Although stainless steel exhaust systems have reduced the number of exhaust system replacements, it's important to remember that regular exhaust system inspections remain an integral part of import service. When inspecting an exhaust system, remember that the exhaust pipe and muffler must:

1. Help "tune" the engine for peak performance;
2. Reduce noise to legal levels;
3. Meet current emissions requirements; and
4. Safely vent exhaust gases away from the passenger compartment.

   If the exhaust fails one or more of these tasks, it's a candidate for service or replacement.

   EXHAUST SCIENCE
   Many modern import engines use a "tuned" exhaust to improve the volumetric efficiency of a high-performance engine. The operating principle is simple in concept: When exhaust gases exit the exhaust port at very high velocities, a mild vacuum is created in the cylinder. Engine builders therefore use intake and exhaust valve timing to take advantage of this vacuum effect to increase the intake charge entering the cylinder.

   Since most of the torque generated by the combustion process takes place by 90 degrees after top dead center (TDC), the exhaust valve usually begins to open before bottom dead center on the power stroke. The residual combustion pressure in the cylinder is then released into the exhaust port, through the exhaust system, and into the atmosphere.

   In most naturally aspirated applications, the intake val...