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Old 08-21-2021, 11:18 AM   #1
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Depowering a Turbo DD?

If this question has already been addressed my bad for not finding the thread. My question is this: Am looking at a boat withe DD 671 TI 485HP
This feels like way more power than needed and the fuel consumption at optimal efficiency (I'm informed) is 12.5 gph. Is there a simple way to drop some HP and fuel consumption by changing injectors and eliminating the turbo? Seems like i might reduce maintenance and increase longevity but that might just be more wishful thinking. Any experienced insights in this regard? Thanking you in advance for your time.
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Old 08-21-2021, 12:15 PM   #2
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Asked a friend about this. He attended many of the Detroit Diesel classes offered in Tampa, and says NO. No way, no how, and it won't work. Dockmate attended many courses with a bunch of Coasties (the Coast Guard runs DD671's and DD871's, DD1271's and DD1671's) ... according to my friend this came up numerous times. It's simply not possible. Sorry.

Actually it is possible, IF you take the motor down to the bare block. So, though "possible" this is both impractical and will be exceedingly expensive.

I'm sure the fellows here can tell you the whys and wherefores. But basically this is a one way operation, from naturally aspirated to turbo...

Friend also suggested strongly that you call Detroit Diesel in Tampa. They offer courses, and are a worthwhile investment even if you have others do the work. Cummins also has classes if you're so inclined. Best wishes.
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Old 08-21-2021, 12:38 PM   #3
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I'm sure the fellows here can tell you the whys and wherefores. But basically this is a one way operation, from naturally aspirated to turbo...
Not doubting your friends but I'd be interested in hearing why. Especially when OEM non Ti versions are available.
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Old 08-21-2021, 01:03 PM   #4
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We have 6-71 TI's rated at 425 HP pushing a 46'/25 ton Chris Craft. I typically run at about 1500 RPM's which gets the operating temp's up to where they need to be. This will make about 8.5 mph in still water and we're fine with that speed. GPH is between 5 & 6 with about 1.5 mpg. I imagine if we ran it hard in the 2000 RPM range our fuel burn would go up considerably but I don't know about 12 GPH. I've never even considered de-tuning but it sounds like more trouble than it's worth. Hope this helps.
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Old 08-21-2021, 01:53 PM   #5
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Easy. Make sure you reach WOT+ rpm fully loaded. Then cruise at no more than 70% of WOT RPM. That will be around 50-60% of max horsepower. No change to engine required.
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Old 08-21-2021, 02:34 PM   #6
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If your motors are the 6 cylinder DD 71 series motors they are two cycle motors.
There is no such thing as a NA two cycle motor.
ALL two cycle diesel motors require a pressurized crankcase to fill the combustion chambers. DD71s do it with a gear driven SUPERCHARGER. Now if you have a turbocharger it resides at the inlet of the supercharger.

What you have in essences is a gear drive blower being feed by an exhaust driven blower. You could in fact remove the turbo and a lot of plumbing, and the motor would still run at 238 HP after you change the injectors to the smaller size for the NON-TURBO engine.
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Old 08-21-2021, 03:21 PM   #7
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DD Depowering

Thanks everyone... very much for the input. I don't have a hard and fast answer yet and as the different views expressed... there are no doubt more than one solution. For what its worth a very experienced mechanic with DD's just wrote me that duplicated what the last post says about smaller injectors.... more interesting is it extends the life of the engine considerably in terms of wear and tear. Will be happy to report on what i learn from DD once I get down to decision making time and action.Thanks again, and I like the idea of adjusting RPM,temp and speed as the other alternative.
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Old 08-21-2021, 03:59 PM   #8
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At full power the boat mentioned could use 30 gallons/hr per engine.
Turbo Detroits have pistons with a lower compression ratio and have no top end. You need a fixed amount of fuel to run a certain rpm under a certain load. With smaller injectors, the governor just opens them wider to reach the needed amount of fuel for the rpm and load.
Best way is rebuild the engine as a natural, but that costs more that the fuel you'll probably save.
Or go slower, use less fuel.
At full power the boat mentioned could use 30 gallons/hr per engine.
My twin 671s in an 83' boat burn about 8.5 gallons/hr @ 10 knots total.
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Old 08-21-2021, 04:19 PM   #9
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You need a fixed amount of fuel to run a certain rpm under a certain load. With smaller injectors, the governor just opens them wider to reach the needed amount of fuel for the rpm and load.
That's why de-tuning the engines would need to go along with re-propping to allow them to reach rated RPM at a lower power output.
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Old 08-21-2021, 06:52 PM   #10
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A 485hp 671TIB pushing a boat at say 9kts will burn basically the same amount of fuel as a non turbo 671 rated at 250hp pushing the same boat the same speed.

The turbo basically does nothing at low power settings.

You would have to change a LOT to convert to non turbo.

Not worth it.
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Old 08-22-2021, 06:49 AM   #11
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Quote:
Originally Posted by Chase View Post
If this question has already been addressed my bad for not finding the thread. My question is this: Am looking at a boat withe DD 671 TI 485HP
This feels like way more power than needed and the fuel consumption at optimal efficiency (I'm informed) is 12.5 gph. Is there a simple way to drop some HP and fuel consumption by changing injectors and eliminating the turbo? Seems like i might reduce maintenance and increase longevity but that might just be more wishful thinking. Any experienced insights in this regard? Thanking you in advance for your time.

No need to modify the engines, just run the boat often at max theoretical displacement hull speed, or slightly less. With occasional bursts of higher RPMs, maybe 75-80% load, e.g., near the end of a longer slow-speed trip.

I can't guess actual fuel consumption without knowing boat and those particular engines, but I can tell you we ran our previous planing-hull boat a lot at "trawler" speeds, often approaching 2 NMPG.

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Old 08-22-2021, 07:58 AM   #12
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The local DD dealer if given the engine serial number can tell you how the engine was put together.

He will have weather the cam was advanced or retarded , weather the pistons are dished for the turbo and the injector size.

If the compression ratio was not lowered , removing the turbo could be done , but the cam might have to be reset and smaller injectors fitted.His literature will show if this was done to less powerful engines and usually he will have a variety of build power graphs and gear and prop info.This would cost nothing if done at rebuild tine.

JUST , Pulling back on the stock engine you have would save loads of effort.

I would check the turbo is at least making some pressure , not being an exhaust obstruction, at your lowest cruise RPM.
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Old 08-22-2021, 10:34 AM   #13
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It's also an option to underprop the current engine a bit so it never reached full power before hitting governed rpm. Other than having the big injectors, that would likely give similar behavior to a lower rated version of the same engine with a turbo.
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Old 08-22-2021, 02:48 PM   #14
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For years Detroit had offered several injectors to detune or enhance an engine. Engine life is increased as the smaller injectors are used as a bonus.
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Old 08-22-2021, 03:44 PM   #15
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Smaller injectors eliminate the ability to use the upper rpm range if the load is the same.
Instead of spending on new injectors ($1000+), why not just pull the throttle levers back and slow down? You get the same result and still have full power for emergencies.
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Old 08-27-2021, 02:56 PM   #16
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Many years back I had a Bertram 42 also with twin 6-71 TI engines. They were rated at 335 HP each. These Detroits, in addition to being supercharged, were turbocharged.

My Detroit Diesel mechanic, who had decades of experience with Detroits told me he could increase power to 450 HP (IIRC) by changing to larger injectors. However, he warned me that it would shorten the life of the engines before a rebuild would be needed.

It seems logical that you could depower by changing to smaller injectors, but the answer to this question should readily be available by speaking with an experienced DD mechanic.
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Old 08-27-2021, 05:06 PM   #17
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Remembering that we are talking about marine engines, presumably driving a prop, isn't talk of changing engine HP to affect fuel usage and engine life pointless? Surely a prop turning at X revs requires the same amount of HP regardless of the driving engine's rating and that amount of HP contributes to engine wear. Am I missing something??

I'd agree that short of changing engine, transmission and prop specs the best way to improve economy and fuel burn is to simply slow down.

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Old 08-27-2021, 05:22 PM   #18
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Quote:
Originally Posted by Lepke View Post
Smaller injectors eliminate the ability to use the upper rpm range if the load is the same.
Instead of spending on new injectors ($1000+), why not just pull the throttle levers back and slow down? You get the same result and still have full power for emergencies.
+1 and not out of pocket expenses.
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Old 08-28-2021, 08:46 AM   #19
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Remembering that we are talking about marine engines, presumably driving a prop, isn't talk of changing engine HP to affect fuel usage and engine life pointless? Surely a prop turning at X revs requires the same amount of HP regardless of the driving engine's rating and that amount of HP contributes to engine wear. Am I missing something??

I'd agree that short of changing engine, transmission and prop specs the best way to improve economy and fuel burn is to simply slow down.

Cheers
Yes, what you say is totally logical with the exception that low and high HP engines will each be matched with propellers that allow the engines to reach their rated maximum RPM. So the fuel consumption at a specified engine RPM will not be the same for low and high HP engines, which naturally affects fuel consumption for each combination.

I don't know what the impact is of running the higher HP DDs at low RPM in the long run, but it is possible that running at low RPM may not be healthy in the long run for the 485 HP engines. Perhaps some DD experts can weigh in on this.
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Old 08-29-2021, 05:28 AM   #20
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"but it is possible that running at low RPM may not be healthy in the long run for the 485 HP engines"

DD operates prime gen sets at 1200 RPM, or 1500 RPM, , low speed is not the problem , low load is.

DD solved that by offering engines with different cubic inch sizing , as well as many different cylinder lay outs.

The requirement for long life was a 60% load most of the time.

This is NOT 60% of wide open throttle , but 60% of the load at the operating RPM.

For a true ocean crosser I have always dreamed of a LST style twin pack tranny.

Two engines operate one shaft , either can be disconnected with the pull of a lever.

This would allow a 2-71 or 3-71 for offshore or a 6-71 for ICW big wave making.

Sadly taking a hacksaw to a 6-71 or 8 -V71 wont duplicate the factory builds.
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