Detroit Diesel 4-71 Engines

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brian eiland

brian eiland

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Joined
Jul 17, 2013
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USA & Thailand
Vessel Name
RunningTide
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37 Louisiane catamaran
A vessel has been brought to my attention that is equipped with twin 4-71 DD engines' It quoted as have 660 hp total,....330hp each.

Is that even possible with these size engines??
 
Theoretically, yeah... sure. HP is a function of engine speed and torque, which itself is determined by how much fuel you can burn (thus releasing energy) and how fast you could burn it.

BUT...

Since the X-71 series DD usually makes something in the area of 40hp/cylinder and 330hp is roughly twice that (~80hp/cylinder) I would be thinking that either:

a) Something got lost in translation,
b) Someone is lyin' his 4th point of contact off, or;
c) I don't want to be near that over-sized claymore when it's running.


EDIT: This is just an off-the-cuff opinion, which is subject to revision upon receipt of new information. I'm just saying that what I said is true, as far as I know.
 
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I saw this on an engine forum
DS, to give you an idea, I have seen 4-71's rated from 82 to 325 horsepower.
Usually they run in the 160-200 range, but marine applications with the DDEC chips are getting 300+ horsepower out of them.
Click to expand...

...and another referenced an inclined or pancaked 4-71

and/or with turbo charging
 

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Covington and J&T did lots of things to the 71 and 92 series engines. A 4/71 at 330 hp sounds like one of those. Probably as good as the 8v71 at 650 hp. If ran hard expect very low hours before needing refreshed. If ran conservativly they can last a long time, but, at low loads they tend to "load up". They are definitely turbo'd/inner cooled and more than likely have blower bypasses. Not a good "slow" engine.
 
MC Escher said:
c) I don't want to be near that over-sized claymore when it's running.
/QUOTE]

I've had lots of 471's in lots of applications from 1946 GMC semi tractor to Oliver 99 farm tractor to mid-70's Ford straight trucks and had fun with all of them. When I think of the suffering that must be going on in a 71 series with 80+ HP per cylinder and the pressures that would require, I don't think that would be a fun engine unless the fun was to blow it up. Of course, I could be wrong about that.
Click to expand...
 
I recently read about an outfit that specializes in the old detroits converting a 4-53 to DDEC using the electronic injectors and a ECM from a 4-71. They said it ran great but the rpm limit was 2300, they wanted 3000 and had not figured how to do it yet. I dont know if the DDEC was an improvement or not but the article was interesting. I love the parts interchange on the old Detroits. And simplicity/durability.
 
IF you want longevity 30-40 Hp per 71 cylinder is the norm.

At 1200RPM , prime gen set use, the engine life gets extreme!
 
Dd-4-71

Just my view. Way to much HP to ask from that engine. Must have high RPM and high compression ratio to result in that HP. Short engine TBO.
 
Lower compression actually, lots of turbo boost, big injectors. Rpm is usually 2750 max, not much more than standard.
 
Kulas44,
Brake mean effective pressure (BMEP) is what applies here. But you are absolutely right .... Lots of pressure and heat.

High rpm by itself shouldn't be a problem if the engine is designed to operate continuously at that speed. Excessive load and temps could easily be obtained w that engine IMO.
 
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And brake specific fuel consumption. Lower compression allows the input of more air from the turbo and allows the engine to utilize more fuel to make the requested power. Cylinder pressure needs to be controlled, to much and they go boom. The problem with this approach is that instead of 19 or 21 to 1 compression ratio the high output DDs are usually around 17 to 1. Thats ok on a new engine but with a little wear they get really hard to start because of it. You'll see lots of white smoke on startup and for longer periods of time, due to low cylinder temps, which is why it was hard to start. A lot of DDs were fitted with ether injection systems for this reason. Lower compression ratios are also one of the reasons these high output engines dont do so well at low speed and low loads. Being a 2 stroke they are basically aircooled at these loadings and cylinder temps are low, resulting in unburnt fuel.
 
High rpm by itself shouldn't be a problem if the engine is designed to operate continuously at that speed.

Most of the 71 series I have seen MFG charts for show a 2100 RPM engine , except fire trucks .

>Rpm is usually 2750 max, not much more than standard.<

To me 2750 is a Loooong way from 2100.
 
The 4-71TI is rated at 325hp (310 prop curve) at 2600rpm. That engine is straight from DD, no aftermarket mods.
 
RickB said:
The 4-71TI is rated at 325hp (310 prop curve) at 2600rpm. That engine is straight from DD, no aftermarket mods.
Click to expand...


Well, I'll be...
Learn something new every day.

I'd be quite curious as to what sort of factory upgrades were done to the bottom end, because a doubling in power implies a doubling in cylinder pressure.
 
No upgrades at all in the crank or rods. Pistons were trunk type two peice (IIRC). cam timing was different, for the injectors anyway. DDs have 3 settings on the cam, advanced, standard and retarded. All of which are dependent on the injector used and hp rating. Turbo engines had different ports in the liners. Any engine could be changed to any other config. easily. The DD was the most known diesel engine of all time. If you need 100 hp constantly in the most economically possible way a DD is the way to go. Not so good above or below this need, or any quoted hp requirement.
 
I'd be quite curious as to what sort of factory upgrades were done to the bottom end, because a doubling in power implies a doubling in cylinder pressure.

Running 80 Hp from an engine that normally does 30-40 hp per cylinder with a turbo and inter cooling simply reduces the engine service life.

Many of the Sport Fish fold think 1000hours to overhaul is a great engine!
 
FF said:
I'd be quite curious as to what sort of factory upgrades were done to the bottom end, because a doubling in power implies a doubling in cylinder pressure.
Click to expand...

That ignores the fact that the high power engine develops its rated output at a higher rpm and ignores the fact that the engine uses a bypass blower that at high power is essentially non-existent compared to the large parasitic load imposed by the standard blower.

Also, don't for one moment think that BMEP is actual cylinder pressure.

Consider that the larger number of power strokes resulting from the increased rpm above the NA version, along with the availability of the power that is consumed by the blower on the NA versions, along with higher volumetric efficiency and better scavenging contribute greatly to power output.
 
Fuel Consumption

I ran across this thread and I do have a couple of 1985 471T, N90 injectors (rated at 325bhp) engines J & T converted. These engines run great, no smoking and very little oil consumption.
I was hoping someone would have some information on fuel consumption as my manual offers no tables.
 
Welcome aboard. Maybe check boatdiesel.com.
 
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.
 
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.
 
"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 .
 
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.
 
FF said:
The rule of thumb for non hot rod DD is 1 gph will give about 16 hp .
Click to expand...


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.



 
Lepke said:
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.



Click to expand...

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 i’m 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.
 
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..
 
Sailor of Fortune said:
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..
Click to expand...

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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