1977 Evinrude 70hp vs 75hp

[jclute]

Hi all,<br />After looking through the specs in my Seloc manual I see no differences between the 70hp and the 75hp Evinrude except for the increased RPM's from 5000 to 5500 and 1 degree on the advance. Does this mean I could run my 70 at 5500 RPM's and effectly have a 75hp?<br /><br />Thanks all,<br />-jc

 

[Chinewalker]

Re: 1977 Evinrude 70hp vs 75hp

Hi John,<br /> Not exactly. The 75 has slightly different port tuning - a little fatter at the top end, with the 70 a little fatter in the mid-range. There is no easy upgrade... Best bet with a motor that old is to make sure you're getting all the rated power - decarbon, synch carbs, set timing to spec, etc.<br />- Scott

 

[jclute]

Re: 1977 Evinrude 70hp vs 75hp

Thanks Chinewalker. I was getting greedy!<br />-jc

 

[ob]

Re: 1977 Evinrude 70hp vs 75hp

Blue Blazes ,Your 70 hp should be propped to turn up 5500 to 6000 at WOT.

 

[jclute]

Re: 1977 Evinrude 70hp vs 75hp

ob, are you sure? My Seloc manual shows the WOT RPM's to be 5000 for the 70HP. I know manuals are often wrong but 5500 to 6000 seems a little high.<br />Any information is appreciated.<br /><br />-jc

 

[Dhadley]

Re: 1977 Evinrude 70hp vs 75hp

Lug it to 5000 and it will die a horrible and premature death. Prop it to 6000 to 6200 and it will live long and happy.<br /><br />Good luck!

 

[jclute]

Re: 1977 Evinrude 70hp vs 75hp

OK guys. Now I'm confused. <br />Is this typical? How does one know at what RPM to prop if the books are wrong?<br />Is this information written down somewhere or is this just learned by experience?<br />I guess I am just a little paranoid to prop it that fast.<br /><br />Again, thanks all!<br />-jc

 

[ledgefinder]

Re: 1977 Evinrude 70hp vs 75hp

Doesn't mean you have to run it that fast all the time - back off to 3/4 throttle once you get going.<br /><br />The idea is to keep the load on the motor low, let it rev a little freely & don't lug it. Lugging - say running it wide open throttle, continuously, at 4200 rpm - encourages preignition & bangs away at the motor. <br /><br />If you look at the factory recommended ranges for the 65-75hp OMC 3cylinders, you will see several with 5000-6000 range. They have virtually the same bottom end as yours (cranks, rods, case, bearings, case), just a little different porting. <br /><br />If you want some experience on what happens when you run 6500-8000+ (though there are racers here), ask the same questions on 'scream and fly' performance boat website.