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Operating Mercury 75hp 2 stroke above published WOT rpm range

gboy99

Member
From what I understand Mercury's 75hp 2 stroke is a de-rated 90hp. The WOT range for the 75hp is 4750 - 5250 rpm but for the 90hp it is 5000 - 5500 rpm. My 75hp 2 stroke is fitted with 13" diameter 18 degree Vengeance s/s 3 blade prop. and runs at 5550 rpm at WOT. Will this damage my outboard?
 
5550 is a little high.

It probably won't cause any sort of damage and when the air gets a little more hot/humid your rpms will "naturally" fall since it will be harder for the motor to breathe.

However, if you think you are gaining anything by running above the WOT range for a 75 (thinking maybe you can squeeze a few more horses out), you are incorrect there.

The maximum horsepower for any given motor is always at it's maximum right "smack in the middle" of the WOT range. As you pass the midpoint the horsepower actually starts to decrease.

So by running above WOT you could well be turning your 75 horse into a 70 and burning quite a bit more gas to do so.

Your carbs just can "feed it" enough to get more than the rated horsepower. So while it's a de-rated 90, it's alot more than just the RPM's that make it a 75....

(edit to add hp curve diagram)
 

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If the prop is undamaged, and otherwise suits how you use the boat, I would simply keep the throttle back a hair for wide open. Now if you were exceeding the WOT range by 500 or more rpms, then I would consider more pitch - you don't want to powerhead to "run away" and fry itself, but as I indicated, a couple hundred isn't the end of the world...
 
From what I understand Mercury's 75hp 2 stroke is a de-rated 90hp. The WOT range for the 75hp is 4750 - 5250 rpm but for the 90hp it is 5000 - 5500 rpm. My 75hp 2 stroke is fitted with 13" diameter 18 degree Vengeance s/s 3 blade prop. and runs at 5550 rpm at WOT. Will this damage my outboard?
I had a 4 blade 13x15p on my 84 up merc 2 cycle on my 18' mod v bass boat. rpm's went to 5800+ rpm at wot. trying a 13x19p 3 blade. I was told when comparing 4 vs 3 blade props to add 2* of pitch for the 3 blade. we'll see when I take it out this weekend. I'll most likely sell the 4 blade later. the 4 blade has a whole shot out of this world! something like 3-5 seconds jumping out of the water!!
 
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update on the prop upgrade. the 13.3"x19 pitch prop works great! with a load(me-260 lbs,buddy 330 lbs, brother 200lbs)790 lbs,it rpm'd to 4800 rpm. before I loaded the boat I took it our with just me. it pulled 5200 rpm. if anyone needs a 4 blade solaras prop(13"x15p) let me know. $60 bucks. it was only used twice, vgc, just not enough pitch for a light boat.
 
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update on the prop upgrade. the 13.3"x19 pitch prop works great! with a load(me-260 lbs,buddy 330 lbs, brother 200lbs)790 lbs,it rpm'd to 4800 rpm. before I loaded the boat I took it our with just me. it pulled 5200 rpm. if anyone needs a 4 blade solaras prop(13"x15p) let me know. $60 bucks. it was only used twice, vgc, just not enough pitch for a light boat.
still have the prop for sale 765 252-8529 thanks steve
 
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