The Outcome
Though bad weather prevented quarter-mile testing of our now-GMPP-equipped Cobalt, seat-of-thepants feel was definitely improved an amount commensurate with the dyno charts. The car sounds a lot more performance-oriented as well, with a throatier intake snarl and a whole lot raspier exhaust note. (Those not as fond of four-cylinder tunes may want to opt for the Touring version of this exhaust.) This is to say nothing of the improved underhood and exterior looks these parts have provided.
Make sure your GMHTP subscription is up to date, as you won't want to miss our next installment, where we'll be adding some truly massive Ecotec power-FWD traction be damned!
Nearly invisible under an array of plastic covers, this Cobalt's 2.4L LE5 features variable valve timing and is the largest-displacement Ecotec currently offered in any GM. It's also the same mill used in the non-turbo Sky and Solstice roadsters, though of course mounted sideways here. This was the first Ecotec to use a stronger Gen II block, and features high-strength items like forged connecting rods, oil-jet piston cooling, and chain-driven (not belt-driven) camshafts. This little sucker is tough! | 
In the interest of gaining some flow and improving on a near-nonexistent exhaust note, we're bolting up a Performance cat-back exhaust made by GMPP. Carrying PN 17802111, it can be yours for $489. This kit includes everything seen here with the exception of the tip, which is sold separately and available in a few different styles. We chose slash-cut (PN 17802112, $69) to help keep a stock appearance. For the more mellow amongst us, a quieter Touring version of this exhaust is available for $100 more. | 
The stock exhaust isn't the prettiest to look at, consisting of a skinny resonator and huge muffler with all sorts of shielding around it. The stock cat pipe (or downpipe, if you will) is 2.25 inches OD. Then, the stock cat-back system (a strange bird) begins at 2.25 inches for the tiny bit fore of the resonator, bumps up to 2.5 inches for a couple of feet, and finally shrinks to 2.25 OD again the rest of the way to the muffler. It's then 2.5-inches OD from the muffler to the tip. |
The factory exhaust aft of the cat is all one-piece welded, and were it not for this econo-car's non-independent rear suspension, the entire system could come off as one piece. Short of dropping the rear axle assembly, getting the exhaust off involves cutting. Support the muffler with a trans jack or willing assistant, and bust out a reciprocating saw. (Cobalt-tipped blades are a must!) We chose to cut the system just after the muffler, as this provided the most weldable location should the need to reinstall the stock system ever arise. | 
The entire stock exhaust is supported by rubber hangers, all of which will be reused with the GMPP system. Some silicone spray on each will help them slip off with only a slight tug. After removing two such hangers just ahead of the exhaust tip, the muffler drops out. The cat pipe unbolts from the cat-back via two studs and nuts just forward of the resonator. (You can see this connection in the far bottom right of the photo.) The resonator/over-axle pipe is snaked out after its own two hangers are undone, one just ahead of the muffler's location and the other just aft of the resonator. This completes the stock system removal-piece of cake! | 
The resonator pipe is the first part of the GMPP exhaust to go up. Insert the two bolts that hold the front of the pipe to the existing cat pipe (a new gasket is provided for this junction), then slip the stock rubber hanger on at the rear of the resonator. Don't tighten anything yet. Just forward of the cat is a flex coupler that causes the pipe to be significantly bendable, so swinging things around to aid installation will be easy. |