I have a 95 Nordic 28' heat with a gen IV bbc, 563hp and a Bravo I and a 26p prop. The problem I am having is the engine dyno'd up to 6200 rpm and in the boat it won't turn more than 4200 at 63mph. Any idea why it Dyno's out great but in the boat it sucks! More info: The last motor I had in it did the same thing. I have had other boats along side me and the speed is accurate, and I have also checked the tach against a hand held tach at idle and it is with 20rpm. The ignition has a 6200rpm rev limiter in it, but doesn't seem to have been a problem on the dyno. Any ideas?


Optional Information:
Make (of engine): GM
Model (of engine): 468
Year: 1995
Horsepower: 563

Already Tried:
New engine, dyno'd ok, carb was gone through also. The only difference between the dyno and the boat is the fuel delivery system and the exhaust system. The exhaust system is new Eddie Marine manifolds. The only thing I haven't changed is the remote mechanical fuel pump that is belt driven with the remote water pump. The ignition system is MSD 6M-2L and a 85501 locked out advance distributor.

Heeeeelp!

At what rpm is your peak torque...and how much torque do you have.

Boats like torque.

ie...a 500 hp rotory motor making 500 horsepower at 9000 rpm wont perform very well in a 5000 lb boat.

My guess is that your motor might make the kind of horsepower you were seeing on the dyno...but doesnt produce a wide enough torque curve, and that the engine is spinning the prop to its peak torque rpm.

Also...you should definately dyno your motor with the wet exhaust you have on it now...it will drastically effect how the motor performs.

Chris

I have
558 ft lbs at 4100
563 4200
567 4300
570 4400
569 4500
570 4600
573 4700
574 ft lbs at 4800
572 4900
567 5000
560 5100
554 5200

And
563 hp at 5900

Boat is 4600 lbs dry

Also part of the problem is the engine won't rev more than 4200 rpm at 63 mph in the boat.

Would have been nice to dyno with wet exhaust but the place didn't have the means.

Your torque peak seems to happen at pretty high rpm for a boat engine..
One thing to check would be stock intank fuel pickup and lines may be too small especially if it still has the stock anti-drainback checkball. The water pump mounted fuel pump should be fine up to around 600hp but a fuel pressure gauge would be a good way to check it.
26p is a small prop for that much hp and you should have no problem spinning it a boat that light .
I was running a 26p in my 27Carrera which is about 1000lbs heavier with only 500hp and ran 69.8@5000rpm

Yah, tomorrow is fuel test day, I have a clear line and a couple pressure gauges hooked up so I can see what suction side before the pump is and another after the pump. The clear line is to see if there are bubbles. I have 1/2" line from both tanks to fuel pump, with new in line filters and a new water seperator also.
I'll be back tomorrow to post results and look for more ideas.

I was also told to take a closer look at the dist, but it worked good on the dyno, so I'm not sure about that being a problem.

I didn't see anything on where you have tried different props. Have you tried different props? What prop are you running now?

x2

You definately want to remove the stock check balls which are located in the 90 deg fitting coming off the top of your fuel tank...where the fuel line attaches to the tank....they are very restrictive.

Chris

Is 4100 the lowest RPM you could stabilize the engine on the dyno? IF so thats pretty high.

As Chris says(CB-BLR) you want torque more than anything as in a boat the water grabs the prop and doesnt let the engine flash up to where it can even start pulling.

Let sus know about your fuel flow.

Try something like a 20" pitch prop and see if you can get past the "hump" into a a higher RPM range to see if it pulls.

Bad fuel killed a first time outing for me once as sis a bad ignition module - at the dyno they use their own so I simply installed mine and didn't catch it.






Uncle Dave

For outdrive boat apps I try select a cam/head the package I can stabilize early and that starts to pull at around 3K.

Here is the curve on my 406.



Uncle Dave

What is the drive ratio ? If it's 1.36 that could be your problem.