The V12 with a displacement of 5577 cubic centimetres which had been used during the aborted record attempt in October 1937 was completely stripped down and optimised. Its reliability was to be increased, e.g. by means of a richer combustion mixture and reinforced piston pins. Other improvements included nickel-plated piston crowns to reduce the risk of charring. The piston clearance was also increased. And finally Georg Scheerer in the engine workshop of the racing department ensured a steady supply of fuel/air mixture to each cylinder.
Test bench trials confirmed the success of all this work. On 29 December 1937, an output measurement prior to installation in the vehicle showed an output of 525 kW (714 hp) with no additional slide carburettor. On 6 January 1938, Scheerer minuted that with these additional carburettors, the output would be around 563 kW (765 hp). This was 21 kW (29 hp) more than the engine had achieved during the record-breaking attempt eight weeks earlier.