TOPIC: HP Ratings 50s/60s vs. Current Documentation

Hello, I understand vintage motors and current motors have their horsepower rated differently. I located the vintage OBC horsepower to size recommendation chart on this site. Does anyone have a published chart that shows what the conversion factor is or an "old HP to new HP" chart or can point me to one? Thanks in advance for your assistance, I did some searching here but didn't find one.

Isnt the difference that the older models rated HP at the crank while somewhere in the 80's they started measuring the HP at the prop? If this is true then there really wouldnt be a conversion for it would there? As each make/model would loose different amounts of HP going through the lower units.

From everything I have been told, it is about 15%. Using that, an older 100 HP is about the same as a newer 115. It does vary some between motors, depending on lower unit gear ratio ect.

All Dunn II wrote: From everything I have been told, it is about 15%. Using that, an older 100 HP is about the same as a newer 115. It does vary some between motors, depending on lower unit gear ratio ect.

I have read about the same. 15% to 20% (I think this is a stretch) but you have that reversed? now that the manufacturers rate the motor at the prop, the newer 115 actually have more hp at the crank? meaning the older 115 hp is about the same as a newer 100?

This is one quote I picked up from another forum that seems a lot more reasonable, "the difference in flywheel and propshaft ratings came out to be 3-4%. You'll hear a 10-15% difference thrown around a lot, but I've never seen any justification"