QSB 6.7 Re-Rating 480 to 550

[Joe F]

Hello All, I’m helping a family member in Florida with his boat, a recreational sportfisher with twin QSB 6.7 480HP@3300 (CPL 4191) with ZF 2800 series pods.

New it turned 3350 at WOT. Now, with added gear, dirtier bottom, etc it is operating above the Cummins prop demand curve at cruise and only reaching ~3200 WOT.

One option is to change props ($$ x 4).

Is it feasible to change motor to the intermittent duty rating of 480HP@3000 RPM or HO of 550@3300? If feasible, and the pods can accommodate higher HP, is this a viable approach to ensure the engines are operating within Cummins fuel curve specs?

Thanks
Joe

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Brad

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[Tony Athens]

Steve,

I think you are over thinking this, or maybe you are thinking this way ??

Yes—all of the CPL4191’s are the same engine internally so put them all on a dyno at 15 GPH @ 2600 RPM and they will all “wear out” at the same rate.. IMO, the prop curve fuel burn has nothing to do with it………..But with that said, (another IMO), in general, the higher RPM you pull that HP at (ABOUT 300HP) , the easier it is on the engine ( like on the 3300 RPM versions)—WHY? Again, another IMO—BMEP is lower, & you are pumping more coolant, pumping more oil, more seawater, more oil thru the piston cooling nozzles, etc etc..

Tony

P.S. The 550 Version does have an extra oil cooling plate in the oil cooler to help keep sump temps within spec when at very high loads. At 20 to 24 GPH of fuel burn, I do not think it matters..

[Steve Lewis]

CPL 4191

Tony and team,

I was thinking about this thread while I was mowing the lawn(Yes I have issues). If the QSB 6.7 480 and the QSB 6.7 550 are the same CPL AND Engine Config (D313011MX03) Then are these the same EXACT engine? Therefore can we look at a QSB 6.7 480 as if it is a QSB 550 and use that fuel burn graph as opposed to the QSB 6.7 480 Fuel burn graph with no ill effect?

Or put differently:

The fuel burn difference in the Cruise RPM range (2400 – 3000) is any where from 10% to 15% more on the 550. SO…………. We can look at a QSB 6.7 480 that is over propped and generating fuel burn numbers that are BETWEEN those published on the 480 and the 550 graphs and still be “just fine”? As you have indicated before, it is fuel burn that really matters. Stay at OR BELOW the fuel burn curve in the cruise range. So the 550 curve is our mark on both engines??

OR are we looking at the same Diamond450/480CE discussion down the seaway?

The reason I ask goes to boat hunting. There are boats out there with QSB 6.7 480’s that are propped more like the 550(those Zeus Pods) and I have frowned at that. If this is well and truly the same engine with just different fuel delivery mappings then I can include the 480 equipped boats in my search and give them a good solid look.

Thoughts??

Steve Lewis

[Tony Athens]

Here is a 2nd “practical view” based on engine “wear & tear”…………..Re-rating will not change that —Things will stay the same that way… Only a prop change will do that..

Do you really need to do anything–IMO, NO as long as the GPH at, lets say, 2200-2600 RPM stay the same as time goes by….

Tony

[Joe F]

Thanks Tony. Yes, low annual hour vessel.

Top end speed is not a concern, and it is currently operating below the prop demand curve for both the 550@3300 and 480@3000 ratings.

We will investigate the cost of re-rating as well as prop modifications.
Joe

[Tony Athens]

Basically both re-rate options can be done as the CPL is the same… The 480 @ 3000 is below:

FR95035 FUEL RATING
RATED POWER: 0473 BHP @ 3000 RPM
TORQUE PEAK: 1165 FT LB @ 2000 RPM
GOVERNED POWER: 0473 BHP @ 3075 RPM
CPL: 4191
CERTIFIED BY: IMO ,IMO ,EPA ,EU
CAL RESTR: NONE
POWER RATING: INTERMITTENT
RATED POWER IS 480 MHP AT 3000 RPM (353 KW)

Plus the props could be changed or re-tuned. Cost–No clue.

From the practical stand point and my view– Practical=Go with what works best for the pocket book……….. My View = best for the engine & do the prop thing–go for 3400RPM.

I am assuming this is a low annual hour vessel, correct?

Tony

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