Correct rpm range for 1986 115

[redjmp]

I have read on the prop selector here at iBoats the the correct wot range for my 86 115 is 4750-5250.
Is this correct.
Other years of I6's have 5000-5600 listed but if mine is indeed supposed to only be run at lower rpm, then that is where I want to run it.
This year has threaded con rods with small threaded rod bolts as opposed to the old nut and bolt design of other years.
And I have read that someone has thrown a rod through the block with this design perhaps because it was being over revved?

 

[emckelvy]

Re: Correct rpm range for 1986 115

The later-style "cracked cap" rod is actually a stronger design than the older machined-style, the person you mentioned had a problem likely because he re-used the stretch-type rod bolts which are only intended to be used once.

Your motor has essentially the same innards (except maybe for the rods) as a power-ported late-model 140HP and they were quite happy to run at the rated rpm of 5300-5800. If you prop yours to 5500-5800 at WOT lightly loaded she's not going to complain!

HTH........ed

 

[redjmp]

Re: Correct rpm range for 1986 115

Mine has low dome pistons. I think the 140's had high dome.I have heard guys running these in excess of 8000 but they are willing to rebuild them often.
Still I know that they can run at that range but if the selector says otherwise I have to question why.
What did mercury spec this year of 115 to run?

 

[OldMercsRule]

Re: Correct rpm range for 1986 115

My Seloc indicates 5000-5500 for the 1982 to the 1988 115; ALL THE SAME. The 1978 to 1980 140 s were specked out at 5300-5800 in the same manual and the older 115s were the same 5000-5500 RPM range. The change from crank rated to prop looks like 1982 so that would also support what Ed told you as the later 115s were the same as the 1980 140. The previous poster: (emckelvy) knows these motors as good as anyone arround so I would follow his recomendations. JR

 

[Clams Canino]

Re: Correct rpm range for 1986 115

What you prop if for (the WOT range)... and what you RUN IT AT are two totally different things.

All the inlines outta be propped for the high end of the reccomended range... so prop it to make 5200 - 5500 WOT in order to make sure it's not overpropped (too tall a "gear") and therefore lugging too hard.

Does that mean you outta acutally run it there as an SOP? Absolutely not.
The inlines cruise happiest at about 4000-4200 rpm (assuming they are propped right as per above). Running it at WOT all the time is like putting your car down in L2 and running it around at 5500 RPM all day. Sure, the racers do it all the time - they also overhaul them after every important race.

Summary... running around at WOT wastes gas... and will seriously shorten engine life - just like any other motor.

-W

 

[redjmp]

Re: Correct rpm range for 1986 115

What about gearing it for 5200(21 p) and running at 3500 (30 mph) on small 1600lb boat as this seems to give best fuel economy, better than 6mpg.
If I do this, what do I do to prevent damaging engine or at least to warn me of impending doom. How do I monitor combustion chamber temp or am I still safe running it this way?

 

[Clams Canino]

Re: Correct rpm range for 1986 115

Nothing wrong with running at 3500 - so long as it maintains the boat on plane without struggling at all.

-W

 

[redjmp]

Re: Correct rpm range for 1986 115

Thanks so much for the advice guys!

 

[redjmp]

Re: Correct rpm range for 1986 115

please delete