Accuracy is relative in absolute terms -- any dynomometer can give you repeatable results.
If you mean the "true" power, then unbolt the motor. On a chassis dyno, it is always an estimate.
If you want to get estimates that will be incredibly stable from shop to shop, the dynojet (interia dyno) is king. HP and tq figures are calculated based on how quickly the wheels can accelerate the known constant weight of the drum.
For eddy current (load bearing) dynos, there are lots of "fudge factors", so readings will tend to be more variable from shop to shop.
Personally, I prefer dynojets simply beause the results from different shops are more easily compared. If a guy at x shop in CA put down 296 on a dynojet with such and such mods, odds are he'd put down right about 296 on a dynojet at y shop in PA. Not so with a Mustang dyno.
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