Importance of engine running at max RPMs

[Bondo]

Re: Importance of engine running at max RPMs

running at 4000rpms. I have a 4.3l merc. alpha 1. I ran with this prop for an hour or two. The next day the engine or outdrive had more vibrations and hummed a little. Could I have already caused damage?

Ayuh,.. A Compression Test will tell you that answer...

 

[Span 24]

Re: Importance of engine running at max RPMs

Absolutely it will cause damage. Specifically to the valves first. Since "lugging" causes excessive combustion chamber heat, the valves get too hot and deform. This is called "tulipping" the valves. This will cause them to not seat properly, and you will lose compression on the affected cylinders. Loss of compression will lead to the affected cylinders to not fire properly.

OK, so please expand upon this. You do mean that ~prolonged~ lugging will cause this damage, right? I mean, a two hour trip on the wrong prop which runs WOT at 400-600RPMs below the manufacturers spec does not ~ruin~ the valves, right?

My issue (described in detail above) certainly did no permanent damage to my engine. I can believe that an engine which is already questionable and poorly maintained, old, and perhaps headed for scrap anyway could be irreparably damaged by running 400-600RPMs under designed WOT for a solid hour but really, should we all make certain that even our spare prop is pristine and suitable for all load ranges?

Michael

 

[QC]

Re: Importance of engine running at max RPMs

He is concerned about prolonged operation at WOT when the engine does not reach recommended WOT RPM. If say it was 400 RPM under at WOT and then you ran the boat prolonged at 1000 RPM under, much less of a big deal although still not optimal.

 

[David Greer]

Re: Importance of engine running at max RPMs

I'm confused. Are you all saying that you ought to run your engine at full speed all the time or you will hurt your motor? And its more efficient? WOT is "wide open throttle" or "floored", right?
But on my performance chart from Yamaha I get 8.8 GPH/3.57 MPG at 4000 rpm but 18.6 GPH/2.4 MPG at 5300. I shouldn't run at 4000 or I am "lugging my motor"?

 

[Span 24]

Re: Importance of engine running at max RPMs

I'm confused. Are you all saying that you ought to run your engine at full speed all the time or you will hurt your motor? And its more efficient? WOT is "wide open throttle" or "floored", right?
But on my performance chart from Yamaha I get 8.8 GPH/3.57 MPG at 4000 rpm but 18.6 GPH/2.4 MPG at 5300. I shouldn't run at 4000 or I am "lugging my motor"?

David

As indicated by SteelSpike in post number 4 of this thread I don't think that anyone is suggesting that operating your engine at any setting other than WOT (throttle lever full forward) is damaging your engine, only that the engine should operate within the recommended WOT range while at WOT.

I see it as relative. Meaning that if your engine is properly tuned (considering many things including weight, mass, fuel, ignition, and prop) this will be proven because it will run within its recommended WOT range while at WOT. If this is so then it will be properly tuned and so will not 'lug' at any throttle setting within its power range, idle to WOT. Does that make sense? Did I get it right oh marine gurus?

Michael

 

[MikDee]

Re: Importance of engine running at max RPMs

Getting the right prop on your boat, is like having the right gear to keep your engine in the sweet spot of it's maximum rpm range. This keeps it running the cleanest, and most efficient, throughout it's full rpm range. Delivering best power, & fuel economy, keeping nasty combustion deposits at a minimum, and preventing excess stress on all it's parts. Basically, It takes the load off your engine, prolonging it's lifespan, no matter what throttle setting it's at

I don't know how to word it any more clear, & concise?

 

[QC]

Re: Importance of engine running at max RPMs

Yes, what Span the Man said, and ignore my WOT comments unless you want your brain scrambled . . .

 

[David Greer]

Re: Importance of engine running at max RPMs

Thanks Span for 'splaining it to me. Set it up to hit the top numbers and it's good through the whole range. Looks like I get to take the clipboard out on the river and do some testing!

 

[QC]

Re: Importance of engine running at max RPMs

Set it up to hit the top numbers

With a typical load and trimmed for max speed . . .

 

[David Greer]

Re: Importance of engine running at max RPMs

The trim tabs part is a new dynamic to me. Although I've always had power trim, these were smaller boats (16-19', 50-70hp, light flat hulls) and I trim by feel and ear. Now I have a heavy 21', deep V, trim tabs and 150 hp. I enjoy having the tabs but find I can tweak both the tabs and the motor trim to affect the plane and thus the mph/rpm. I just need to play with it to find the best trim on the motor and then adjust with the tabs. My level guage doesn't work but I don't see what use it is, really, with the other factors in play.