@trenzterra. I copied your latest reply above to Shambles to my friend and asked for comments. He replied:
"No, still think it's the turbo catching up, could be something to do with the wastegate sticking , but doubt it.
"It actually only seems to be about a second till it responds to the throttle input, and for the log to be of any more value it would need to be logged at around 0.1 second intervals max, not 1/2 second intervals."
I think that has exhausted his input on the subject but when/if you find out what the issue was it would be nice if you could post on the forum and I will let him know how close or not he was.
Knowing how good he was with this data logging stuff I suspect I'll be buying the next lunch.
Thanks for helping me check on this and on following up! I thought it was turbo lag at first too, but perhaps the data log does not tell the whole story.
So what I am experiencing is that - on a consistent press of the pedal (and not very heavy footed), the car accelerates up to 13km/h but somehow stops accelerating further even though the pedal remains depressed. The feeling is akin to an engine braking feeling.
You can see this at the start of the graph where speed tapers off (the vertical axis numbers are inaccurate as I was trying to fit 4 lines to scale) although accelerator pedal pressure remains consistent. After noticing this (probably about 1 second in?) this is when I try to increase pressure on the pedal, but even doing so the car does not respond at all for about 3 seconds. I noticed also that RPM dips a little bit during this "lag" period.
I corroborated this with my dashcam footage and the lag period seems to be about 3 seconds long. I have experienced this intermittently and each time the lag period is about the same.
99 times out of 100 I do not experience this issue which makes it hard to troubleshoot even at the workshop. On the rare occasion it rears its head, it is normally in heavier traffic (which makes it problematic).
My impression of turbo lag is that it should only last about a second and most and the car should still continue accelerating (slowly?) while waiting for turbo to kick in. But yeah happy to hear if your friend has more insights to share!