theZ, R1 etc., all had a large opening at rear of cowl.
theair flow e.over the cowl, produced a low pressure in th cowl--
th wascommon method ofcooling the big radials then
later, the pressure cowl, cowl flaps, etc; were added to reduce drag
the current radials, also extend the prop as far forward the cowl aspossible--to minimize-cowl-prop interferrence.