Supersprint - Exhaust Anatomy

On their best runs, the RS4 produced 372 hp and 312 lb-ft, while the E92 M3 put down 416 hp and 301 lb-ft. In part three, we will show the complete before-and-after development curves.

A performance exhaust system should consist of a specific diameter of pipe tubing that optimizes back pressure and exhaust gas velocity for maximum horsepower and torque. This diameter is related to an engine's displacement, performance output and operating range. But there's no magic formula to calculate what should be used. Also, the tubing need not necessarily be constant from the downpipe to the tailpipe (primary header tubes being the exception), due to pressure and temperature changes through the system. Also, bends and curves must be as gentle as possible, so as not to create turbulence or restriction. However, available space on a modern vehicle is often limited.

By analyzing the baseline pressure and temperature data throughout the stock exhaust system, Supersprint engineers determine what pipe diameter to use and where maximum performance gains can be made. At each measurement location (or exhaust component) there is a unique back pressure characteristic. The graphs on page 82 and this page show pressure characteristics at the four locations in the OEM system of the RS4 and E92 M3 respectively.

Exhaust pressures increase with revs at each point, decreasing as the exhaust moves through the system. Note that an average vehicle with a 6000-rpm redline will have a 'Pressure 1' value in the 7.3 psi range (dotted black line on each plot), while more sporting models will measure in the 3.0 to 5.5 psi range at 6000 rpm and continue to increase toward their higher redlines. The RS4 and M3 fall into this sporting/performance range and therefore have systems that are well optimized from the factory. However, the new M3 has higher Pressure 1 values, indicating a good candidate for an aftermarket header application. Incidentally, Supersprint is developing a step header for the new M3.

In the next installment of this three-part series, we'll describe the testing and fabricating of a prototype performance exhaust system, and build on our understanding of exhaust flow characteristics and exhaust system design. Then will it become clear how Supersprint engineers can extract more power over a free-flowing factory exhaust system.

Stock baseline TV sound test results for Audi RS4 and BMW E92 M3