Popular Exhaust Modifications

by: Bill Gregory (sansho@rennlist.com)

Date: August 9, 2002

There are 4 changes that can be made to the 964 exhaust system: Change the headers, take out the catalytic converter, replace the primary muffler with another muffler or a bypass pipe, and replace the secondary muffler with another muffler or a bypass pipe.

 The headers on a 964 are not very efficient. In fact most of the near 22 hp gain made on the first 993’s was due to a more efficient header system.  There are several vendors (B&B, FVD, Cargraphics, GHL) who sell replacements headers, some which can include catalytic converters, and, with mufflers, they tend to be in the $4K-$6K range. Some people have also adapted 993 headers to the 964, which requires turning one bank of header flanges 90 degrees.

 Replacing the catalytic converter, of course, affects emissions. There are bypass pipes that look like catalytic converters as well as those that are straight pipes. Cargraphics sells a catalytic converter that replaces the primary muffler, too. There are also “sport” catalytic converters available. If you replace your catalytic converter, make sure the replacement has a port for the oxygen sensor.

 Replacing the primary or secondary muffler is a typical upgrade. It’s said that replacing either one will gain 8-10 hp, however, replacing both of them with bypass pipes yields no additional horsepower, but does yield lots of noise. A “cup bypass pipe” to replace the primary or rear most muffler runs around $275-$360, and saves weight at the farthest rear point in the chassis, which is a great place to save weight. A “U” pipe to replace the secondary muffler is a little less expensive. There are vendors that also make a variety of replacement mufflers for the primary and secondary mufflers.  Figure 1 presents a photo of the primary "cup" bypass pipe as installed (perspective is from under right side of rear bumper).

Figure 1. Dansk "Cup" primary by-pass pipe.