Cummins QSM 11

[Mongo]

I am trying to find out what the difference is between the 660hp (very common) and the 715hp (uncommon) QSM 11. I have a boat here with a pair of 660hp QSM 11 and there is one like it with 715hp QSM 11. Cummins shows that the 715hp engine gets better economy than the 660hp engine. Does anyone have experience with these two engines? Thanks.

[seapower]

M11's are a well known, time tested mid-range engine made by Cummin's. Generally speaking the more horsepower you get out of a given block, the less total engine life you'll can expect. Depending on the application, how many hour's a year your planning to use the engine and the amount of load required, would determine my choice. Just remember, horsepower isn't free. Frank

[Nomad]

I am trying to find out what the difference is between the 660hp (very common) and the 715hp (uncommon) QSM 11. I have a boat here with a pair of 660hp QSM 11 and there is one like it with 715hp QSM 11. Cummins shows that the 715hp engine gets better economy than the 660hp engine. Does anyone have experience with these two engines? Thanks.

I have the 660's in my boat, but considered specing the 715's. To make the decision (which was about a year ago, so I may be a little off on my recollection), I studied the power and fuel consumption curves very carefully. From what I recall, the 715's are rated to a high rpm (2500 vs 2300?), but actually put out less power at 2000 or 2100 rpm. I do not recall that they were more efficient, at least below 1200 rpm (which is were most of my use is). I actually considered going with the 450 hp wet turbos, but concluded that at least for resale, I would need the high hp. Also, the additional speed that would be obtained by a 10% hp increase is, theoretically in most cases, not more than 5%. For my use at least, I concluded that I would prefer the presumably longer life 660's.
Good luck. Rick

[PacificBlue]

I was able to run both in the same boat model and the benefits were minimal at best. Stick with the QSM11 670's, as the 715 is mostly a "paper" gain on hp, allowing them to add another model. You will also burn more fuel. Same is true for the QSC8.3 550 vs. 600.

Remember, whenever you are comparing two engines with different RPM, compare the data at percent throttle to see how they stack up.

You will get longer life (between overhauls) out of the 670 version.

[jrzcape]

I put the qsm-11@715 hp in my new 45'Jersey Cape and love them,30knt. cruse burning 44gph (+or-) so far a great motor with a great background, We also use the qsc540hp in the 36' and have tried the qsc500hp and the qsc600hp and have found the 540 to give us the best performance.The 600 runs at a higher rpm and burns more fuel but goes a little faster.
I hope this helps you, Wayne

[JPH]

The QSM-670 has proven to be an excellent engine in the field and I’d expect the QSM-715 also to build an excellent reputation over time. Your question as to which engine to select can only be answered by asking; what is more important to you, cruise speed or top speed? If top speed, select the 715. If cruise speed is more important, select the 670.

Here’s why? The 670 is a 2300 RPM engine and the 715 is a 2500 RPM engine. Now, ASSUMING the same transmission ratio, the 2300 RPM engine (670) will swing a wheel with more pitch compared to the 715. The 2500 RPM engine will swing a wheel with less pitch compared to the 670. At every speed through 2300 RPM, the 670 will be faster because the wheels have more pitch than the wheels on the 715. Only when you get above 2300 RPM will the 715 be faster, but marginally so, because the wheels have less pitch vs. the 670 wheeled boat. Boats are moved by wheels in the water, not engines!!

If torque is a big issue and is badly needed due to the hull characteristics, there is a slight 715 advantage despite the wheels with less pitch vs. the 670.

Whichever HP rating you have, prop them to see greater than their maximum recommended RPM, do the preventative maintenance, and enjoy them! They are both good engines!