Power difference in 15hp's

[swfl mike]

I'm looking into upgrading to a 15hp for my jon boat and the older johnson/evinrudes are a dime a dozen on craigslist.

What I'm wondering is since they changed the method of rating horsepower in the 80's, how much of a noticeable difference is there between a 15 rated at the crank and one rated at the prop?

Most of what I can find in my price range is late 70's to mid 80's. If there is a good deal less power in an older one than a newer one I'll probably wait till I can find a newer one, but if it's not all that noticable than I'll scoop up a cheap oldie.

Thanks for any responses

Mike,

 

[JB]

Re: Power difference in 15hp's

Performance differences are undetectable except with fancy equipment and then may still not be detectable.

 

[DJ]

Re: Power difference in 15hp's

Anything after 1983 is prop rated. On a 15, you probably couldn't tell the difference.

 

[swfl mike]

Re: Power difference in 15hp's

alright thanks guys

 

[ChrisAG]

Re: Power difference in 15hp's

I kind of disagree... OMC in their brochures for 1981 claimed a big torque boost for the 15 in that year, in which a speed increase was realized in a typically loaded boat (two fishermen and their gear).

Lots of modifications were made that year, including tuned exhaust and timing changes, so I would think there was a difference, but how much would be open to debate.

 

[JB]

Re: Power difference in 15hp's

The question was about differences between crank rated and prop rated outboards. All 1981 models were crank rated.

 

[ChrisAG]

Re: Power difference in 15hp's

If you want to be absolutely "on paper" stringent then yes. Clearly, however, the OP has an interest in a motor that might have slightly more power, and I just thought I'd inform him the 1981+ editions of the OMC 15 had more internal changes than other years which accomplished this. In fact, it is possible they didn't need to make any other changes whenever the prop rating "officially" came into effect.

How is it possible a 1981 "crank rated" 15 is more powerful than a 1980 "crank rated" 15? Manufacturers can increase the power while leaving the "rating" unchanged through taking the reading at different rpm, or taking advantage of the "fudge factor" permitted by law (+/- 6 or 10%, I forget which). If the 1980 model happened to be slightly over-rated (at the crank) and the 1981 model was under-rated up to the maximum allowable (at the prop) there would be a significant difference between the two.

They may be motivated to do this when new competitors enters the market; by 1980 or so there were more players in the marketplace.

Further, if OMC knew the prop-rating would come into effect in 1983, it makes sense they would address a few of their models ahead of time, rather than go to the trouble of changing the whole lineup in one year.

 

[swfl mike]

Re: Power difference in 15hp's

Thanks, thats great info. I think I'll aim my search towards something newer than '80