What makes power in an engine? Fundamentally, it's all about air, fuel, and a well-timed spark. But if it's the stuff going into an engine that really counts, why is exhaust system design so critical?
What it ultimately boils down to is restriction. The more restricted the flow of exhaust, the more power gets left on the table. Jason Fenske of Engineering Explained is here to show us how Koenigsegg's clever engineers mitigate the restriction imposed by emissions equipment.
In essence, a good exhaust system is half of the formula for creating the optimal environment for combustion. From a performance standpoint, the ideal exhaust setup is a straight pipe. Traditionally, this means having optimal-width exhaust piping that goes straight from the exhaust manifold (or, in the case of some crazy setups, directly off of the cylinder exhaust ports) to the open air, where the gases can then dissipate.
The term "straight pipe" is derived from this ideal path, but in terms of exhaust design, it can generically refer to any exhaust system that is free of additional restrictive components, even if the piping itself is not technically straight.
While this may be the perfect solution for making lots of horsepower, it's not good for air quality. The government took steps to correct this in the 1970s with the Clean Air Act. It forced the introduction of components which inhibit the flow of exhaust gases in the process of scrubbing them of their most harmful contents.To scrub these harmful gases out and get emissions levels into compliance, automakers employed the catalytic converter. Unfortunately, catalytic converters require a lot of heat to operate efficiently.
The solution to this was a smaller, denser catalytic converter, which was installed before the main converter in the system. This is referred to as a pre-cat, and it is designed specifically to shoulder the burden of cleaning up emissions immediately after a cold start.
Unfortunately, this highly restrictive element impedes the flow of exhaust gases out of the cylinders, which means some are left behind, reducing the purity of the air/fuel mixture, and robbing the engine of power—in the case of the Agera RS, as much as 300 horsepower!
Koenigsegg has addressed this with two significant patents. The first, dubbed the "Rocket Cat," places the pre-cat inside a larger pipe which allows exhaust to bypass the restrictive component. The second is a series of bypass valves which isolate the Agera RS' pre-cats from the main flow of exhaust completely after the engine is warmed up.
Fenske explains both in detail, along with the logic behind Koenigsegg's process, in terms anybody can understand. Check out the video above to see for yourself.