Exhaust

The Exhaust System

Back to the exhaust system design, if we look at an exhaust system off of a race car designed to run at high rpm, you will find long primary pipes that ultimately merge together somewhere down the line. The idea here being that with a long primary tube length, the exhaust gases pulses from one cylinder are effectively isolated from the other cylinders. Once you get to the collector, where all the primary pipes come together, the pulses from opening and closing valves are effectively smoothed out, and the exhaust gas is traveling at a steady and uniform speed. Combine all the primary pipes at this point, and you have the significant benefit of the neighboring exhaust gas stream vacuuming or extracting the exhaust from the other cylinders.

Exhaust systems can be optimized for varying engine speeds by changing the primary tube length. The longer the primary tubes, the higher rpm the maximum scavenging affect occurs at. Of course, the opposite is true as well, the shorter the primary tubes, the lower rpm that maximum scavenging occurs at. In practice, a production engine that must meet emissions standards is extremely limited in the primary tube length by the requirement for the catalytic converter to be mounted very close to the exhaust port. This is one reason why modern cars can achieve such significant gains by moving to a true long tube header with a more appropriate primary tube length. This requires movement of the catalytic converter further from the exhaust ports and results in slower warmup of the catalytic converter, therefore strict emissions standards for the much scrutinized warmup cycle cannot be met. Maybe one day somebody with enough technical background can convince the people who make regulations to modify the rules to allow a longer warmup cycle, which will allow OEM’s to vastly improve engine efficiency (read fuel economy) through better exhaust system design. I doubt it will ever happen, so the only option is to install long tube headers (and be out of emissions compliance) if available.