Weird thing.

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sloopjimd

Cadet
Joined
Jun 23, 2010
Messages
23
Ok so I hae a 1976 15 hp Evinrude and a 1984 15 hp Johnson. The 76 was on my boat and I wanted the newer motor just because it was newer. The 76 ran like a dream and after a little tweaking the 84 runs very well also.
The question is this these motors are almost identical in terms of parts and such. Why does the 84 out perform the 76.
The 84 is faster of the line and top end is much better.
I love the 76 its my baby and I thought it ran very well before I got this 84.
Seems like a dumb question but why is the 84 more zippy than the 76?
 
Lion hunter

Lion hunter

Lieutenant Commander
Joined
Apr 9, 2005
Messages
1,529
Re: Weird thing.

At some point in the 80's they began rating HP at the prop instead of the flywheel. The newer motor may have 1-2 more HP than the old. Have you compared compression? I'd lean toward that (and maybe better tuned).
 
R

reelfishin

Captain
Joined
Mar 19, 2007
Messages
3,043
Re: Weird thing.

Things like prop pitch, carb size, as well as compression and exhaust porting differences will make a difference in power output.
I've also run across many 15hp motors which were running 9.9 carbs. The original carbs were most likely robbed to upgrade a 9.9 motor at some point. The venturi size is different between the two carbs.
 
S

sloopjimd

Cadet
Joined
Jun 23, 2010
Messages
23
Re: Weird thing.

Normally I would agree with you Reel but in the interest of science and curiosity I swapped the carbs and it didnt change one thing. I did do compression test on both and they were beleive it or not about 2 or 3 psi with in each other.
I agree with Lion hunter about the hp ratings. But is 1 or 2 hp at the prop going to make the difference?
Im not having any concerns just a healthy discussion.
Maybe ill put them both on my boat and make a twin screw (im totally kidding of course)
Thanks
 
TN-25

TN-25

Chief Petty Officer
Joined
May 27, 2008
Messages
607
Re: Weird thing.

There are threads on here that detail the various changed to the 15 over the 9.9 that were made over the course of their production. It was sort of deduced that the original 1974 15 probably put out closer to 13 horses; later OMC did some exhaust housing tuning on the 15 to help it deliver a little more. That alone probably can account for the performance difference you are finding between the 1976 & 1984 versions.

Strangely, in later years that change was also incorporated into the 9.9 so who knows what the 9.9 was putting out at that point.
 
Bob_VT

Bob_VT

Moderator & Unofficial iBoats Historian
Staff member
Joined
May 19, 2001
Messages
26,022
Re: Weird thing.

1984 is the key year! That is the year that OMC changed from powerhead HP to prop HP.

Just as an example ....... 1983 100 HP was actually about an 85 hp at the prop...... about a 15% loss

In 1984 a 100 HP was a true 100 at the prop ;)
 
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