Here is a good example of another flaw in the factory design: The exhaust Y-pipe is crushed for ease of installation. This creates a bottleneck in the system, restricting exhaust flow.
Doug Thorley has spent over 50 years designing tuned headers and exhaust systems. The company's Tri-Y exhaust systems are designed to maximize both torque and horsepower from the start of the rpm range up to 4,500 to 5,000 rpm. This is achieved due to the specific diameter and length of the primary and secondary tubing for each application. This allows for excellent exhaust flow from the engine through the cat back, creating just a little backpressure. Thorley believes that a little backpressure helps equalize exhaust pulses, which improves scavenging.
Doug Thorley produces numerous patented Jeep and SUV header and exhaust applications. We wanted to install the latest and greatest exhaust system on Project Grand Caddy since it has received its newly rebuilt 5.2L (318ci) powerplant.
The primary collectors meet at a secondary collector, which leads to the factory cat and new muffler.
The type of material used for both the flange and the tubing is carbon steel, grade C1008. The flange thickness is 3/8 inch, and the electric-welded tubing is 14-gauge (0.083). The primary tubing is 1-5/8-inch OD. The secondary tubing (14-gauge, 1-3/4-inch) will go into the final 2-1/2-inch collector. The headers are ceramic-coated inside and out with 0.3 mils of thickness inside and 0.5 mils of thickness on the outside. The ceramic coating is rated up to 1,200 degrees F and is said to retain up to 40 percent of the engine's heat, giving longer life to the engine, hoses, wiring, and other heat-sensitive items.
After installation of the Doug Thorley Tri-Y headers and exhaust system, dyno results have proven that the setup works by producing the additional horsepower and torque we were looking for. The system went into the vehicle without the headaches other systems often inflict, and we're sure this high-quality package will give us years of trouble-free service.
Dyno results showed that the Doug Thorley system definitely produced more horsepower and torque. The baseline was before the system was installed. At the rear wheel to the ground, the 5.2L engine produced approximately 160 hp and 211 lb-ft of torque. After installation, the engine produced approximately 175 hp and 222 lb-ft of torque. Keep in mind this is real-world power, meaning this is power at the tire on the ground. Factory power ratings are always taken from engine dynos and don't account for parasitic loss through the drivetrain and oversize tires. We may retest the vehicle with factory-size tires once the new engine is fully broken in. We're sure the results will be far more horsepower and torque. We'll keep you posted. | 
To the left of the starter was one of the factory exhaust's restriction points. Now, the exhaust gases have a less restrictive and much more direct line of escape. With each bend and kink of the factory pipe, the more force it takes to expel exhaust, which in turn makes for a major power loss. | 
Doug Thorley Header's cat-back system with Aeroflow muffler. The muffler utilizes a straight flow-though design. This system is also CNC mandrel-bent with aluminized tubing. |