This article was originally posted at the GRM Homepage

Clearly the most bang for your performance buck in the horsepower and torque race is to get rid of the engine exhaust gases in an efficient manner. Problem is that many aftermarket performance mufflers or exhaust systems will drive you up the wall on a long trip with their constant droning.
However, now you can have it both ways: reasonably quiet stock sound for normal street and highway driving, and straight-through performance for the drag strip or autocross that still meets all noise level requirements. How? Read on, McDuff!

STAGE ONE

Replace the heavy stock muffler with a large diameter "turbo" muffler that has been slightly modified. The anatomy of a typical reverse-flow stock muffler is shown in Figure A (next page). Note the gases must negotiate at least two 180-degree turns within the confines of the muffler's expansion chamber.
Figure B on page 48 depicts a modification to the muffler which allows gases to vacate directly out the tailpipe in a straight line. Now I ask you, if you were one of the hot little molecules of exhaust gas, which route would you rather take?
Now let's look at the photos on page 48. Photo 1 shows the donor muffler before minor surgery was performed to attach the additional outlet. (Checker p/n 17715, Dynomax 21/2" inlet, 21/2" outlet-about $15.00) Photo 2 depicts the 21/2" plug with pullout hook attached. Photo 3 shows how simple it is to secure the plug with a single screw for normal day-to-day driving.
If you're concerned about the legality of this modification, SCCA Solo II Stock rules allow that any or all parts of the exhaust system from the catalytic converter to the rear of the vehicle can be changed or modified.

STAGE TWO

Rather than attaching the modified muffler that we built to a small-diameter stock tailpipe, we should consider going all the way and increase the cross section of the plumbing with a "cat-back" system. (See Photo 4.)
The car for this project was a Nissan NX 2000, which comes from the factory with a 13/4" tailpipe and a very heavy muffler and resonator. We had our local muffler shop bend a new duplicate pipe of 21/2" diameter. Crunch the geometry numbers and you will see that we doubled the effective cross section by adding only 3/4" to the pipe size.
Unless your muffler shop has access to a mandrel bending machine which maintains constant pipe size, you may have to sacrifice a little efficiency due to the slight restrictions your muffler shop bends into your new tailpipe. Not to worry; your original pipe had the same restrictions. The larger diameter will easily make up for these minor restrictions. The rear flange of the catalytic converter of the NX2000 has a 21/2" outlet. You can use a standard 21/2" flange blank to weld to your new tailpipe. Bending the new tailpipe, obtaining short pieces to modify the muffler and a 21/2" flange cost $40.00. With my trusty Miller MIG welder I put it all together in about two hours. The original muffler hangers were used to support the new system. How does it work? Great sound for street use and highway travel and straight-through performance for the track. What more could you ask? Have a nice "quiet" day.

Photo 1	Photo 2
Photo 3	Photo 4