Originally Posted by Mertz
I am working on a 65 283 that is rated at 220hp with power pack heads. How does the 1965 horsepower rating compare to the current ratings? Is there a conversion factor that I can use?
The difference between the "old" ratings (better referred to as SAE Gross Ratings) and "new" ratings (SAE Net, used since 1972) is that SAE Gross was measured while the engine was under perfect conditions. There was literally the bare minimum of accessories attached and the exhaust and timing used would generally not reflect what it had when it made it in the car.
SAE Net is tested with all of the accessories, exhaust and emissions equipment intact to provide a more realistic number. Both were measured at the flywheel.
Generally, I would just multiply the Gross HP figure by 0.85 to get a rough estimate of the SAE Net HP rating at the flywheel. That would put the 220hp engine at 187hp at the flywheel
. I don't claim it's the most accurate way of doing it but it'll get you in the ballpark.