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2013 75hp max RPM?

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wijames

Member
I'm looking at Mercury motor models and I'm seeing that the 75hp four stroke is listed with an RPM range of 4500-5500 and all of the other size motors, both smaller and larger, list 5000-6000 RPM. Why the difference?
 
Longer stroke?

Jeff

PS: Waiting to hear how many 4 strokes blow up, or need an early rebuild, after running 5 grand plus for years. You can easily get away with that with a 2 stroke, but...
 
This will be long, and maybe a bit confusing, but- the 75 is exactly the same outboard as the 90 and 115. The only difference is the PCM calibration and the decals on the cowl. Usually, in an engine group like this, the easiest way to reduce hp is to re-map the fuel delivery programming, but in the case of the 75, Merc found that re-mapping alone would not reduce the hp enough to make the 75 hp threshold, without some kind of engine problem (they never did say what the problem was, but my guess was that by reducing fuel flow, a lean condition occurred- just a guess). In order to still have an outboard in the 75hp class, the easiest way to lower maximum hp is to lower maximum rpms. The reasoning behind the hp grouping is economics- every manufacturer wants to have an outboard for every boat made, and it's a whole lot cheaper to make one outboard and "de-tune" it than it is to make 3 separate outboard models.

Oh, and Jeff- about the lifespan of high revving fourstrokes: I have a 2006 60 EFI that has 1150 hrs, and runs 5900-5950 rpms WOT, and I run at WOT a lot. Still have 140-150 psi compression across the board, no engine problems, and no noticeable power loss. (a couple of gearcase problems, but those where impact related ;) .)The new stuff will definitely take high revs and last. As an example, I drive the wifes' Nissan Sentra once in a while, and it scares the bejeezus out of me when the tach hits 7 grand, but it doesn't seem to hurt it any.
 
I'm looking at Mercury motor models and I'm seeing that the 75hp four stroke is listed with an RPM range of 4500-5500 and all of the other size motors, both smaller and larger, list 5000-6000 RPM.
Click to expand...
Don tknow where info came from but below is off Mercs website.....

75 EFI FourStroke Specifications

HP/Kw @ Prop
75 / 56

Fuel Induction System
4 valves per cylinder, dual overhead cam (DOHC)
Full throttle RPM
5000-6000
82 x 82mm

[TD]Bore and Stroke (in / mm) [/TD]
[TD] 3.23 x 3.23in [/TD]
 
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..."about the lifespan of high revving fourstrokes: I have a 2006 60 EFI that has 1150 hrs, and runs 5900-5950 rpms WOT, and I run at WOT a lot. Still have 140-150 psi compression across the board, no engine problems, and no noticeable power loss. ("

Truly happy to hear that. I've been...concerned that people would have costly rebuilding problems with the 'new' four strokes.

Jeff
 
The " 75hp four stroke is listed with an RPM range of 4500-5500 " That is the operating range that will develop the 75hp .
It can run 6000+ but optimum performance is at 45-5500.
Most outboards were designed to run at wide open. But at WOT it uses a lot more fuel for the end results.
My motors at 4400 average about 2.2-2.4 miles gal. At 5500 about 1.7 or less.
My twin 88/85's still have 145# compression after 26 years.
The thing I noticed is the props actually wear down.
I compared a new prop to my old and it was about 1/8th to 1/4 in smaller.
Not much difference but it can make a difference in performance.
Also the right prop makes a huge difference in fuel economy.
I change to a 19 from a 17 , wanted more speed. Got 2-3 mph more and the usage went up by 20-25%
Hope some of this helps, early morning coffee buzz!
 
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faztbullet said:
Don tknow where info came from but below is off Mercs website.....

75 EFI FourStroke Specifications

HP/Kw @ Prop 75 / 56
Fuel Induction System 4 valves per cylinder, dual overhead cam (DOHC)
Full throttle RPM 5000-6000
82 x 82mm
Click to expand...

Not sure what site you looked on, but WOT specs for the 75 are definitely 4500-5500, 500 rpm less than the other outboards in this hp group.
 
bilge rat jim said:
This will be long, and maybe a bit confusing, but- the 75 is exactly the same outboard as the 90 and 115. The only difference is the PCM calibration and the decals on the cowl. Usually, in an engine group like this, the easiest way to reduce hp is to re-map the fuel delivery programming, but in the case of the 75, Merc found that re-mapping alone would not reduce the hp enough to make the 75 hp threshold, without some kind of engine problem (they never did say what the problem was, but my guess was that by reducing fuel flow, a lean condition occurred- just a guess). In order to still have an outboard in the 75hp class, the easiest way to lower maximum hp is to lower maximum rpms. The reasoning behind the hp grouping is economics- every manufacturer wants to have an outboard for every boat made, and it's a whole lot cheaper to make one outboard and "de-tune" it than it is to make 3 separate outboard models.

Oh, and Jeff- about the lifespan of high revving fourstrokes: I have a 2006 60 EFI that has 1150 hrs, and runs 5900-5950 rpms WOT, and I run at WOT a lot. Still have 140-150 psi compression across the board, no engine problems, and no noticeable power loss. (a couple of gearcase problems, but those where impact related ;) .)The new stuff will definitely take high revs and last. As an example, I drive the wifes' Nissan Sentra once in a while, and it scares the bejeezus out of me when the tach hits 7 grand, but it doesn't seem to hurt it any.
Click to expand...
OK, so if I'm reading this correctly, I could feasably turn my engine into a 90 by changing the control unit?
 
That rpm should not damage anything since the higher hp versions run much higher revs. Not sure on the limiter, though.

On the hp de-rating issue, one would assume the camshaft is mainly how it's done, since the rev range is so much lower.

Jeff
 
The rev limiter will activate at about 150 rpms over recommended WOT (5650 rpm). The guardian system will kick in, the alarm will go off, and the outboard will drop rpms by dropping spark to 1 or 2 cylinders. Very, very annoying ;).
 
fastjeff said:
If I am correct (hah!) raising the rpms won't help IF the camshaft is made milder to lower the hp.

Jeff
Click to expand...
I did somr further checking into the motor and they didn't change the camshaft. They de-tuned the motor with the control module and a restrictor plate but the motor was still a little over the 75hp range, so the lowered the max RPM to get it to fall into specs.
 
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