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Old 09-09-2019, 02:38 PM   #21
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The original natural engines were over built as was the style in the 1930s for heavy duty diesels. So there was room for more hp. But run hard, the high hp engines need frequent rebuilds. Physical engine size was less of a consideration in those days. Many boats in the 1930s had engines of the same hp as Detroits but were 3-4x bigger.
I've owned lots of Detroits, and when buying new ones, fit a bigger natural rather than a turbo or worse yet a double turbo engine. So I ran 12v71s instead of a high hp 671. I have run my share of turbo Detroits, but baby them and keep the EGTs low.
Detroits naturals are my favorite for reliability. All mechanical and no injector pump.
Got a pair now.
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Old 09-09-2019, 08:04 PM   #22
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A 330hp 4-71 d.d. is a great engine. The hp to weight ratio and fuel consumption per hp are impressive, more so when you consider the age of the design. Longevity was not a consideration for this package, it was the sacrifice. Longevity had to be sacrificed to get the hp/weight ratios. This is not a trawler engine, it is a planning hull engine. The idea is you have the hp to get her on plain and then you back her down for longevity. The competition was 350 hp gassers, they too had no longevity. Gassers were expected to live 1200 hrs between overhauls and 330 hp 4-71’s were expected to live 2400 hrs between overhauls.

To a slow boater these engines look like ticking time bombs. To the sport fisherman these engines look like long lasting work horses. It’s all a matter of perspective and goals.
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Old 09-10-2019, 06:05 AM   #23
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"I was hoping someone would have some information on fuel consumption as my manual offers no tables."


The rule of thumb for non hot rod DD is 1 gph will give about 16 hp .
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Old 09-10-2019, 07:31 AM   #24
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The rule of thumb for non hot rod DD is 1 gph will give about 16 hp .
Yes, I recall calculating about 16.3 on my twin Jims
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Old 09-14-2019, 03:16 PM   #25
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I have a pair of DD 671 turbos on my boat. Yes it is designed to go fast, but she will run all day at 1400 rpm, maybe a few minutes at 2600 rpm (about 25 kph) to get around slower boats.
Mine are 450 hp each total 900 hp. Not a trawler, but as much fun.
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Old 09-14-2019, 06:09 PM   #26
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The rule of thumb for non hot rod DD is 1 gph will give about 16 hp .

Depends on the hull, props, how well the DDs are tuned, and what you use for fuel. My ancient 671 naturals normally cruise my 83x17x5 hull at 10 knots and burn 8.5 gallon/hour at 1800 (max continuous) so I'm using 136 hp of about 200 x2? I don't think so.



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Old 09-14-2019, 08:51 PM   #27
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Depends on the hull, props, how well the DDs are tuned, and what you use for fuel. My ancient 671 naturals normally cruise my 83x17x5 hull at 10 knots and burn 8.5 gallon/hour at 1800 (max continuous) so I'm using 136 hp of about 200 x2? I don't think so.


Props and hull shape have nothing to do with how much HP an engine makes out of a gallon of diesel. What you use as fuel does affect things, however iím Sure every one is using Diesel #2. If you are burning 8.5 gallons per hour total then you are making 136 hp total or 68 hp per engine. How much speed you get out of the 136 hp is affected by hull shape and props.
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Old 09-14-2019, 09:28 PM   #28
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I would love to have a boat with a 4 or 6-71... They aren't sexy,efficient,quiet or modern, but at the end of the day,feed them clean fuel and they won't fail you. Many engine diapers required..
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Old 09-14-2019, 09:54 PM   #29
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I would love to have a boat with a 4 or 6-71... They aren't sexy,efficient,quiet or modern, but at the end of the day,feed them clean fuel and they won't fail you. Many engine diapers required..
I had 6V53s in a previous boat. Ran like tops but I would always buy all the oil absorbent pads that WMP had in stock...
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Old 09-14-2019, 10:17 PM   #30
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I had 6V53s in a previous boat. Ran like tops but I would always buy all the oil absorbent pads that WMP had in stock...
I have lots of experience with 71 series DD's but none with 53 series. I know they have the same rep as 71's.
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Old 09-14-2019, 10:19 PM   #31
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They ran great but they did use/leak some oil. About a gallon every 24 hours run time. I had a mechanic make up a homemade Airsep and that helped with the oily mist problem.
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Old 09-14-2019, 10:41 PM   #32
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all the DDs leak. The "ho"made remedy was to put a milk jug on the end of the crancase breather tube and put a "diaper" in there. I never had a DD with an AIRSEP, so I can't speak to that. I'm sure Ski and Dmarchand can speak to this issue.
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Old 09-14-2019, 10:52 PM   #33
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That is pretty much what we did. Used a fuel filter without any element in it. The oil would stick to the sides of the filter canister. It had a petcock in the bottom so I would drain it every 20 hours or so.
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Old 09-15-2019, 04:59 AM   #34
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I would love to have a boat with a 4 or 6-71... They aren't sexy,efficient,quiet or modern, but at the end of the day,feed them clean fuel and they won't fail you. Many engine diapers required..
I disagree. Nothin sexier than a pair of Detroitís in a big ass sport fish.
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Old 09-15-2019, 09:33 AM   #35
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They ran great but they did use/leak some oil. About a gallon every 24 hours run time. I had a mechanic make up a homemade Airsep and that helped with the oily mist problem.
I don't understand that. My twin 6-71's were from a WWII minesweeper and the little soda-can drip cups I hung on the breather tubes collected "drips" but no way was it close to a huge amount. I ran at 1350 rpm. Are you running yours at 1800 full throttle, or perhaps they are ready for an overhaul??
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Old 09-15-2019, 02:09 PM   #36
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Detroits don't have to leak large amounts. I run my current 671 naturals at 1800 (max continuous) and don't have a noticeable mist. I have pans under the engines that don't seem to collect oil. Several POs ran at 2100, but 10 knots is fast enough for me. The airbox will always leak a small amount. Excessive amounts can be from worn parts. The basic 671 block was designed for about 200 hp. If you're pushing 450 hp the rings and sleeves aren't going to last as long and you'll get more leaks.
The engines are heavy duty and the design allows for ways to ensure each cylinder puts out the same power continuously. Blocks are marked with numbers by each bore. The numbers refer to minor machining differences in manufacturing. They correspond to a correct sleeve for that cylinder. Some mechanics just put the same sleeve in all cylinders so while some cylinders are carrying the load, others are loafing.


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Old 09-16-2019, 11:27 AM   #37
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My twin 4-71's have 60,000 hours on them. First started in 1980. Commercial use mostly at about 1300 to 1400 rpm. Fuel consumption abot 3.5 gph each at 1400 rpm. While rated at 1800 rpm continuous duty 115hp. I am taking about 65 hp out of each engine at 1400rpm. 34 inch 4 blade propellers. 3:1 gears. In nearly 40 years use these engines have never had their heads off. Still running great. The Detroit diesel 71 series non tubo engines are one of the best least problematic engines ever made. They just don't break.
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