Hunting a mystery Porsche engine tick

I’m a former Apple camera engineer, and presently a Gentleman Scientist (e.g. retired). One of my new projects has been to study the noise signatures of Porsche engines to identify sources of ticking. I began my investigation with an unusual chaotic tick that many other owners have observed, but that no one could explain.

I tried to use standard broadcast microphones at the outset of the project. However, I couldn’t isolate the engine’s ticking from the sounds of the fans and exhaust. Therefore I switched to the Cortado mic, which is perfect for high-frequency impulsive noise. I attached the mic to a ferrous bolt on the bottom of the engine by epoxying a magnet to it. I simultaneously recorded the exhaust note with a broadcast mic from behind the car in order to place the ticks relative to engine rotation.

What the data shows

• Exhaust phase plot — The exhaust note over the duration of the recording: six ignitions over a complete engine cycle (one camshaft revolution and two crankshaft revolutions). The stability of the pattern demonstrates that the engine phase is being tracked correctly.
• Engine tick phase plot — Engine ticking recorded with the Cortado mic. The green samples were recorded with the clutch engaged (pedal out) while the blue samples were recorded with the clutch disengaged (pedal in).
• Event separation — The time separation between ticks shows that the ticks arrive periodically, but with many long skips between arrivals. This indicates that the phenomenon depends on the phase of some other object in the system, not just the engine.

The Cortado made it possible to pull a faint, fast, structure-borne signal cleanly out of an extremely noisy mechanical environment — exactly the kind of measurement a contact mic is built for.