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911 Twin Pipe Muffler Conversion

By Steve Grosekemper

One of the best things about a 911 is the sound; it is what draws many people to the model in the first place. Anything that can improve that sound can only add to the personality of the car.

This is no recent revelation on my part; companies have been selling aftermarket exhaust systems for 911s since their beginning. The problem with making a better exhaust system for your 911 is that the factory did such a good job in the first place.

Many aftermarket units are excessively loud and possess an annoying resonance. Then there is that excessive price tag… “Hey that muffler will buy me a new set of tires”!

The solution I came to is simple, just modify the stock muffler. There are two basic types of mufflers for 65-89 911s. The first type is the dual inlet, single outlet type. (65-74 911) The second type is the single inlet, single outlet type found on ’78–89 911s. 75-77 911’s will use different versions of these two types of mufflers depending on what emission control systems the cars were originally equipped with.

Now most people are familiar with the dual outlet mufflers made famous by the ’73 Carrera RS. (Two pipes straight out the back about ten inches apart) They are great systems with great flow and sound, but they are very loud!

We are going to do something a little different. Those mufflers have several baffles cut out and only use the center silencing chamber, where the exhaust then exits out the back.

We will not do any baffle cutting on the early type muffler, and minimum cutting on the late muffler. The great thing about this conversion is that we will not be cutting into the body of the muffler at all. Our surgery will be arthroscopic, all done through the hole made for the second tail pipe.

The one down side with this conversion is that the rear valance needs to be modified. We are going to install our second pipe on the right side of the car in the exact opposite position as the left side. So it is best to take care of the valance first. This is not a difficult proposition, but best left to a body shop for those not familiar with the process.

Early muffler modification-

•  Remove the muffler and make a 2.25-inch hole in the exact center of the right side of the muffler. A hole saw works best for this. You will notice three pipes inside. By installing a right side exit we are reducing backpressure and allowing the exhaust to exit without going through all the silencing chambers.

•  Re-install the muffler back in place.

•  Spot-weld your new right side tail pipe in place. Be sure to center it so it is symmetrical with the other side. Porsche sells a replacement tail pipe for 911 mufflers, which works great for this application. The great thing about using the Porsche part is that it is the exact same size as the left side and the chrome tip will fit perfectly. The part number is 901.111.287.00 and costs about $30.

•  Now remove the muffler and neatly complete the weld.

•  Reinstall the muffler one last time and take the car for a test drive. The exhaust will rumble at idle and howl with your foot planted, but it will not be loud and will pass the strictest driving event noise restrictions.

Late Muffler modification-

The late muffler modification is the same as the earlier version with one exception. After the hole is cut into the end of the muffler you will find a pipe that makes a U-turn.

•  Reach inside the muffler and remove the wrapping from the pipe. A pair of long needle nose pliers work well for this job.

•  Take a cutting torch and remove the U-turn pipe from the end chamber.

•  Remove all debris from the end chamber. If pieces are left in the end chamber an annoying rattle may result.

•  For an additional exhaust “growl” make two 1.5-inch holes in the chamber wall on each side of the two existing pipes. These are the pipes that used to have a U-turn pipe attached to them. The cutting torch works best for this as well.

Attach the tail pipe as described above and get ready for your test drive.

Don’t be surprised when people turn their heads, smile and say; “Now that’s what a Porsche is supposed to sound like”.

Good Luck