This is taken from "The Straight Dope, September 10 1976"; In tests at the Bronx Zoo in 1924, a dynamometer--a scale that measures the mechanical force of a pull on a spring--was erected in the monkey house. A 165-pound male chimpanzee named "Boma" registered a pull of 847 pounds, using only his right hand (although he did have his feet braced against the wall, being somewhat hip, in his simian way, to the principles of leverage). A 165-pound man, by comparison, could manage a one-handed pull of about 210 pounds. Even more frightening, a female chimp, weighing a mere 135 pounds and going by the name of Suzette, checked in with a one-handed pull of 1,260 pounds. (She was in a fit of passion at the time; one shudders to think what her boyfriend must have looked like next morning.) In dead lifts, chimps have been known to manage weights of 600 pounds without even breaking into a sweat. A male gorilla could probably heft an 1,800-pound weight and not think twice about it. I didn't see an exact orangutan reference but FOX Network TV aired a tug o war between a large Sumo wrestler and an orangutan. The Sumo went flying like the other end of the rope was tied to a car.

I have heard for a chimps it is between 4-7 times stronger, and gorillas estimated at anything between 10-20 times stronger, but I don't know for orangutan. I remember reading about the first man to try studying chimpanzees in the wild, and he was photographed after with both of his arms ripped out of the sockets as he had approached at the wrong time, and this is a favoured attack by chimps, to rip your arms off.