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Jeep Grand Cherokee - Easy Power With Doug Thorley

Posted in How To on August 1, 2007
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Project Grand Caddy received Doug Thorley's ceramic-coated Tri-Y headers and cat-back exhaust system. The exhaust system without question increased the engine's horsepower and torque. The bolt-on system (welding optional) is 50-state smog-compliant. All systems utilize factory hangers.

One of the most tried and true methods of increasing your vehicle's engine power is replacing the factory exhaust with well-designed header and exhaust systems that properly and efficiently increase flow. The simplest way to prove this is to put your hand over your mouth and try to exhale. It's a little difficult, isn't it? That's what your factory exhaust system is doing to your vehicle. The hard part about squeezing additional horsepower from your engine is selecting the right exhaust system.

The exhaust ports cannot just be opened up to gain power, proper flow must be present, especially in modern engines. Though some techies may question the theory that exhaust systems must maintain proper and slight backpressure to retain low-end torque and power, it's a generally accepted rule. The exhaust systems on older, carbureted engines aren't as complicated because they don't have the sensitive electronics of modern vehicles, but they still need accurate attention and proper, unrestricted flow. One cannot simply bolt on the largest exhaust pipe available without taking these things into consideration.

The factory intake and exhaust systems on some vehicles are so restrictive that it's hard to understand why they're designed in such a manner. Actually, it's a matter of manufacturing economics. A factory exhaust manifold simply collects all the gases from the engine heads' exhausts ports and jams them into one tube and hopes they flow smoothly out the end of the tailpipe. When it comes to this design, "hope" is the key word. Why does this cause a loss of power? Because your engine has to work harder to expel the burnt air/fuel mixtures. A header helps the gases flow out of the engine much more smoothly and efficiently. With a little extra effort and a few more manufacturing cents, an increase in engine power, performance, efficiency, mileage, and consumer appeal would be easy. The manufacturers' neglect of performance design is a windfall for aftermarket companies like Doug Thorley Headers that take great pride in producing high-quality exhaust systems.

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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.

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Doug Thorley Headers
Corona, CA 91720

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