550 Foot pounds per second equals one horsepower.

   The unit goes back to James Watt, who needed to sell his steam engines.  Watt was trying to tell customers than just one of his steam engines could replace so many expensive and hungry horses at their mines or mills.  We can assume that Watt chose a small and weakly horse for the comparison to make his engines look their best. 

  Many many years later, Detroit used the idea, and the unit to advertise the power of car engines.  Everyone knew that more power gave a livelier car.  They started out measuring the power of an engine removed from the car and mounted on an engine dynamometer.  The engine was tested “bare”, no generator/alternator, no power steering pump, no air cleaner on the carburetor, no exhaust manifold or mufflers.  All of these things took engine power to run, said engine power subtracted from the power shown on the dyno. 

   About 1971 the big three in Detroit got together and agreed upon a new test procedure, engine was to be equipped with all power robbing accessories, an exhaust manifold, a muffler, air cleaner, the works.  The idea was to give car buyers a more accurate idea of how much usable power their new car might have.  The other effect was to reduce the advertised horsepower of all cars by about 15%.  Although car salesmen groused about the reduced horsepower claims, all makes suffered about the same amount, so things came out more of less even. 

   Things trundled along for many years.  Until in very recent years, some imported cars began advertising huge horsepower out of very tiny engines.  One of them was claiming 200 horsepower out of a 120 cubic inch engine.  For comparison a Chevy 283 cubic inch two barrel carburetor V8 only claimed 180 horsepower when it came out back in 1956.  And that was under the more generous rating system in use up until 1971.

   The long and the short of it is, I don’t believe the incredible horsepower claims from some tiny engines.  As a rule of thumb, an engine can produce about one horsepower per cubic inch of engine displacement.  So 180 horse power from a 283 cubic inch Chevy V8 is believable.  200 horsepower from a 120 cubic inch engine is not believable.