Cliff, one of the great things about the Davis heads is that normally you don't have to change the compression setting unless there is a huge change in ambien temp. My ST .51 conversion turns a 12x6 at 10,500. The Diesels are most effecient if you prop them for 8,000 to 8,500 rpm. Your .40 should turn a 12x6 three blade in that range.