2003 Pontiac Grand Prix - Dyno Testing - Six Sense

This angle really shows the size difference in the outlet to the intermediate pipe when comparing the two. The manifold outlet is less than 2 inches, while the header merges from 3 inches down to 2.25 inches.

Here's the completed front header installation. Many people and gas station attendants have since praised the clean-looking headers. We remind them the headers have also increased gas mileage and performance.

The larger diameter (2.5-inch) intermediate pipe is mandrel-bent and ceramic-coated the same (inside and outside) as the headers. This bigger pipe flows into a humongous 3-inch collector on the rear header.

There's an amazing size difference between the rear header and stock manifold. Notice the extra thick flange to prevent warpage and gasket blowout, and the built-in 3-inch flexjoint.

Our W-car was equipped with the California air pump package. SLP has seen only a couple others with it on the East Coast. Here, Ken "King of Skunkworks" Estelle welds on a stainless tube with the original flange for stock location and retention of the emissions equipment. New Jersey can be tough and the state may get worse on emissions inspections (underhood and tailpipe), so we wanted to retain the emissions controls.

The rear header goes in from the top while the rear of the motor is lifted up to make room to squeeze it in. Plug wires, EGR pipe, air pump hose, motor mounts, and engine lift brackets were removed before we tried this header hurdle.

Featuring an extra-large collector and a 3-inch downpipe, these tubes dwarf the stock pipe and cat. In that area, the flow restriction is reduced tremendously, helping the exhaust to scavenge more effectively, which adds torque and horsepower.

This area with the U-pipe and stock cat is ultra-restrictive. The stock U-pipe shrinks down to less than 1.75 inches, causing a major reduction in exhaust flow.

SLP's PN 30073 high-flow cat or the PN 30072 race pipe is 3 inches in diameter to fit the PN 30070 headers (we used) and SLP's other W-body exhaust systems. They eliminate that restrictive stock U-pipe and cat. The high-flow cat has a bung for the O2 sensor and the race pipe comes with an O2 simulator.