Porsche often does things backwards—just look at the 911, keeper of the rear-engine flame. But that philosophy extends to smaller things, too, as Jason Fenske of recently illustrated.
In his latest video, Fenske examines the design of the intake manifold on Porsche's 718 Cayman and 718 Boxster. This model recently switched from naturally-aspirated flat-six power to a variety of turbo flat-four engines. And the intake design on this new range of engines is something to behold.
As Fenske explains, most modern engines are designed to generate high-pressure air in front of the engine's intake valves. The length and shape of the intake tract creates pressure waves that push a blast of high-pressure air against the intake valve right as it's opening. This crams a little extra air into the intake stroke, even on a naturally-aspirated engine.
Porsche's turbocharged four-cylinders work the opposite way: each intake tract has an expansion chamber where the intake pressure decreases before the intake valves open. This sounds completely counterintuitive, right? Wouldn't the expansion chamber mean less air gets into the combustion chamber?
The thing is, the expansion chamber allows the intake air to cool down before it goes into the combustion chamber. Cooler intake air allows an engine to run a leaner air-fuel mixture (which is good for fuel economy), and it allows for more spark advance (which improves economy and power). And since these engines are turbocharged, Porsche was able to tune the turbo pressure to work with the intake expansion chamber to optimize the whole system.
The result? Porsche says its unconventional intake design reduces cylinder temperatures by 20 degrees C, and leads to a 12 percent reduction in fuel consumption at full load. Pretty nifty!
For an even more thorough illustrated explanation, here's Fenske himself: