Choosing a camshaft is no easy task, and when you throw in turbocharging as a factor, things can get even trickier. To help engine builders better understand the challenges of picking the right spec cam for their turbocharged application, Comp Cams recently put together this explainer video to layout the basics of turbo cam selection, featuring Engineering Group Manager Billy Godbold. To understand the unique needs of a turbocharged engine, you have to understand a few basic concepts, like how the air and exhaust flow through the cylinder, how backpressure effects this flow, and how the intake/exhaust cam profiles and valve timing should be used to control it.
Pressure ratios for the intake and exhaust sides of the cylinder are pretty close to a 1:1 since it’s close to the same as the ambient atmosphere in a naturally aspirated engine. So, they have one-atmosphere pressure in the inlet and one-atmosphere pressure at the exhaust. All of the motion of flow is created by piston motion or wave tuning.
Naturally aspirated engines rely on something known as wave-tuning to improve efficiency. This is done by using precise geometry to reflect pressure waves from the exhaust valve and back in a particular time in the cycle. This process is critical to naturally aspirated engines because it is the only way to create charge motion other than with piston motion.
If you add a supercharger to the mix, the ratio of pressure all the sudden changes to favor the intake side with a ratio of 1:3 now. This creates two, three or even more atmospheres of pressure on the intake side, while the exhaust side is still close to ambient. In this kind of setup, airflow movement is initiated before piston motion comes into play.
Turbocharged engines will also raise the intake pressure above ambient, however, the restriction caused by the turbine wheel while driving the compressor causes back pressure in the exhaust that changes the ratio to 2:1, while also affecting cylinder filling. In older, less efficient turbo system, the exhaust side might get twice as much pressure over the intake side, making selecting a cam to work with this setup pretty complex. Overlap becomes a major concern at this point. If the intake is opened while the exhaust is still opening, pressure differential will cause a backflow.
This is why you don’t just throw a camshaft meant for a naturally aspirated engine into a turbocharged car. Comp Cams has a few off-the-shelf options for turbo engines, and the difference is both power, and health of your build.