I have one like mentioned by Jezmo. It cost $129, it's very well made, but you have to assemble it. I bought mine, then got divorced, so it's still in the box. I'll sell it for $50. The way it works is it mesures the torque the engine produces at a given rpm, that gives you the HP. If you want one like you see on TV, I don't know if anyone makes one, but if they do plan on spending thousands.