Scope for increasing Exhaust Lap on valves with long steam lap

A question was asked (by Chris Newman) as to how can it be deduced from Fig 50 (in Wardale’s book The Red Devil and Other Tales from the Age of Steam) that there was scope for an increase in exhaust lap for valves with longer steam lap?

The actual wording referred to appears on page 170 of the book, where it says:  “Fig 50 suggests that there was ample scope for further increase in the exhaust lap with valves having a long steam lap”.  Fig 50 is reproduced below:

Wardale answered as follows:

“Because with longer steam lap the exhaust opening is much larger than for the standard engine, so more exhaust lap can be used without restricting the exhaust opening to below that of the standard engine.  The need for more exhaust lap when the exhaust is improved is because the compression starts from a lower pressure and is generally inadequate, as proved by 3450’s indicator diagrams.”

See also the reference to Exhaust Lap on the Valves and Valve Gear page of this website.