At the time of Secretariat's death, the veterinarian who performed the necropsy, Dr. Thomas Swerczek, head pathologist at the University of Kentucky, did not weigh Secretariat's heart, but stated, "We just stood there in stunned silence. We couldn’t believe it. The heart was perfect. There were no problems with it. It was just this huge engine."[18] Later, Swerczek also performed a necropsy on Sham, who died in 1993. Swerczek did weigh Sham's heart, and it was 18 pounds (8.2 kg). Based on Sham's measurement, and having necropsied both horses, he estimated Secretariat's heart probably weighed 22 pounds (10.0 kg),[20] or about two-and-three-quarters times as large as that of the average horse.
http://en.wikipedia.org/wiki/Secretariat_(horse)
That engine drove Secretariat to a feat at the Belmont that is for the ages.