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Could comparing the rate of change of RPM between two acceleration pulls (same gear) be used as a quantitative way to determine if adding timing is being productive? Can one assume that if the engine is making more torque it will go through the rev range faster?
That how work an inertia dyno to determine the torque.
But it's only true if you get a consistent load (something hard to get on the road for example) and it will only give you a good trend at best.
Your theory is absolutely correct, however the small changes you will be looking for with a change of perhaps 2 degrees of additional timing will likely be very hard to spot on a data logged acceleration run on the road. This is the theory behind many of the 'virtual dyno' software packages, where an accelerometer is being used to measure longitudinal g force vs engine rpm. The problem is that when you are looking for such small changes (2 degrees for instance may only represent a change of 3-4 kW), there are many factors on the road that can mask this - Surface condition, incline, wind etc.