Aftermarket exhaust systems are usually at the top of the list in any performance buildup. When your buildup involves a turbocharged vehicle, the benefits of a well-engineered cat-back exhaust system are immense. Significant increases in turbo response, power output and reliability will be the result of just bolting on the exhaust system on nearly all turbocharged vehicles. However, some vehicles will not show significant benefits from the addition of the exhaust system by itself. On these applications, the factory engine control unit (ECU) may reduce boost pressures, change timing curves or change fuel delivery after the exhaust system is added. As a result of these adaptive ECU's and their uncooperative nature to see more power generated, little or no power is seen at the wheels on some vehicles. On these vehicles, the OEMs are making it harder to increase horsepower. Fortunately, the speed bumps to performance can be crossed as long as our intelligence is not lower than the bump itself. Understanding what to look for in an aftermarket performance turbo exhaust system and knowing how to beat the roadblocks that your factory ECU may throw at you will allow you to realize the maximum performance from your vehicle.

Seven Points of Exhaust Flow
The primary function of a vehicle's exhaust system is to muffle the sound coming out from the exhaust port of the cylinder head and to direct the exhaust gas out the back of the vehicle. If we follow the flow of the gases from the cylinder head to the tailpipe on a turbocharged vehicle, we find seven points of interest along the way. First, the exhaust port of the cylinder heads feeds its flow into the exhaust manifold. Unless this part has been upgraded, the chances are that the factory piece is a cast-iron manifold that supports and locates the turbocharger while directing the exhaust flow into the turbo. Second stop is at the turbine housing of the turbocharger. At this junction, the exhaust flow is directed through a nozzle (turbine housing) that increases the flow velocity to power the turbine wheel. While providing the power to turn the compressor, the turbine section also has a side benefit of dramatically reducing the exhaust noise. Essentially, the turbine section acts as a very effective muffler. Once the flow makes its way into and out of the turbine housing it finds its third stop at the downpipe. The downpipe provided the exhaust flow connection and direction into the catalytic converter. The catalytic converter provides our fourth point of interest. The converter aids in the conversion of exhaust gases into less polluting products. After the converter, one finds all of the "cat-back" elements. The cat-back components include the fifth, sixth and final stops (B-pipe, muffler, tail-pipe or tip). From the converter, the exhaust hits the Before-muffler exhaust pipe or B-pipe. Some B-pipes may use a resonator or pre-muffler, but it is pretty rare on turbo exhaust systems. The B-pipe feeds the sixth stop, the primary muffler. The mufflers job is to "muffle" or reduce the exhaust noise further. Finally, the exhaust meets the atmosphere through an exhaust muffler tip or actual tail pipes on some applications.

Objectives of the Turbo Cat-Back
The objective of a properly-engineered, aftermarket cat-back exhaust system is to provide additional performance while still delivering adequate sound control. Unlike an all-motor exhaust system, a turbo exhaust system suffers no ill effects from going as big as possible. Bigger is better in this case. The bigger or larger diameter exhaust pipes allow the back pressure to be significantly less than the factory exhausts system. As a result, the difference in exhaust pressure before and after the turbocharger is increased. The increase in the magnitude of the pressure difference allows the turbocharger to reach higher shaft speeds at lower engine operating rpms. As a result, boost response increases and boost pressures increase. More boost pressure at the intake manifold results more power at the wheels. Can a turbo exhaust be too big? A turbo exhaust is too big if it drags on the ground or is too large to provide adequate sound suppression. For peak performance, most aftermarket turbo exhaust manufacturers will use the largest diameter tubing that can be properly routed underneath the vehicle. A muffler will be matched that allows an adequate amount of sound suppression.

Sight, Sound & Performance
In this evaluation, we have covered both the looks and the performance available from most of the performance exhaust systems available for the EVO-VIII. One are which we cannot address with paper and ink is the sound of the exhaust system.

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