I've removed, torn-down, cleaned, lubricated & re-assembled the BPV (35mm turbosmart re-circulating). It seems to operate smoothly and normally in response to pressure/vacuum changes. About 6 months ago out of desperation I even replaced the cut-down spring with a full-sized supercharger spring from Turbosmart (which is far too strong for the GTM setup, locking the valve closed most of the time and causing a lot of flutter, but certainly ruling out the BPV recirculating).
I did speak to Sam but then he stopped responding, so I'm battling this one on my own now.
I was also thinking maybe a boost leak where the plastic manifold meets the head, but that doesn't explain why the throttle and timing are so different (I would have thought that, boost leak or no boost leak, the throttle & timing should behave the same). I've removed and reseated the intake manifold at least 4 times over the past couple of years, including replacing the gasket, and it all seems completely fine.
I'm most confused about the throttle and timing differences between the two runs - I just can't understand how or why the ECU is doing what it is doing, especially as the MAF sensor readings are quite similar between the two runs.. It's like there is some unknown third variable the ECU is using to decide how to perform
