Buick Regal GNX Exhaust Upgrade - 3-Inch Turbo Buick Exhaust Test

The final remnant of the GNX suspension-the front torque arm bracket-must be removed now to fit a standard exhaust system. It's for the better, as the driveshaft can make minor contact with the bracket over big bumps-never good. Werner puts the Sawzall to work; quick and dirty, but we have to hit TTP's dyno before they close today. I'll make it pretty eventually.

Check out the size difference between the 3-inch Cotton's exhaust system (left) and the 2.5 incher I'm removing. Coupled with Cotton's huge 3.5-inch downpipe this sucker will really flow!

The front section is held in position and bolted down loosely with the four bolts. We set up a jack to hold up the front, then Chris slips the new mufflers on and lightly snugs the new 3-inch front clamps.

Werner slips the tailpipes over the axle, and inserts them into the mufflers.

I attach the new hangars with a washer, nut, and bolt. The nuts are run down but the rear clamps are left slightly loose.

The Cotton's system utilizes a frame hole behind each rear wheel where its tailpipe mounting tab lines up. With the system still loose, we angle the tailpipes until these holes line up, and using bolts through Cotton's supplied rubber isolators, we mount and tighten them with nuts.

With the tailpipes mounted, Chris makes a couple of minor adjustments to ensure that the big pipes clear the upper control arms, etc., then the front pipe's mounting surface gets some RTV and its four bolts are tightened, and all of the clamps are tightened down. The car is started and the lift is raised to check for any leaks. The driver-side after-muffler pipe is leaking just a bit, but tightening the clamp helps that. With the final check complete, the Buick is set down. The pipes have a really good angle of exit behind the wheel.

Baseline and modded dyno testing took place at TT Performance Parts. TTP's Dynojet was a new type of dyno for the Power Trip Buick, and I figured on seeing higher numbers from our previous SuperFlow testing due to a different dyno type. Also keep in mind that previous dyno testing was done with a 3-inch downpipe; Cotton Performance's massive 3.5-inch downpipe would surely improve the pump-gas numbers. All testing would take place with the coolant between 165 and 180 degrees, a locked-up torque converter clutch, and 17 pounds of boost.

Power Trip's best baseline run saw 461.9 horses and 455.7 foot-pounds of torque, SAE. Averaging the three pulls made for 454.0 horsepower and 453.6 foot-pounds of torque, SAE. With Cotton's 3-inch exhaust system, the best run was a 472.5 horse, 461.9 foot-pound SAE pull, and the modded average was 472.4 horsepower and 458.7 foot-pounds of torque, SAE. That's a peak-to-peak gain of 10.6 HP/6.2 TQ, and averaged out, a gain of 18.4 HP/5.1 TQ. Incidentally, the first two pulls with the new exhaust (which were not factored into the average numbers) showed how free-flowing my system had become, with the boost over 19 psi and nearly 500 HP/500 TQ showing up at the rear wheels before turning the boost controller down to compensate!

In comparing the third-run 2.5-inch exhaust log with the third-run 3-inch exhaust log, note that no XFI tuning was done between exhausts. Here, the 02 correction is showing -8.6% on the 2.5-inch exhaust (but was -4 to -6 in adjacent frames). On the 3-inch exhaust log, there is much less correction at -1.6%, and it even went into positive percentages in nearby frames, indicating that more fuel was needed for the increased airflow of the larger system. And though the Hartline Performance tune's air/fuel ratio information is proprietary, I also saw a slightly leaner-but still very safe-air/fuel ratio with the 3-inch system. Additionally, the injector duty cycle was much lower for the 2.5-inch at 52.2% compared to 56.5% for the 3-inch.