Bort62 is dead on. There should be no guessing with a dyno chart.
Run in high (1 to 1) and start around 2000rpm go full load and run it up to the max rpm.
Many things will change from day to day, dyno to dyno.
Did the operator correct his temps and baro pressure to a standard? If not the exact same engine on two runs hours apart can vary widely.
If you have pinging (detonation) yes it will seriously kill power.
Also, at what rpm per second is the dyno set for? If your increasing rpm at 300 per sec then run a back to back run at 900rpm per sec, the faster one (900) will produce less hp and torque because some is wasted accelerating the reciprocating weight.
In other words, to slowly spin an engine up will require 25 lb's from 2000-4000 (exponentially of course) but if you try to spin it quickly, it requires more torque to do that (50 for example) the difference of 25 lb's will be reflected in the dyno chart and you will think the second pull lost power!
Now if you moved the timing on the second pull, you might incorrectly assume that the timing change caused a power loss when in fact the improvement was overshadowed by a bigger loss caused by the increase in acceleration.
Boil this all down to......
Bort62 was right
