Maybe the first few times the cars were on track they used the factory rating, after that, they never based the ratings on actual factory HP in stock elim. It was always a level the playing field approach. If one car ran significantly under the index, it got re-rated. Nothing to do with an observed dyno output, except track data. Same thing they did in pro stock. One engine/combo dominates, factor it out of existence by adding weight to it so no longer competitive.
They are still rated at levels for many combinations, far below the current outputs.
