Detroit Diesel question

[moparharn]

I am cost conscious . Boats with Detroits seem to be less expensive. Can Detroits be run at low rpm to save fuel? Can I take a 671TI and remove the T and the I and put in smaller injectors and get a fuel efficient engine similar to a Perkins or Lehman? Does the two cycle need to run at higher rpm to generate enough heat to run properly and get good life? In short, can I make a Detroit get fuel economy and good life somewhat similar to a 4 stroke like a Cummins, Lehman, Perkins, Yanmar? Is the two stroke Detroit by its nature not able to compete fully in the fuel economy war? I am willing to live with slower speeds as long as the hull can handle all conditions similarly to when it had higher HP.

It seems a semi displacement trawler with 671TI's can plane, ut use about 42 gallons an hour combined in doing so. If you simply slow down to hull speed you will improve economy, but to what level I am not sure. I am told that Detroits do not like to run at low rpm for very long and need to be blow out and heated up every hour at least. (give or take and not really my point here) If I de-tune this same boat to run at low rpm hull speed, will the engines give me economy similar to what it would have had with Lehman's? Can Detroit naturals compete in this way? Thanks for any help I m might get. Bill

 

[DavidM]

You don't need to modify the 671TI to remove the TI stuff. Just pull the throttle back and go slow. It will then act like a 671N.

And yes you can run a low loads and get decent fuel economy comparable to an older Lehman, Perkins, etc. Just run it up once a day to blow it out.

David

 

[Mako]

Jimmies really aren't much less fuel efficient than the other older mechanical engines you mention. Although my WW2 Gray Marines generated about 16.1 hp/gal-hr, the "newer" ones with 4 valve heads provide about 18.3 hp/gal-hr. Your Cummins, Lehmans, etc., aren't much better than that - maybe about 19+ at the most.

You can detune with smaller injectors easy enough. But if you drop the turbos then there are other modifications needed including higher compression pistons. Don't forget you'll need to re-pitch your props as well, which ever way you go.

Regarding loading of Jimmies, tons of various opinions from lots of experienced persons on these threads. Personally I don't like running below about 50% because that big blower is cooling off the block too much, and I'm not a believer in the "run at 100% one hour a day to blow out the carbon" argument. But that's just me.

But honestly, they like to be run hard. If you do detune them back to 176hp naturals, then you can run them at 80% all their lives.

 

[bayview]

The specific fuel consumption of two stroke is higher than four stroke that wont change.

The question with buying a high output engine is how hard was it run. There must be some data somewhere showing the prop demand fuel consumption of different detroits at the same output.

It is generally not the HP you have but the HP you use that determines fuel use.

 

[Woodland Hills]

Turbo Detroits have a lower compression ratio than the naturals and they don’t spool up the turbo enough to provide boost until you get up around 1500rpm. Running at hull speed around 1000 to 1200 rpm they all are natural engines!

Just run them up to boost rpm until they clear the smoke and run clean once in a while. That will also get the oil hot enough to burn off any moisture too.

 

[moparharn]

Thank you very much for all of your replies. Can someone offer an opinion on what the best mile per gallon I could hope for from a pair of DD 671TI"s would be on a 42-48 foot semi displacement trawler around 30,000 to 40,000 lbs. I am guessing that a pair of Lehman 120's at one knot under hull speed (approx 6-7 knots) would deliver a combined fuel burn of 4-6 gallons per hour. Is a 671TI capable of this, or once again am I trying to have my cake and eat it too. Thanks again. Bill

 

[rslifkin]

I'd expect better than 2 nmpg at a knot below hull speed in a boat that size with any diesels. Better setups will manage more like 3 nmpg. So more like 3 gph total, if not a little less. 6 gph at 6-7 kts is twin gas big block territory.

 

[Woodland Hills]

I burn under 10gph @ 10mph plus another 1gph for the generator. 63’ lwl/18’ beam at 100k lbs. it works out to 1mpg at 1050 rpms.

 

[sbman]

I burn under 10gph @ 10mph plus another 1gph for the generator. 63’ lwl/18’ beam at 100k lbs. it works out to 1mpg at 1050 rpms.

Which engines does your boat have?

 

[Woodland Hills]

12-71 TI @ 650hp and 36x36 props

 

[GoneDiving]

How much are you saving by buying a less attractive (to the general buying public) vesssel?

In my case, I saved close to $100k vs a sexy boy racer vessel. That pays for A LOT of diesel. The fact that I got a fuel efficient ex-commercial vessel is a bonus.

Good luck

 

[Lepke]

I buy boats because they have Detroits. I like the reliability. Fuel economy doesn't mean much when your engine quits out to sea. No bleeding the fuel system, no injector pump, a little air in the lines is a pia, but rarely stops the engine. They will run for years, even worn out. You get smoke and a sheen on the water, but they run. A 671 will run on 5 cylinders, not smoothly, but it will get you home.

I've owned several turbo Detroits and run even more. I prefer naturals in a yacht for the economy and longer life. But turbo engines can last much longer than average by going slower and keeping the oil clean. Converting a turbo to a natural is like doing a rebuild. You need to change the pistons, the heat exchanger and oil cooler are probably too big, as are the injectors.
I use to be a commercial fisherman. I trolled salmon and tuna. During salmon season, once on the grounds, I would troll as long as 14 hours, shutdown and drift the night, and do the same routine for many days until the boat was full or out of supplies. I saw no harm in running slightly above idle, about 700, for days on end. Detroit generator engines run at 1200 rpm, and will do it for decades whatever the load.

Changing injectors will help in fuel economy but not greatly and cost $1000+ for a 671. Slowing down will give similar results. I've been running Detroits since 1960 and rebuilding them since the late 1960s. I've played with the economy and do a few things that improve the economy, but the biggest move in economy is go slower. If you're going for longevity of a turbo engine, keep the engine below 80% of hp and the oil clean. Add a bypass filter that cleans down to 1 micron. Or a centrifuge.
My current boat has 1947 twin DD naturals and burn about 8.5gh @ 10 knots in the ocean with the usual PNW swells. The 10 knots is at 1800 rpm and is the max continuous rating. Boat is 83x17, about 80 tons.

 

[Juliet 15]

I have 4-71Ns in a 58' LRC that's pretty heavy; at haul out with low fuel and no water she weighed a tad over 87,000 lbs. She carries 500 gal water, and 2390 gal of diesel. I keep the water and the fuel full, so I'm roughly 110,000 lbs.

With these engines I cruise at 1300-1400 RPM, burning between 2.5 - 3 Gal an hour each (say 6 Gal/hr), and make about 7.5 kts thru the water.

I had Cummins 6BTA 5.9M3s in another boat. They were good. But I absolutely love these Detroit engines. I wind them up for an hour at the end of each days run after I learned why we do this from my old 30-year Detroit mechanic.

 

[tiltrider1]

I never ran my 42 at 6-7 kts. I always ran her at 9 kts and burned a gallon per nautical mile. If I slowed to 6kts I bet she would have gone 2 nautical miles per gallon. I had way more money than time so 1 nmpg was always acceptable to me.

 

[Lostsailor13]

453n

My first Detroit 453n with a 72 series velvet drive @ 2.57:1 turning a 26x21 prop and at 1500 rpm I'm getting 7.5 knots burning about 2 gph

 

[FF]

Before comparing engines fuel burn it would pay to investigate a fuel map or BMEP map.

These look like rough bullseyes with the lowest fuel consumption in the center , HP produced RPM and fuel flow are on the edges of the info..

There is usually a good GPH /HP location somewhere near the torque peak as the graph expands out the burn to HP gets worse and worse.

To operate efficiently the boat should be powered and operated near the bulls eye.

Sadly most engine assemblers do not provide the fuel map and a mere propeller / hp graph / will help not overload the engine , but does zilch about efficiency..

Claiming this engine does x HP/GPH may be correct , but only in a limited range of HP and RPM.

 

[rslifkin]

Knowing the BMEP graph is useful, but unfortunately, if the engine was sized to operate away from its efficiency peak at cruise, there's not usually an easy fix.

But for estimating efficiency, a few general assumptions can be made. 4 stroke diesels will typically have a flatter efficiency curve than 2 strokes. Turbodiesels will typically have a flatter curve than N/A diesels. And any diesel has a much flatter efficiency curve than a gasser.

 

[sbman]

Charts are available for many of the Detroit marine configurations. This chart matches up with the configuration found in my Uniflite, 671N w/ N80 injectors and advanced cam timing. Rated power 257 HP.

If I pick a spot, say 60 horsepower at the propeller that is 1450 RPM burning 4 gallons per hour. Chart produced in 1970.

Here is a chart made 30 years later for a Cummins 6BT Marine engine

60 HP with the Cummins propeller curve, happens at about 1700 RPM, and burns just under 4 GPH, maybe 3.75

0.25 GPH better for the Cummins? If you run your Detroit at similar power levels to whatever other engine you want to compare it to, I think you'll find it's relatively comparable in fuel consumption and fuel consumption likely isn't worth worrying about between the various marine diesels.

Any diesel when run at low power levels for long durations can suffer from carbon build up and could benefit from a high power level run to ramp up temps and burn things clean. The exception to this might be very modern electronically controlled common rail engines.

 

[moparharn]

Thank you! I am really enjoying the replies. So many good posts on this thread. So many experienced people. This is very helpful to me hearing things first hand and real world. Bill

 

[FF]

Remember the prop charts are ALL theoretical , some are more accurate guesses than others.

The fuel map or BMEP is measured performance, observed not a guess.

 

[Creek]

Back in the 1980s I had a Bertram 42' sport-fisherman with twin Detroit Diesel 6-71 TIs turning 36"x36" propellers. The engines were rated at 335 HP each (IIRC).



The boat was a bit under powered with these engines due to its approximate 40,000# weight and deep V design that made it an excellent sea boat but less than optimally fuel efficient. Nevertheless, when planing it delivered 1.75 NMPG at 1700-1800 rpm which was +/- 16 knots.



I loved the reliability, ease of repair and sound of those engines.

 

[moparharn]

Generally the 671TI's are rated at 425hp. I wonder if your engines did so well in their economy because of the lower HP rating? I also wonder what the differences are between the 335hp 671 and a 425 671 are? Likely just injectors? I am guessing your Bert fell off plane around 13-14 knots so 16 knots is very good speed for that kind of economy. What was your cruise and WOT rpm? Thank you for your input. Bill

 

[Creek]

I cannot say definitively what the differences are between the 335 and 425 HP engines are, but larger injectors are definitely necessary to deliver the extra fuel needed.



There was also a model of those engines identified as 6-71 TIB, in which the "B" added a the blower bypass option. As you probably know, these engines are supercharged as well as turbocharged; with the B engines, when the engine speed is sufficient to spool up the turbos, the bypass would cut out the supercharger which by then was unnecessarily scavenging horsepower. Mine were the TIs.



On the injector issue, I had an excellent mechanic who told me he could jack up the horsepower with much larger injectors (they did this in WWII on the landing craft) but that it would shorten engine life. I decided against it.



As to my Bertram's planing speed - you have it just about right; I almost always ran it at 16 knots because it was optimal for the engines and fuel economy. If I recall correctly, WOT was 2100. Good luck to you.

 

[markrkrebs]

.38lb/hp-hr

All those older diesels run similar efficiency. Detroit turbos actually slightly better than Lehman's. (2-stroke isn't less efficient by nature). I've owned both. Remember power goes as speed CUBED so most engines are loafing at hull speed. Example: twin Lehman's running 8kt, 49' defever (50klb) got better than 2.6 nm/gal. BUT this meant only 27hp each, at 1400 rpm or so. That's pretty low. My solution: overprop. I added 2" pitch and this turned out to be a great decision. I ran them at 1500 at 9.3kt burning 4.8gph total: 1.98nmi/gal, 44hp each. This was a great setup though with the big props it couldn't achieve 2200 wot anymore. If you don't need to run wide open throttle or really want top speed then I don't think it's good to have a little prop and the engine freewheeling at high RPM at Hull speed.

 

[CaptBud]

Detroits need to be run high enough to generate enough heat to prevent "wet stacking" raw fuel entering exhaust manifold. Putting in different injectors requires a skilled mech to adjust the fuel log, not many around with the skill. My advice is if it aint broke don;t mess with it

 

[markrkrebs]

This was a great post with the Cummins charts, and notice at 2000 RPM it comes out to 17 horsepower per gallon per hour which is .35 pounds per horsepower hour. They're all close in performance.

Also noticed the prop load power curve is far less than the rated power, a consequence of drag going with the square of speed. So at all your low RPMs you're going to be using very low power.

 

[JimC]

We have a 471T on LilyD. She is 40 ft long and 15 tonnes loaded. Over the last 9 boating seasons we have averaged 1.04 litres/ nm at 6-7 knots, running at 1100 to 1200 rpm.
The engine has never missed a beat in that time, fires up instantly and no smoke. (Well except for when we run her up to 1800 rpm to clean things out )

 

[CaptBud]

I had a 471 on my last boat and found what looked like black oil under the alternator...there obviously is no oil in the alternator but was told by a Detroit expert that "wicking" was taking place...raw fuel was seeping down a fitting in the exhaust manifold and getting picked up by the alternator cooling fins and slung on the floor below the alternator. Recommendation was to run at a higher RPM for the heat and that solved the problem.. Prior to this advice I hired another mechanic and by the time it was over he said I needed a complete overhaul.........load of bull

 

[Tolly Roger]

Slight Thread Drift......
But I am really enjoying the YouTube Channel Dangar Marine from Australia.
There are several really good videos involving the rebuild of his Detroit Diesel (and assorted projects on his steel trawler)
Today's entry is the final adjustments and getting the 471 started (again) after the second rebuild.

 

[JimC]

When we first installed the 4-71T in 2008, we had a similar problem with black goo leaking from the turbocharger as it is the low point in the exhaust system. The DD mechanic diagnosed it as unburnt fuel "ends" due to too low exhaust temperature. He had me put all of the engine cooling water into the injection elbow to put more back pressure on the exhaust and raise the exhaust temp. And he told me to run it harder.
Incidentally, the 4-71T is at least 10 dB quieter than the 6-71N it replaced.