[AT] Horsepower Question

[Ben Wagner]

  Here is a puzzler that maybe someone with a better knowledge of 
engines can answer.   I was asked this question, and had to announce 
that I needed to look it up.  What better place than the AT Mailing?

I have a Super A with four cylinders, producing c. 20 HP.

I have a lawnmower, with an engine that says it is also 20 HP, with only 
two cylinders.

What is going on?  Is this two different measures of HP?  Or has 
technology improved that much?   What makes a little two cylinder "small 
engine" have the same HP as four cylinder "tractor" engine.

Thanks,
Ben Wagner