The change to poppet valves beginning with the Fifteen of 1933.
The valve core is a poppet valve assisted by a spring.
Double-beat poppet valves became widely used during the nineteenth century.
But for most uses, he said, poppet valves perform acceptably at a reasonable cost.
This loss is unavoidable on intake ports due to the nature of the poppet valve.
In the early days of engine building, the poppet valve was a major problem.
These have a stacked-tube construction, built around a poppet valve, that is opened when struck by a force.
The engines had poppet valves and reversing was by sliding camshaft.
If the engine uses the common form of overhead poppet valves, these would also be difficult to fit inside the limited space.
Use of poppet valves also increased the speed because they gave very accurate steam delivery to the cylinders.