low RPM under load

I have a 6-71 TI, 486 hp in a 48' Ocean. New boat dont know how long this has been going on. Engines will not turn over 1950 RPM under load (should be 2500 full load). No load is fine 2650 RPM. I have gone through the tune up procedure on both engines, checked fuel pressure, cleaned intercoolers. No smoke until 1850 then light grey smoke. Need somne suggestions!

I'm sorry, but none of these numbers look right to me. Granted, it is completely in the realm of "possible" to bolt on a big enough turbo and large enough injectors to deliver the fuel in order to produce 500 hp in one of these engines, I just don't see it as "sustainable".

Detroit Diesel built the 71 as a "sub 300hp" engine. I know they've been hot rodded but to get almost 500 hp out of one is asking a whole bunch.

I no longer have any numbers for the inline engines but what I have for the 6-V71 lists 220 hp @ 485 ft.lbs. torque as an upper number for a "late model" (mid 80's), turbocharged engine.

The maximum loaded rpm was never more than 2300 as these engine's performance curve falls off drastically after that for a diminishing return of horsepower and torque for the fuel being burned. These engines can move mountains and they do it most efficiently at around 2100 rpm.

As expensive as it is to repair these old girls these days, I'd be wanting to turn them down, not up. Your no load rpm of 2650 is just shy of valve float and, in my opinion, engine life is being shortened.

As far as just plain addressing your question, you need to look first at fuel QUALITY. If you are convinced that your fuel is clean and contains the necessary BTUs to burn, then I would be looking for things like air getting in or low flow.

A spill test would reveal both and would be the next logical step in your troubleshooting sequence.

Good luck.

jgmo
Thanks for the reply, typo on the horsepower, its actually 485, the other numbers are correct. I think I may have determined the problem. The injectors in the engines are the M-95, they should be "M-95 Special" (discontinued now M15 or 115). Significant difference in flo rating. Im going to correct this issue, reset the timing and rack and see.

I dont plan to run them at 2500, but the lack of RPM concerned me. My understanding is that they should be run at 80% of full load at cruise.
Thanks again!