How to Install a Cat-back Exhaust System ?

Something is missing in your life. And it's probably not a heavier workload on the job. Nope, you need something fun for your car--like the snorty growl from dual exhaust pipes. Okay, it's not like anybody really wants an exhaust system that makes car alarms go manic or requires the girlfriend to wear earplugs. But upgrading an exhaust lets you hear the magic under the hood. And to us gear heads--that's music, man. You can legally upgrade your exhaust to gain some power and efficiency. But federal emissions regulations won't permit just any mod. One upgrade that's almost universally permissible is in the exhaust system downstream of the second oxygen sensor. Because that sensor is always located right after the catalytic converter, we refer to these systems as "cat-back." So let's hear what we've been missing.

Max Flow

We went to Snake Pit Performance in West Babylon, N.Y., to install a Corsa cat-back system on a 2007 Dodge Magnum SRT8. If ever there were an engine that begged to be heard, it is the 6.1-liter V8 in Dodge's SRT8 Magnum. Our system wasn't cheap. The total approached $2000, including labor. But the Corsa system is well-engineered and fabricated from 304 stainless steel. Best of all, it fit like the stock exhaust. The installation process took 6 hours, including a few dyno runs. In your garage, with the car up on jackstands, the job might take an hour or two.

How Exhaust Works

The exhaust system is basically a long funnel to expel spent gases from the combustion process. The gases dump from individual cylinders into the exhaust manifold. On cars with a V-type or boxer engine, there are two manifolds, one on either side of the engine block. Inline engines have only one manifold. The manifold's snakelike tubes collect the gases and, on newer cars, send them down a pipe (or two if it's a dual exhaust) and into the catalytic converters. The catalytic converter is a major component responsible for reducing emissions. At the front half of the cat is a reduction catalyst which uses platinum and rhodium to remove NOx emissions. In the second half, the cat burns carbon monoxide and hydrocarbons. The result? Up to 90 percent of these damaging gases never reach the sky. But cats won't scavenge this stuff until they reach operating temperature. So automakers mount them as close to the exhaust manifold as possible to capture engine heat quickly.

After the cat, the exhaust travels on to the resonators and mufflers to cancel sound. Without mufflers, your car would be as loud as a Sprint Cup Car. Cat-back systems often include new middle pipes, resonators and mufflers­--and are often less restrictive (less backpressure) so they can build more power and make better noises, too.

What's Legal

In the good but smoggy old days before emissions standards, you could freely modify your exhaust, as long as the local constabulary didn't object to the noise. A tuned exhaust, along with a bigger carburetor, high-flow intake manifold and high-lift, long-duration camshaft, increased the amount of air and fuel available to the engine--voilà , more power. Times have changed. And California's strict smog regulations have influenced the nation. Aftermarket speed parts built for newer vehicles are almost all 49-state-compliant. In the Golden State, they must pass stricter California Air Resources Board (CARB) standards. So, unless you own a smog-exempt car (1975 or older in California), it's verboten to modify anything that might change the emissions profile. And that's why the most common exhaust modification is a cat-back system.

Getting Dirty

Our gleaming Corsa system looked great. But unless you're going to install it immediately, put the parts back in the protective foam. Because odds are you'll scratch them. Once our Magnum was on the lift, we removed the stock exhaust system behind the cat. That's not always easy. First, douse all of the mounting hardware with penetrating oil, and let it soak in. Start removing nuts, bolts and clamps. Got a stubborn bolt? Try tightening it first instead of loosening. This hardware is heat-cycled constantly, which adds up to corroded, heat-tempered parts. Chances are if you tighten them too much, they'll just snap off. I have a rule: Never reuse any exhaust system bolt, nut or clamp.

Ditto for rubber exhaust hangers--­­splurge on some new ones, dude. The new system should come with all-new fasteners, but probably won't include hangers or their hardware. At the factory, some exhaust systems are installed before the rear suspension is bolted into the chassis. Ever try to snake an 8-ft. exhaust pipe out from above the axle? To avoid removing the rear suspension, just cut the old pipe. Acetylene torches make lots of dramatic sparks, but a hacksaw will do the job. Check the original output pipe for roundness and burrs; it may prove difficult to slip the new pipes over them. A grinder, file or even some coarse sandpaper might help.

Hands On

Now you can finally hang those beefy pipes and mufflers from the new rubber hangers. Slip-fit everything and install all the clamps, but don't tighten anything yet. Slide everything around until it fits. Keep the clamps indexed on the pipe so you can get a socket on the nuts or bolts. Start at the front; shake, rotate and tap the pipes into place. Remember, the pipes are hung from rubber--they'll need some clearance to account for normal motion. One very important thing: Be sure the tips center on the exhaust cutouts, and that each one sticks out the same distance. Now tighten all the clamps and hangers with a torque wrench. Carefully smear a thin coat of antiseize compound on the overlap in the pipes--it'll make disassembly years later much simpler. Use some on the clamp's nuts and bolts, too. Be careful, though--it will stain all that shiny stainless you just paid top dollar for.

[1] CAREFUL TORQUING of the clamps joining the new system to the old cat is necessary to prevent leaks, rattles and crimped pipes. [2] A STRAIGHT-THROUGH MUFFLER can still be surprisingly civilized at moderate throttle settings and in traffic. Our system is optimized for low noise around town and drone-free expressway cruising, while fattening up a V8 engine's already porky torque curve. [3] CORSA uses an exclusive design to cancel noise without adding backpressure. Other mufflers use fiberglass packing and simple systems of baffles and perforated pipes to achieve the same end. [4] THE STOCK SYSTEM (top) from our Dodge Magnum has minimally smaller pipes, but the mufflers and resonators are more restrictive than in the Corsa--and the stock system is quieter. The new system is a powerful attraction on our muscular SRT8.

Dyno Test Results

We ran our Magnum on a chassis dyno before and after the new system was installed. This Dynojet has a large, heavy flywheel spun by the car's wheels, all monitored by a computer. The car's ability to accelerate the flywheel is recorded, giving us an excellent graphic representation of how much power we're making at any rpm. Aside from hearing the sweet sounds as we ran through the gears, we saw increased power. That's great, but not when it comes at the expense of midrange torque on a street-driven vehicle. The Corsa system increased peak power by only 5 hp, but fattened the torque curve throughout rev range by nearly 20 lb.-ft. And it's torque that makes a car accelerate, particularly at moderate speeds, not horsepower. A less well-designed system might have increased power and torque at some revs, but left a hole in the power curve elsewhere. That makes a car harder to drive and slower coming off corners. The increased torque also provides better fuel economy--it takes less throttle to maintain any given speed, and that means less fuel is burned.