1984 Mec 90HP power curve vs. 115HP

[Cogburn]

I've been working on a 90hp Merc that I bought from a guy off craigslist and have it running good now.


The prop I was running on it was an ancient 48-32750-19 12 1/2"x 19p. It turns 5200 at around 35+ mph on my boat(it turned a little faster before I detuned it for todays gas...it went 36-37mph before(I blocked off the high speed lean out circuit, removed the bowl vent jets and timed it to 21?btdc.)).

It went 41-42mph on the 14ft runabout I tested it on.

My fishing buddy brought me a Merc 48-72764A4 stainless prop that is 13 1/4"x17p. He claims he got 54mph on his gps with it on his 115. He doesn't have a tach-I 've been in his boat and it is fast and is similar in size to mine.

My boat is an old Lund Mr. Pike 18ft his is an old 18-19ft Bluefin-both side console fishing boats.

My Lund...

Bluefin kinda like his...

Anyway with the increase in prop efficiency and diameter but reduced pitch I expect it will probably drop me into the recommended rpm range of 4000-5000rpm wot. I might find out tonite.

The recommended RPM range of the 115 is 4500-5500RPM. I take it this is because of different porting in the block? If he was running 54mph with the 17p prop how many rpm was THAT?

Anyway what is the difference between the 90 and 115 mercs?

Am I right thinking that this prop will drop my wot rpm or not?

Any comments on the speed difference in boats? Hull better on the Bluefin vs. Lund?

 

[79Merc80]

Re: 1984 Mec 90HP power curve vs. 115HP

I have a 84 90hp Mariner. I wasn't aware that the RPM range was 4-5000, I thought it was 5-5500. My tack isn't reading correctly, so I don't yet know what I am actaully getting. I do know that the 90 is the same engine as the 82 and earlier 115's, and the 90 hass a lot more low end tourque than the 115 of the same year.


Craig

 

[Cogburn]

Re: 1984 Mec 90HP power curve vs. 115HP

Went to the lake and the prop change didn't change much on the top end. 5100rpm at 36mph. I saw 37mph and 5200 for a short time.

The hole shot though is way better. Hit the throttle and it turns 5000rpm instantly and bogs down to 4200 when it planes out a second or two later and then rpms and speed build pretty quick up to 5100 and 36mph. Didn't tear any more seat mounts out though(they were creaking).

It will cruise at close to 30mph at 4000rpm.

I shut it off at the ramp and then when re-started to trailer it idled fine and throttled good. The local bait-n-gas station has a couple for 13 1/4x17 Hustler props on sale for 40$-I might pick them up and a prop hub kit and see what they do. Too bad they don't have any 19p props.

 

[Cogburn]

Re: 1984 Mec 90HP power curve vs. 115HP

I bought the Hustler 13 1/4"x17p props from the gas station. The were on closeout and I got them for 40$ a piece.

The Hustler has vents and the blades are swept back quite a bit, I had to grind the anode for clearance.

I had set the carbs a bit leaner and replaced the switchboxes and it idles really good but alas it is too lean to really get out of the hole. I will have to open the idle jets a bit-I closed them 1/4 turn and I thought it was a bit rich before.

The WOT speed is up a tad to a reliable 37mph and the WOT rpm is down to around 4700rpm. The prop loads the engine noticeably harder than the Mercury 13 14"x17p prop. Cruise speed is also up a couple to around 32mph at 4000RPM.

Anyway I'm calling it done other than a slight idle adjustment. The amount of good information on this forum is terrific. I probably would not have bought this motor from the guy had I not found this site and learned that the problem it had was probably a stator(he is a mechanic and had it to a marine mechanic attempting to fix a top end miss.).

It has taken a stator,rectifier,switchboxes, plugs , and a little link and sync to make this motor perform great. I am very happy to have found this forum and the tremendous amount of information on it and some good advice from it's members.

 

[Cogburn]

Re: 1984 Mec 90HP power curve vs. 115HP

I raised the motor a hole and it responded by gaining a little rpm to 4800+wot and 38mph+ with a normal load-2 people and a load of fuel and fishing gear. This is the first time I've seen 38mph in this boat and it does it pretty consistently. 4200rpm cruise is 33+mph. Cavitation plate is visible in the water at wot.

Turned the idle mixture back out 1/4turn and the idle is good with prolonged idling at 2.5 mph 650 rpm and will take off immediately when I jam the throttle ahead.



This is a big improvement over 2 35hp Evinrudes and 27mph(at best).