The "horsepower loss" story is pretty much of a myth, unless you're running a really small engine. Even with a very heavy heat load and high head pressure, it shouldn't take more than 1 or 2 horsepower to cool an area the size of a truck cab. A 5 HP compressor cools my entire house, all 3,000 square feet of it!
Jerry
HRL probably knows more about this then I do. He has run them on Dynos & such. The newer cars have variable disp. compressers but the old A-6 & comps. from a few years ago pulled a lot of power from an engine. Ive heard as much as 25 HP. I know you can sure feel the difference & that belt better be really tight or it will squeal.
