The secret to making big horsepower numbers isn’t really a secret at all—the first step is lowering restriction both in and out of an engine. Emissions controls have been choking the smooth flow for almost 50 years now. Koenigsegg has two patents on exhaust designs that allow big airflow without allowing big pollution, and Jason Fenske from Engineering Explained is here to tell us how they work.
Before understanding how Koenigsegg frees up horsepower, we first have to examine why modern exhaust is so choking to engine airflow. The worst offender is the catalytic converter, which contains a mixture of metals on a tight honeycomb-like structure. As the exhaust flows through the metal-coated honeycomb, the NOx gases which result from combustion are broken down into more benign gases. Catalytic converters require heat to work properly, which is why many modern cars require a “pre-cat” that is located close to the engine for cold-start, since it will heat up faster and begin to function faster.
The trouble with all this environmentally friendly restriction is just that—restrictions. It takes pressure to force the exhaust gases through the honeycomb of a catalytic converter. That resistance to flow can keep exhaust in the combustion chamber, killing efficiency and therefore power. Koenigsegg has a solution, though, and it involves some airflow trickery.
The first patent was the “rocket cat,” which changed the design of the pre-cat to allow airflow around the honeycomb structure. At low engine rpm, there is not enough exhaust flow for the gases to divert around the honeycomb structure. Only at raised rpm can the exhaust gases divert around the restriction in the pre-cat and be treated in the main catalytic converter as usual. It is a pretty good solution to the problem, but like the rest of their engineering, Koenigsegg shot for the moon and made the system even better—and more complicated.
By adding a few butterfly valves to the exhaust, the engine control computer can modulate at will where the exhaust gases travel—essentially creating the ideal conditions in the exhaust at all times. Thanks to those valves, the path of least resistance is always available, and that helps the Agera RS’ turbocharged five-liter V-8 to crank out 1160 horsepower while still meeting modern emissions requirements. Unfortunately, it is probably not a system you can install in your garage with that wrench set you stole from your dad.