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Author Topic:   1996 Johnson 70-HP Maximum Engine Speed
gmoulder posted 06-30-2014 04:51 PM ET (US)   Profile for gmoulder   Send Email to gmoulder  
I would like to know at what maximum engine speed my 1996 Johnson 70-HP two-stroke engines can be operated. I have seen statements such as "high operating range." I would like to know when can I expect to hear expensive noises coming from the transom. The 1988 Outrage 18 has the engines at the correct height I raised them from when purchased, and then lowered them to a point where the cavitation plate rides just on the surface of the water. The props are Viper 19". The rough boat speed and engine speed readings are: 25 = 4000-RPM. 32 = 5,000-RPM, 37 = 5,400-RPM. This is with four adults and half-full tank of fuel. I trim in and out to achieve the highest speed for a particular rpm. The boat has very good pull and torque in the 4,000-RPM range, and there is "jump" from 5,000 to 5,400. I am interested in knowing where the maximum speed would go if I went WOT; and, if I were to re-prop for speed only--which I am not--what would that speed likely be? I believe that, ideally, the boat should run out of power to accelerate and increase speed at the point of maximum RPM with a light load. The boat feels as though it will run right through its maximum operating rpm limit (if that rpm is 5600) before the going forward forces are equaled by the going backward forces. Thanks, Glen
Teak Oil posted 06-30-2014 09:24 PM ET (US)     Profile for Teak Oil  Send Email to Teak Oil     
Looper (loop charged) outboards like yours like RP<. You can spin those motors up to 6000 RPM without fear of damage.

You are propped too high right now. You should be running 17" props and getting up around 5,800-RPM with a light load. You may also see a little more speed since you will be letting your engines produce full power. Those 70s are great motors

tedious posted 06-30-2014 11:06 PM ET (US)     Profile for tedious  Send Email to tedious     
Glen, it's impossible to predict maximum RPM and speed perfectly - it would be easier to guess if you can provide info on what speed and RPM you get at WOT. All we know is that 5400 = 37 mph; is that your WOT reading?

The motor's redline is 6000 RPM and you'll get best performance if you choose props that let you hit that, or even a whisker more, when you're running light and fast - say with just you in the boat, and 1/2 tank or less of fuel. I would not change props until you run that experiment.

Those motors can overheat if lugged - you really want the WOT RPM to be right around 6000. If you can't get there, running light, you'll want to drop down in pitch.


gmoulder posted 07-01-2014 09:13 PM ET (US)     Profile for gmoulder  Send Email to gmoulder     
Tim and Teak Oil,
Wow, had no idea that max rpm was 6000. Will take the boat out on Friday [before watching the fireworks at the Kemah Boardwalk] and see where WOT ends up. My rough math says that the boat gets 7 mph for each 1000 rpm [at that range]; so, 32 + 7 = 39 at 6000. We will see. Will report this weekend. Thanks again. Glen
dgoodhue posted 07-03-2014 09:51 AM ET (US)     Profile for dgoodhue  Send Email to dgoodhue     
OMC operating range 5000-6000 rpm, suposedly it has 6700 rpm limiter. My motor does about 6200rpm at WOT. I have seen as high as 6400 when I was playing around with props but with it being an older engine (mine is a 1989) I don't like to rev it that high.
gmoulder posted 07-06-2014 07:31 PM ET (US)     Profile for gmoulder  Send Email to gmoulder     
Tim and Dgoodhue,
After the decarbonizing treatment, and running 2 gallons of fuel with 2 cans of Seafoam through the port engine, took the Outrage onto Clear Lake this morning for speed tests, now that I know what the maximum rpms are. First, all the treatment did world's of good for the engines, most of the issue with the port engine stalling at low rpms has gone; second, turned the boat loose and found WOT for starboard at 6000 and port at 6200; gps was saying 39 / 40 with small chop and bimini up. This is what was calculated, the speed increase to rpms is linear. Thanks also to Tom Clark for the recommendations of the 19 Vipers, they seem to be just the trick. In addition to WOT and max [advisable] rpms coinciding, a really neat thing is that from 4000 rpms the boat really jumps. Thanks to all, Glen
Peter posted 07-07-2014 07:13 AM ET (US)     Profile for Peter  Send Email to Peter     
Given that the motors are running at max RPM or above at WOT and the 70s turn the prop shaft through a 2.44:1 reduction, I think it would be worth trying 21 inch pitch Vipers.
tedious posted 07-07-2014 10:28 AM ET (US)     Profile for tedious  Send Email to tedious     
No, those existing props sound perfect, judging by the speed run. 6200 is not going to hurt anything, and you'll still be able to keep the revs up even when the boat is more heavily loaded.

You do not want to lug these motors (or really any two-stroke) as the fuel-air mixture contributes a lot to the cooling. Plus that kick in the pants from 4K to WOT is awesome - why spend money to lose it?


Peter posted 07-07-2014 11:41 AM ET (US)     Profile for Peter  Send Email to Peter     
Lugging would be running at 5000 RPM at WOT. Seems to me that WOT top speed on an Outrage 18 should be higher than 39 MPH with twin 70s. Perhaps its a bit faster with the bimini top down which means that the motors would be even less loaded with it down and the RPMs at WOT even higher. That's why I would try 21s to see where the tachometer falls at WOT before I concluded that 19s were the right choice.
tedious posted 07-07-2014 12:25 PM ET (US)     Profile for tedious  Send Email to tedious     
Yes, that's exactly what I was concerned about - 5000 may be about what you get, with the 21s and a heavier load. But no harm in experimenting with the 21s if they can be had for free; I just wouldn't spend money on it.

Hard to say on the maximum speed - so much depends on bottom condition, any water in the hull, drag due to T-top, bimini, etc. Certainly two 70s are not going to go as fast as a single 150 - less overall power, and two gearcases and props in the water. My sister-in-law's 18, with a Honda 150, tops out around 41, with a fair amount of water in the foam and bottom in moderate to rough condition.


dgoodhue posted 07-08-2014 09:52 AM ET (US)     Profile for dgoodhue  Send Email to dgoodhue     
If it was me, I would rebuild the carb on your 70's given stalling issue and that seafoam made a difference. My observation with 3 cylinder OMC's over the years is that most of the failures are from a lean carb.
tedious posted 07-09-2014 07:59 AM ET (US)     Profile for tedious  Send Email to tedious     
Dave's got it right - rebuild the carbs as a winter project and redo the link and sync per the service manual. Then next time you have a problem on vacation, you'll be able to fix it right then and there.


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