A modified hydraulic circuit for an OEM vehicle transmission, which transmission has a forward clutch, a forward clutch hub and a forward sprag having an inner sprag race and an outer sprag race, an overrun clutch and an overrun clutch hub, the forward clutch and the forward sprag providing a link to a planetary gear train to propel the vehicle forward, the forward clutch hub defining the outer sprag race, the overrun clutch hub defining the inner sprag race, a hydraulic valve body having a plurality of passages and a separator plate, and a 1-2 shift valve, a 2-3 shift valve, a 3-4 shift valve and a 3-4 relay valve, and a gear shift valve, includes a blockage in the passage fluidically connecting the 2-3 shift valve to the overrun clutch, a blockage in the passage fluidically connecting the gear shift valve to the 2-3 shift valve, removal of a movable element from the first check valve body and a passage from the gear shift valve to the overrun clutch via the first check valve body, wherein when operating in the D4 mode, the forward clutch is applied and the overrun clutch is applied and are locked to one another, such that the forward sprag clutch is ineffective. A method of modifying the hydraulic circuit is disclosed.