480CE High Fuel flow at Cruise

[Joe Schmidt]

I just completed a survey and sea trial on a boat with twin 6CTA8.3-M (480 CE) engines. During the sea trial I had a cummins tech riding along with his laptop hooked up monitoring the engines. He said they ran good with the exception that they didn’t make full RPM. Once I got his full report, I noticed that the fuel flows during cruise were also way too high compared to the cummins power curve. Looking through Tony’s tips I learned that these engines can have big issues when they don’t make full RPM. Needless to say, I’m a little concerned. I used the prop calculator on Boatdiesel and the prop that is on there is the recommended one, so I don’t think its an over-propping issue unless the boat is way overloaded for some reason. Fuel was only half full, and unless there’s water hiding somewhere it should have been average weight. On another forum one user suggested it could be an injector issue. When I questioned the tech he said it could be a turbo issue. I can’t figure how to post a picture here, so I copied the performance numbers below. Any insight on what I should look at would be very helpful.

SEATRIAL OPERATING AT CRUISE
PORT STARBOARD

CRUISING RPM 2283 2285
COOLANT TEMP 173 170
ENGINE OIL PRESSURE 65 71
ENGINE OIL TEMP 185 181
BOOST 20 20
FUEL RATE (GPH) 21 21
ENGINE LOAD% 85 85
TRANS OIL TEMP 130 132

SEATRIAL OPERATING AT FULL LOAD
PORT STARBOARD

CRUISING RPM 2513 2508
COOLANT TEMP 179 179
ENGINE OIL PRESSURE 64 63
ENGINE OIL TEMP 195 194
BOOST 23 23
FUEL RATE (GPH) 24 24
ENGINE LOAD% 100 100
TRANS OIL TEMP 138 144

Topic

[Tim]

Yep you’re over propped

BTW, I now run 23×25 props vs. stock which were 23×27. Hope that helps!

[Steve Lewis]

A couple things

Your fuel burn at 2280 is what you should be getting around 2500+. So at a minimum you are looking at moving the fuel burn 200rpm. I might argue that you need to move it 300 rpm just to be safe with this engine. You are looking at a pitch reduction of~3 inches. Maybe more if you have prop slip that is out of the norms. I have not seen any indication in your posts about prop dimensions. Do you know the prop specs? Diameter and Pitch. You might get told that you need new props as opposed to being able to modify the existing ones. Be prepared for that as a possibility.

I attached an article that I wrote about the procedure for plotting your fuel burn more completely and then what to do with that data. Give it a read and then do the procedure and post a more complete set of data. 2 data points on a sea trial give you an indication but we need to see the whole chart from 2000rpm to WOT including vessel speed so that we can also see how much prop slip is involved.

About WOT, you need to make some where in the 2680 range for this particular engine. When I sea trialed our boat, we made 2680. After 1 inch of pitch removed we make 2680 easy and we have some throttle lever travel left to go so WOT will take care of itself when you are dealing with your cruise rpm range.

About the fuel burn in the higher rpm range. When you run the engines faster, your boat goes faster and sits in the water better so it makes sense that you are closer to the curve in the higher rpms. What really matters is what rpm you cruise your boat at how how it sits in the water and loads the props.

Smoke is an indication of the air fuel mixture being off. It is usually on the air side. Have your cooling circuit serviced the “Seaboard Way” and you will likely find that the Aftercooler needs cleaning which can restrict air flow into the intake which can mess with the “combustion system” and lead to some smoking. There are other causes but the Aftercooler needing service is higher on the probability scale. you really won’t know until you start working on things.

Propellers are not VooDoo but they are also not a simple calculation. There will be some messing around to dial it all the way in. My situation, as written before, is that we needed to move our fuel burn 100rpm up the rpm range. I need my 2200 burn to move to 2300. I figured 1 inch but ignored the fact that my prop slip was higher than it should be. So when I had the 1 inch taken out, prop slip improved but my fuel burn did not. So I should have taken out 2 inches. The prop shop just wanted WOT rpm specs and was not wanting anything to do with messing with cruise range rpm. So I just looked at him and said “Take an inch out please” and he said “Ok”. Knowing what I know now, I should have asked for 2 inches…….. We will do it again over winter layup. We started with 23×27, we are currently at 23×26 with the goal of getting to 23×25. And based on a couple of other owners with the same boat that I have, 23×25 looks like a reasonable goal.

Also my mechanic now has 2 upgraded heads in his possession so we will get 2 new heads installed over winter layup to go along with the further reduced prop pitch. Lots of work being done this winter……..

Spring launch 2022 will be a highly anticipated event for us.

Keep us in the loop on your progress.

[Tim]

Yep you’re over propped

Disclaimer: Someone here a LOT more experienced than I will likely chime in to help / correct me.

You’re not just shooting for the 13.6gph@2200rpm but also ensuring that you hit your “Maximum RPM” which is approx 100 over … should be around 2700.

So you’re striving for 13.6gph@2200 per engine but also that you actually hit that in/around that 2700rpm mark at WOT.

[Joe Schmidt]

Thanks!

Thank you everyone for the responses. The boat is a 40′ Riviera Convertible with twin disc 1.77 gears. Bottom and running gear were spotless, we hauled before the sea trial.
I agree that it is definitely overpropped, I have been in touch with a prop shop already. My only concern is whether or not something else may be effecting my fuel flows. The port engine was smoking throughout the rpm range; not a lot, but visible smoke at all rpms above 1000. Nothing from the starboard side, but the report shows exactly the same fuel burns. I attached the actual numbers from the report. The fuel flow at 2500 rpm is a lot closer to the 480 curve than the flows at ~2300, which to me seems odd. I would think if it was all from overloaded prop that the fuel flows would be further off at higher RPM’s. I could be completely wrong though.
My other question is how do I figure out exactly how much pitch to take off to get the 450 curve numbers? Is it going to be trial and error, that seems like it would be pretty expensive to get the pitch tweaked multiple times. I plan to do some more runs to get the flows at multiple RPMs, specifically at 2200 to see how far I am from Tony’s 13.6. Any other advice besides use the prop calculators based on a 450 curve to get what pitch it should be?

[Greg Weaver]

35 Carolina Classic here and I burn 34 gph at 2340 and 24.5-25.5kts 69% load

You are deff over propped.

[Tim]

Yep you’re over propped

I have almost same boat 2004 Dancer 460 and it weighs around 40k all loaded up (full fuel, hard top, full water and all of my alcohol we carry around). These boats typically come with 23×27 wheel. I’ve moved to 23×25 wheel and the engines are much happier for sure. It’s still not quite where Tony recommends 13.6gph @2200 but close.

[Jon Tisdale]

Agree with prop issue

I run a 40 express with the 480 CE’s and weigh in just under 40k with a full load. Just barely on full plane at 1980 or so I burn 12.6-12.8 per side. Comfortable high cruise, 2200 or so I see 15-15.5 gph. Could be you have a prop issue, or the condition of the bottom of the boat may be affecting the numbers as well. Make sure you’re not trying to push top end numbers with growth on the hull. Especially if you’re Not sure when it was pulled last or cleaned. Injector issue would cause smoke typically, if you had one stuck open you would see the remnants of Un burned fuel, aka black smoke and typically your fuel numbers would be way off side to side, port running 14-15 and stb running 24, that’s when to check for stuck open injectors. Just my .02….. for the most part I really have had good luck with these engines

[Steve Lewis]

You are over prop’d

What make, model and year boat are we talking and what are the dimensions of your props and what trans ratio do you have? How many hours on the engines? Posting pics on the original post is a bit of a known quirk but you can post pics on your response……

You really are over prop’d if you are burning 42ph total at 2280. you should be in the range of 30gph at 2300.

My 460 dancer weighs in around 30K and that loads the props which loads the engines. Read the article about Props moving boats and engines turning the props. I am still burning too much fuel after having 1 inch of pitch removed from our 23×27 props. I am now swinging 23×26 props and I am burning 29gph total at 2140rpm making 22mph. I make 2680 easy with throttle to spare, but still burn too much at cruise. This winter I will have 1 more inch taken out with the goal of moving that 29gph up to 2250 or get it as close to 2300 as possible.

If you have read Tony’s tips then you know that these engines are known to drop exhaust valve seats. The way to remediate that is reduce pitch on the props until the fuel burn is on the curve for the Diamond 450 NOT the 480CE curve. Then it is highly recommended that you replace the heads on these engines. There are new heads that have upgraded metallurgy to account for the exhaust valve seats. Here is the rub. Good luck finding a set of these heads right now. We bought the boat in April and I am still waiting for 2 heads and I am running slow and easy until we can get a pair and get them installed.

You really need to ask yourself, and a trusted mechanic, about all of this and if they have the time and the parts to make this right. I knew all of this going into the purchase of our boat and lowered the offer to offset the estimated cost of the head replacements. We still bought the boat though. You need to ask yourself if you have the patience and tolerance for all of this. If not, move on to another boat……

My mechanic took our engines apart, serviced the cooling circuit, had the injectors tested/serviced, inspected the turbos and wet elbows. All looked good and he then put humpty dumpty back together. I had the props modified and re-installed while all of this was being done. The engines run just fine yet the fuel burn is still too high. We have our boat in cruising trim and we are over prop’d.

We have 650hours on the engines and I am nervous as all get out every time we get up on plane. There are a bunch of folks on this forum that have done the work and now enjoy their boats with out having to worry about valve seats and the resulting mayhem.

Ask yourself if you are up for all of this………

[Author]

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