[elwesso]

Here's what I don't understand. I don't understand how that device can simply use speed and RPM to determine torque. As we all know, torque = force x distance (lb-ft or N-m). HP is simply torque at a given RPM.

Using one of these scanner tools, you can measure all of those things except a force. The only way this scan tool can calculate that is by essentially back-solving using the A/F ratio, airflow, and air intake temps. My guess is that scan tool isn't setup specificially for any given engine, so assumptions have to be made on volumetric efficiency, thermal efficicency and mechanical efficiency. The one thing that this CANNOT account for is friction inside the engine, which can be significant.

That said, I don't really think dyno numbers really mean a whole lot, as a number itself. They are only useful to compare setups. In other words, you do a baseline run of a car in it's stock form, then install your modifications, then re-run the car ideally on the same dyno at similar ambient conditions. I don't care if the dyno says you make 30RWHP or 3000 RWHP, if you show a 30HP gain, it's still 30HP regardless. That is where I think dyno's are most effective in showing the CHANGE.

Remember you can see EASILY see a 10HP swing just by changes in ambient temperature.. It's two fold because by decreasing the ambient temperature you increase the density of air, which in turn makes the engine injest more air, and by increasing the temperature DIFFERENTIAL of a heat engine, you increase the overall power output and to some extent the thermal efficiency of the cycle.