QSC 8.3 exhaust manifold

[boaterholic]

I was told that on the QSM 11's people are having to change the exhaust manifolds every 4-5 years due to wear and corrosion where it mates to the block causing leaks that are evident by the soot streaks. Is this also the case for the QSC 8.3's?
And on the QSC 8.3 540hp are there any weaknesses to be aware of?
Thank you!

 

[ttmott]

whom told you that?

 

[boaterholic]

I guy I know that had those engines on his boat, and at about the 4 ish year mark started seeing the soot marks where the manifold mates to the block and shortly after had his mechanic look into it. Ended up having to replace the manifold and gaskets, and Cummins told him that they were having issues with those and had numerous one replaced at about the 4-5 year mark. So got my curiosity on the QSC 8.3's as there are some boats I was interested in powered with those and checking if anyone also experienced that.

 

[Shaps]

I've had mine 4 seasons, approx 300hrs. The engines have ~550 hrs on them overall. Have neither experienced that personally nor heard anything like that.
Of course, YMMV.

 

[boaterholic]

good to hear, thanks!

 

[ttmott]

Like most marine engines that start life as terrestrial their power ratings are elevated. Elevated power ratings come with higher temperatures and pressures and stresses. With respect to the Cummins QSM11, your friend is not fully informed as to the principal cause of the issue. The primary cause of the leakage at the exhaust manifolds is the increased load beyond specification that Sea Ray implemented with over-propping of the boats. This is well documented. Specifically, EGT's were becoming excessive in the 1700 to 1900 RPM and causing the manifolds to warp and leak. A contributor to the warping was the manifold bolts were too short and not capable to be stretched within the elastic range enough to accommodate appropriate clamping forces at operating temperatures. The Cummins response in a Bulletin is to change the short bolts to longer. The correct response is to remove pitch from the propellers, install the longer fasteners and cruise around 1900 RPM. I think it's been over five years ago that I removed the manifolds and machined their faces true, installed the longer bolts, and removed almost three inches of pitch from the props; no indication of leakage to date. As far as "corrosion" - the condition isn't corrosion but erosion from the exhaust gases.
Anyhow now you're more informed on the QSM11.

Aren't the 8.3 exhaust manifolds water cooled?

 

[boaterholic]

Like most marine engines that start life as terrestrial their power ratings are elevated. Elevated power ratings come with higher temperatures and pressures and stresses. With respect to the Cummins QSM11, your friend is not fully informed as to the principal cause of the issue. The primary cause of the leakage at the exhaust manifolds is the increased load beyond specification that Sea Ray implemented with over-propping of the boats. This is well documented. Specifically, EGT's were becoming excessive in the 1700 to 1900 RPM and causing the manifolds to warp and leak. A contributor to the warping was the manifold bolts were too short and not capable to be stretched within the elastic range enough to accommodate appropriate clamping forces at operating temperatures. The Cummins response in a Bulletin is to change the short bolts to longer. The correct response is to remove pitch from the propellers, install the longer fasteners and cruise around 1900 RPM. I think it's been over five years ago that I removed the manifolds and machined their faces true, installed the longer bolts, and removed almost three inches of pitch from the props; no indication of leakage to date. As far as "corrosion" - the condition isn't corrosion but erosion from the exhaust gases.
Anyhow now you're more informed on the QSM11.

Aren't the 8.3 exhaust manifolds water cooled?

Good info for me, thanks! Not sure on the QSC being water cooled, I'm in the learning process and will check into that. Assuming they are water cooled and engine doesn't overheat then the warpage should not be an issue on the QSC's?

 

[ttmott]

I think the manifold issue is unique to the QSM11. Regardless, Sea Ray had a propensity to over-prop their yachts for some reason. In all cases with diesel engines this needs to be addressed.
One thing I missed on the QSM11 is to target 17GPH at 1800 RPM when taking pitch out or have EGT available.

 

[Shaps]

Good info for me, thanks! Not sure on the QSC being water cooled, I'm in the learning process and will check into that. Assuming they are water cooled and engine doesn't overheat then the warpage should not be an issue on the QSC's?

I do believe the QSC exhaust manifolds are water jacketed