That cross over point is dictated by the forumla that calculates HP.

And I can't for the life of me remember it off the top of my head. I'll look it up and add it in few minutes.

Power = work/time or power = (force x distance) / time.

James Watt measured the work of a draft horse and found it could lift 330 pounds 100 feet in one minute.

Thus 1 HP = 33,000 ft-lbs / minute.

What we actually measure (on a dynamometer) is torque, expressed in foot pounds (in the U.S.), and then we calculate actual horsepower by converting the twisting force of torque into the work units of horsepower.

Visualize a one pound weight, one foot from a fulcrum on a weightless bar. If we rotate that weight for one full revolution against a one pound resistance, we have moved it a total of 6.2832 feet (Pi * a two foot circle), and, incidentally, we have done 6.2832 foot pounds of work.

Watt said that 33,000 foot pounds of work per minute was equivalent to one horsepower. If we divide the 6.2832 foot pounds of work we've done per revolution of that weight into 33,000 foot pounds, we come up with the fact that one foot pound of torque at 5252 rpm is equal to 33,000 foot pounds per minute of work, and is the equivalent of one horsepower.

Hope this helps.

Bob

1966 7 Litre Convertible
1959 F-100 (under construction)
2005 GT40 (in my Dreams)