This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
One of the most important emissions devices ever invented is the catalytic converter. The converter works by altering harmful exhaust gases into more environmentally friendly byproducts. The first converters used a platinum-coated ceramic honeycomb or aluminum-oxide pellets coated with platinum to convert HC and CO into water vapor (H2O) and carbon dioxide (CO2).
Starting in 1980, a new type of converter design was introduced. These new three-way catalytic converters control HC, CO, and NOx emissions only when the air/fuel mixture is exactly set to 14.6:1. Without going into too much detail about fuel injection design theory, this air/fuel mixture is the absolute ideal for running the engine, and is the overall goal of almost all fuel injection systems. With the installation of these new catalytic converters, the addition of an oxygen sensor is required. The oxygen sensor (Lambda or O2 sensor) is connected to a computer that meters and controls the system to maintain as best as possible, this 14.6:1 ratio. When the car is running at this level, also called the stoichiometric mixture, it has reached a high level of efficiency and the catalytic converter is also working at its best.
Problems can occur for a number of reasons. If the oxygen sensor becomes disconnected or stops working, then the fuel control system can become confused and have difficulty metering the system. Usually this will cause the car to run richer than normal. Running the car in a rich mode instead of at its normal level can cause the catalytic converter to become clogged with soot from the exhaust system. As the converter, or CAT as it's sometimes called, becomes clogged, it can severely affect engine performance.
Troubleshooting CAT problems can be a bit frustrating. Some clues that the car may have a CAT problem include loss of power at higher RPMs and speed, even though the car idles perfectly fine. Many mechanics troubleshoot the fuel injection system first (indeed a smart place to start), but never really think that the problem might be a clogged CAT. If you remove the CAT, you should be able to clearly see through it. Chances are if you shine a bright light on one end, and you can't see any light through the other end, then it's clogged. The 911 CAT has a right angle built in, so you can't really see directly through regardless, but some reflected light should be able to shine through. Cars that have recently had their oxygen sensor replaced are ideal candidates for CAT problems. If you purchase a car that has receipts that include an oxygen sensor replacement, be aware that the previous owner might have driven the car many miles in a rich condition, thus damaging the CAT.
Of course, to inspect the CAT, you need to remove it. The process of removal simply involves the unbolting of the CAT from the exhaust system. You don't need to remove the muffler or the heat exchangers. I would recommend that you remove the left rear wheel, as it will gain you a lot of access room for the job.
It is very common for the nuts and bolts on older cars to rust, and make exhaust components very difficult to remove. This very well may be the case with your CAT. If so, then you might need to grind off the nuts and/or heads of the bolts to get the CAT off of the car. It's not an easy job, and it is complicated by the lack of room underneath the car. If you have extreme difficulty, then take the car to your local mechanic.
Original Porsche converters can easily cost more than a thousand dollars, however, good aftermarket ones are available for about $350. If your car is having problems passing an emissions test, it may be a sign that your CAT is worn out and needs to be replaced. You may also be surprised at the additional horsepower that you might gain. Any clogs in the CAT will directly affect the efficiency and power of the engine.
Another alternative is to bypass the converter completely. While this will generate less restriction than a CAT, the tailpipe emissions will basically go through the roof. Using a CAT by-pass pipe in most states is legal only for off-road use, and should only be used for racing purposes. Removing the catalytic converter from your car will make it at output significantly more emissions than today's new cars. Doing so is not really worth the few extra horsepower that you will gain from polluting the air.
The two options available for replacing your old catalytic converter consist of a new catalytic converter (aftermarket unit shown on top), or a CAT bypass pipe (bottom) which basically doesn't do anything for controlling emissions. Brand new factory catalytic converters cost about a thousand dollars, but there are some very good aftermarket ones that are available for around $350.
Installed on your car, the new catalytic converter should instantly improve performance. Years of use can clog old converters, causing restrictions that prevent the engine from running properly. If your O2 sensor has recently failed, then there is a chance that your CAT has become clogged. Under these conditions, the car has a tendency to run rich, which can seriously damage the converter.