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Are you referring to a chassis dyno or engine dyno?
For my drivetrain setup, the "15% loss" thumb rule is almost exact.
So, if you have chassis dyno numbers, multiply them by 1.15 to get engine numbers.
My 2000 was on the dyno in 2004 by the previous owner. He said the results should be multiplied by 1.03 for the HP & Torque. Is this true?
I guess if it feels good, do it...
Keep in mind:
1. No 2 dynos are going to read exactly the same.
2. A dyno is a great tool to compare engine output before and after changes.
3. Don't get too caught up in the number.
There is zero basis for the previous owner to say his results should be adjusted 3%. Probably something he read on the internet.
He must be talking about some correction factor - that's usually already done at the time by the dyno operator. Either way, dyno numbers vary from dyno to dyno (as noted above), and are nothing more than a tuning tool.