RS Turbo MFI ECU I was thinking of (purely because thats what I drive), although yeah I imagine you are right Chip that is how I would do it also.

I didnt realise that the frequency of Det is present throughout the running of the engine normally, thought it would stick out like a sore thumb! Which I supose it does to the human ear but as you say you would need to have a set of reference tables for any given ECU to compare live readings to and make a 'decision' based on that.

Well it was a nice idea whilst it lasted!