1981 90hp Johnson Power rating at prop or power head

"Folks, When did OMC start rat

"Folks, When did OMC start rating HP at the prop. For example I'm familiar with the 3 cyl 70HP and it wasn't unitl 1986 when the power head changed from a 49cc to a 56.1 cc engine to actual get from 62HP in the older models to the actual 70hp at the prop. When did this happen on the 90HP V4's? I'm specifically looking at purchasing a 1981 model and I'd like to know what gains I might get compared to my 1990 70HP. Keeping in mind the the v4 will be heavier, any ideas on how much heavier? Much appreciated!"

I'm not sure when they cha

I'm not sure when they changed but the '81 would still be rated at the powerhead. Graham probably can jump in and tell you.

"The International Council of

"The International Council of Marine Industry Associations set a common standard for horsepower in 1983. This started the change to rating outboards in terms of Shaft Horse Power (horsepower at the prop) as opposed to Brake Horse Power (horsepower at the flywheel).

In the case of OMC, throughout the 1984/85 model years, some were still rated BHP, while others SHP.

Most notably, what was a 200 horse model in 1983 was marketed as 185 horse during the 84 and 85 model years (same engine).

So an 81 model was most certainly 90 horses at the flywheel, somewhere near 80 at the prop (depending on gear reduction etc).

Your "on boat" performance difference I believe would be minimal - perhaps an increase of a few mph at the WOT end, partially because of the added weight of the 90 compared to the 70.

Additionally, on a comparative basis, although moving from a 70 to 90 horse (almost 30% increase horsepower wise) does not translate into a 30% performance increase.

I poked a fictious boat in my prop calc - 17 feet, total displacement 1800 pounds - all else the same. Top speed 90 horse, 46 mph - top speed 70 horse - 41 mph

So 30% more horses result in what 12% more speed.

The only other 30% increase you will see is in gas burned."

"The 90 is 300lbs and has 99 c

"The 90 is 300lbs and has 99 cubes.The 90 is by far the better motor ( if in good shape ) , smooth and lots of torque.Not only that the crankshaft / rods / pistons / lower unit are the same as the V-4 140 and the term " rugged " comes to mind."

i have owned both a 70 and a 9

i have owned both a 70 and a 90...agree with "student" on 90 being a better motor..you are looking at a 25-27 year old motor tho..i would be very careful before i got rid of an engine that i knew condition of on that deal.you will not see that much difference in performance if that is your goal..