s spec (typically 30–40 ft-lbs for the converter flange, 15–20 ft-lbs for the clamp bolts). Borla’s instructions give exact values. If you misplaced the manual, Borla’s support page has downloadable PDFs for each part number.

After torquing, go back and double-check that all rubber isolators are fully seated on the hanger rods – a partially seated hanger can cause exhaust rattle under acceleration.

Step 5: Check for Exhaust Leaks

Reconnect the battery negative terminal. Start the engine and let it idle for at least 2 minutes to allow the system to heat up and expand. While the car is running, slide underneath (with the engine off if you prefer safety, but a running check is acceptable with caution) and listen for hissing or ticking at each joint. Pass your hand near the connections to feel for escaping air. If you smell raw exhaust inside the cabin, you have a leak.

Small leaks at flange gaskets can often be fixed by tightening the bolts another 5 ft-lbs. For slip-joint leaks, loosen the band clamp, apply a thin bead of high-temp silicone exhaust sealer, and re-tighten. Avoid over-tightening, which can strip threads or crack the flanges.

Pro tip: Use a length of vacuum hose held to your ear like a stethoscope to pinpoint leak sounds more precisely.

Step 6: Lower the Vehicle and Perform a Final Inspection

After confirming no leaks, turn off the engine and let it cool for a few minutes. Carefully raise the car with the jack, remove the jack stands, and lower it to the ground. Perform a visual inspection under the car from the front to rear – look for any exhaust parts that may be contacting the underbody when the suspension settles. If the car has been lowered, the tailpipe may need a slight adjustment to clear the rear valance.

Step 7: Test Drive and Evaluate Performance

Take the GTO for a 15–20 minute drive that includes city streets and highway speeds. Pay attention to several factors:

  • Sound: Borla’s S-Type system is notably louder under throttle but civil at cruise. The Touring system is more mellow. Listen for any drone at 60–70 mph. If drone is excessive, consider Borla’s optional resonated X-pipe.
  • Throttle response: You should notice a quicker rev-up and a more eager pull from 2,500 rpm onward.
  • Check for rattles: If you hear metallic tapping over bumps, an exhaust section is likely contacting the chassis. Adjust the hangers or add a small shim between the pipe and the heat shield.

After the test drive, park on a clean surface and inspect for any oil or fluid leaks (unlikely, but always wise). Recheck the torque on the flange bolts after the first heat cycle – a 24-hour re-torque is recommended by many tuners.

Expected Horsepower and Sound Characteristics

The 15+ horsepower gain from a Borla cat-back is realized across the entire power band, not just at peak. On a 2005 LS2 GTO, many owners report a 12–16 whp gain with no tune. Adding a cold air intake and a custom PCM tune can push the total gain to over 25 hp. The sound is deep and throaty, with a distinctive Borla “bark” at wide-open throttle. For those who want the loudest option, the Borla ATAK cat-back (if available for your year) offers an even more aggressive tone.

If you are considering pairing the exhaust with headers, check with LS1Tech.com forums for firsthand install advice on your specific GTO year – some header brands require minor pipe modifications to the cat-back.

Maintenance and Long-term Care

T-304 stainless steel is highly corrosion-resistant, but road salt and moisture can still cause surface pitting over time. Wash the undercarriage regularly in winter climates. The polished tips (if equipped) can be cleaned with a non-abrasive metal polish. Check all clamps and hangers annually; the rubber isolators may dry-rot after 5–7 years and should be replaced to prevent sagging.

One common issue with cat-back systems on the GTO is the tailpipe hanger breaking due to metal fatigue. Borla offers replacement hanger rods separately, and many owners upgrade to universal polyurethane isolators for longer life.

Considerations for 2004 vs 2005–2006 Models

The 2004 GTO (LS1) has a slightly different exhaust routing and a different rear bumper cutout. Ensure the Borla kit is specified for your year. The 2004 uses a 2.75-inch intermediate pipe and a single tailpipe design; some later Borla kits include adapters. The 2005–2006 models have a larger exhaust tunnel and dual rear outlets. Speedway Motors stocks both versions with verified fitment.

Conclusion

Installing a Borla cat-back exhaust on your GTO is a rewarding weekend project that delivers a noticeable horsepower increase and transforms the car’s personality. By following this step-by-step guide – prepping the tools, removing the stock system carefully, loosely fitting the new components, aligning for clearance, and double-checking for leaks – you’ll achieve a professional-grade installation that performs reliably for years. The 15+ horsepower gain is not a myth; it’s the result of reduced backpressure and Borla’s engineering. Enjoy the deeper exhaust note, the added throttle response, and the satisfaction of having done the work yourself.

Disclaimer: Always follow the specific instructions included with your Borla kit, as part configurations vary. Torque specifications provided here are general; refer to manufacturer guidelines for your exact system. Modification may affect vehicle warranty and emissions compliance in some regions.