Interesting that your description has the start of the inlet and the start of the exhaust at TDC. I though the reverser/cutoff effectively reduced the effect of the inlet valve (effectively the valve throw, and hence the "gain" of the inlet valve), but would not have changed the timing. Hence if the inlet valve at full opening opened at TDC and shut at TDC+70° (ie resulting in a 70° opening period), backing off the valve opening would affect both ends of the opening timing, and hence might achieve opening at TDC+20°, and closing at TDC+(70-20)°, so the valve opening would be shut-off by 40° less period (ie only 30° opening period in this case). This means the exhaust would change timing, too, by a similar effect. I suspect it would take video footage to see that when the cut-off would be advanced, since the timing would be hard to see when the loco was running fast enough to benefit from the change of the cut-off. And to that end, I agree that the chuff timing at lower speed is the main definition; having the back-EMF (error) define the aggression of the chuff might provide a "simple" mechanism, where higher error (ie working harder) gives a more agressive chuff, and low error (eg coasting) would give a softer chuff. I also note your comment about the rotary encoder; this is much more detailed than our most common rotary machine (the car engine) that only has a TDC sensor and evaluates the timing of the other events (sparks) based on % of duration between the TDC events. With both systems, the TDC (Home) event has to be remembered, so loss of power would lose sync of the chuffs until the next TDC/Home event occurs. Thanks for the discussion, Regards,