Optimal RPM for 4BTA

[Stepan Doubrava]

I have a Meridian 341 with 2x 4BTAs. I am trying to find the cruising optimum for my boat. People with this engine (but different boats) seem to cruise at around 2500 rpm. 

In terms of fuel economy my boat seems to have a plateau between 2500 and 3000 rpm (see the attached chart), where the boat is burning about the same amount of fuel per 100km but obviously getting there faster at higher rpm. 

My question is if running the engines at about 2800 rpm for extensive periods of time (1-2 hours) is ok for the engines?

My WOT is about 3150. Thanks a lot!

Topic

[Mark Tripi]

I cruise my 4BTAs at 2400 with a short burst of WOT once in awhile. These are maxed out engines at 250HP and I don’t like running them close to the top. 

[Stepan Doubrava]

Thanks for the link … it is a great article.

Cummins Diesel Ratings & Definitions

If I understand correctly in the intermittent duty mode for an engine rated at 3000 RPM I can only use run at full power (the 3000 RPM)  for 2 out of 8 hours and the rest of the time I should use it at or below the cruising speed which is 2700. 

If my understanding is correct 2700 seems to be my cruising speed, which feels right for my boat.

She is a 2005 model and I do not see any record from the previous owner that he had troubles with the injectors … but never say never.

Thanks again for your advise.

 

[Francis Valerio]

Cummins Rates their engines for what kind of duty.

Basically it comes down to how many hours you can run at 300 RPMs below the rated RPM of 3000 in your case.

You can search here on the sbmar website (not the forum) and there is an explanation of what I am talking about…  Look for “Intermittent Duty”

 

Also early 4BTA engines had injector issues and may have been upgraded on your unit.   Do you have any maintenance of repair logs from previous owner

[Stepan Doubrava]

Thanks Francis! 

Sorry I am not an expert mechanic and not sure what do you mean by CPL, but on my engine tag it says:

“Rated HP (metric) 250 at 3000 RPM” if that has any relevance.

I can actually get to 3150 under load.

I do not think I have any modification to the cooling system, but the temperature when I run the engines at WOT 3150 RPM for about 10 minutes get to 175F (not sure how accurate the gages are).

If I understand correctly the biggest concern when running at higher RPM than usual is the temperature, correct?

[Francis Valerio]

What is your CPL?  2197?

Your RPM rating is @3000RPM I believe.  WOT in neutral would go 3150 I guess sounds about right.  I would say you need to get 3100 loaded at WOT.

 

If above is true than I would say you are propped correctly.  Being propped correctly than you can start talking about what a good safe cruise RPM would be.

Are you monitoring EGT’s?  This would be helpful in determine how your engines are reacting to your loading them up when cruising.

 

Remember 2800RPM is only 200 “off” the rating of the engines.  There is not much room for a sudden temperature spike.

Have you done any cooling upgrades?  sbmar 160F thermostat?

I would say that subtract 300RPM off of the rating speed of 3000rpm which will give you 2700RPM

Modifications to the cooling systems and ensuring all is 100% correct you “may” could squeeze that other 100rpm out of them but I would rather hear that suggestion from Tony himself…

[Author]

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