Twin 1988 250hp 5.9 6BTA, what are the optimum RPM's

[sailusvi]

Hello all, new guy here.

I have a 1988 41ft Defever with twin Cummins, 1000 original hours on both engines. My question is about RPM’s and what should I be running the boat at? Also at what rpm do the turbos kick in? How important is it to run the RPM’s higher for a bit if I’m poking along at 8 knots.
Thank you
Rob

Topic

[Tony Athens]

Read , and reread what Don posted……………………………… This is REAL, not a piece of “paper”form of engines tested in a test cell under controller conditions.. You have a boat, not a “test cell” to gauge engine operating parameters…..

Understand that this still applies–IMO, more that ever as the HP per liter keeps going up……

Tony ………………,,,,,,,,,,,,,,,,

[sailusvi]

Thanks Donald,

Geez, if I’m keeping the ol’ gal I have some time and money to spend, over and above what I was thinking hah! Its boating and I still love it! Especially here at anchor for the night about 50 miles south of Columbus, Mississippi, on a friends boat.

[donald roth]

Since I have been nursemaid to several older 6bta 250’s, I will chime in here to report what I have learned:
The raw water cooling system is barely adequate for any hard use.
Fortunately, most of the parts needed to upsize it are readily available off of larger hp Cummins motors.
Tony is the guru of these mods, hopefully he’ll add some info or point you to it.
The coolant loop can be modified to improve flow too.
Blowby is the aftercooler’s worst enemy, and servicing them is a royal PITA. (get an Envirovent)
The serpentine belt/tensioner can be improved on.
The transmission coolers can be prone to failure.
And last but not least, the overall survival of the motor is 100% reliant on correct exhaust system geometry.

[sailusvi]

Thanks y’all

[sailusvi]

By the way, my hoses are still the original ones….I’ll be needing complete hose kits for each engine.
I’m guessing thats only the beginning

[sailusvi]

Thank you Tony, although I admittedly don’t know much about these engines I am fairly handy with a wench. Knowledge is what I seek and with that knowledge I will be better equipped to dive into the world if early production 250hp 6BTA5.9’s repair and maintenance. Is this early engine problematic? Do people not like them?
This was a fresh water boat for many years on Lake Ontario. In its later years it’s been in the salt but for only 5 or 6 months a year, then goes on the hard for winter.
I am an ol’ 4X4 truck collector, mostly Chevy.
Thanks,
Rob

[Tony Athens]

Sailusvi,

I need to ask.. Do you really want to lean about your very early 6BTA 5.9’s, and then slowly make changes as needed so you can be sure you have done you best to get the most life from your engine ?? Understand the vintage of your engines and they have been operating in salt water for nearly 30 years.

Let’s go there first as with your last post, I’m not to sure what you really want to accomplish here..

And yes many have given it all–Way too many and way too much..–I personally know many of them, and IMO, it was a true waste of their lives as it accomplished absolutely nothing as to what these wars are supposed to be all about..

Tony

[sailusvi]

Thank you Tony, I don’t think I have ever gone past 2200rpm
And don’t know if it will.

[sailusvi]

Thanks Rob, I had no idea.
The ol’ gal had low hours
when I bought her and I
haven’t done much, unfortunately

[Rob Schepis]

The corner tank “worked” ( if that is the right term – or “could get you by”) in low annual hour, low load application when everything was right.. Based on your initial post of running around at 8knots maybe a good amount of your use would be as such. Otherwise, one small coolant /seawater glitch and you have zero chance to do much other than overheat. It does not meet Cummins requirements.. Think of it as not more than a “bulge” in a coolant line with a pressure relief cap and fill. It’s not a true expansion or “header tank” as the Cummins “definition” goes……. As far as sizing – per current requirements of the Cummins install specs the exp. tanks are to be sized at a minimum of 20% of engine coolant capacity plus 5% of total system coolant capacity. That’s ~1.5 gallons……
Think of it like an insurance policy – you don’t know how it will really do you until you really need it…

[sailusvi]

Rob, I had no idea my tank is flawed 🙂

[Rob Schepis]

Corner Tank ——> Expansion Tank

Looks like plenty of room for a REAL expansion tank…

[sailusvi]

A couple more, hope you can read em, thanks!

[sailusvi]

I just found some photos I took awhile back

[Tony Athens]

1988 6BTA 250’s?? A CPL 1247 sounds about right-CAV Injection pump. JWAC engine, low boost turbo if all is original and a MINIMUM seawater pump.————————–shoot for and accurate 2700 -2750 RPM “loaded for bear”, and then you are safe to cruise at 2300 RPM or less if all the rest is RIGHT.. If you cannot not make that, you are leaving HP on the table and also over loading your engine at lower RPM’s…

Tony

[sailusvi]

Thank you for the reply Corey, my boat is up in Maine and I’m on the Tom Bigbee waterway. …dang it. As soon as I get the info I’ll post it.
Thanks again

[Corey Schmidt]

Rob, do you have serial numbers for the engines or perhaps a picture of the engine data tags so we can see exactly how they were setup from the factory?

[Author]

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