Question from an "armchair locomotive engineer". The main advantages quoted for long lap valves are increased steam and exhaust port openings, and this is achieved at most running cut offs. I have steadily built up a library of technical steam literature with all manner of formulae , valve diagrams etc, which mostly explain valve events and the interactions between, lap, lead, cut off, travel etc (including a fascinating recent publication by Adrian Tester). But they all start off with the blanket statement that long lap gives greater port openings (provided the valve travel is lengthened to accomadate it ). I have never seen a diagram or a simple layman's explanation as to why this would be the case. e.g If you increased steam lap by, say 1/4", and increased the total valve travel by 1/2" to accomadate that, then at full travel, with full steam port opening, line for line, the exhaust edge of the exhaust valve would have "overtravelled" 1/4" compared to the original valve. So I can imagine how that would give greater exhaust freedom. But I struggle to see how the steam port opening at, say 45% cut off ,would be any greater than the valve without the additional 1/4" lap. I fear I am missing something simple in trying to visualise this. I'll probably end up doing the sums and just accepting the results, but a visual or practical answer is always nice. Does anyone have a straightforward answer or know of any literature that would provide the answers.