Friction does increase with load. Think sandpaper. If I remember correctly there are exceptions, some plastics demonstrate a reduction in friction with an increase in load (to a point).
The dyno would have to present a greater load to verify the higher horsepower, the greater load would cause greater friction in the gears in the diff and tranny. Things like
synthetic lubes help by decreasing the friction ... so the rear wheel hp can be greater with
synthetic lubes.
I've got a feeling that the relationship between hp and drivetrain losses is not linear (i.e. you can't just use a static percentage of loss). If anything, the greater the horsepower, the greater loss in the drivetrain.
Then again, perhaps the accuracy of the dynos also changes with load. Perhaps the greater the load, the greater of the internal losses within the dyno ... and the greater the inaccuracy of the final reading.
Yup, I've got a 1000 hp 351W ... it's just that there is a 60% sum of drivetrain loss and internal dyno friction.
