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Old 04-14-2017, 08:24 PM   #1
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Turbo or no....

I've never had a diesel....but I'm really curious after trying to educate myself on their benefits, about turbo charging. I understand what turbocharging does.....so please don't go down that rabbit hole.....what I'd like to know is: what's the difference between a 250 ( arbitrary number) horsepower diesel that's normally aspirated and a 250 hp turbo diesel ?? Is it simply a matter of efficiency ? is a smaller engine that is turbo boosted better for weight savings.....does a turbo charger require special maintenance......are they the same in terms of reliability ? If you had access to the Fairy Godmother of Dieseldom....would you want a turbocharged or non-turbo charged engine of a given horsepower ?

( thanks, in advance for your replies, patience and understanding of my limited knowledge...)
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Old 04-14-2017, 10:08 PM   #2
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I am ignorant when it comes to diesels, but I have a turbo-charged Cummins QSB 5.9L engine chipped to 380hp.

Since you know what a turbocharger does, you can probably understand that it allows an engine with a smaller displacement to put out higher horsepower. This allows for a smaller and lighter engine to produce the same HP as a larger, heavier engine.

Many (most?) engines with turbochargers need to use an aftercooler. This is a maintenance item and potential point of failure. The other one of course is the turbocharger itself. For my own application, i.e. a slow boat that doesn't need a lot of HP, I would just as soon have a normally aspirated engine. However, for planing boats that need speed, a turbocharged engine is probably worth the added complexity.
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Old 04-14-2017, 10:24 PM   #3
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Apart form more HP per litre, a turbo also helps to meet emission standards. Turbo's ought to be trouble free, unless you have a wet exhaust with a poorly designed or installed water injection elbow. In that case you can get seawater into the turbo if the exhaust elbow fails. And the elbows are a maintenance item.

I'm happy to have a turbo, but avoided an aftercooler. I did not need the even greater HP/litre then possible. The aftercooler's I would be OK with use coolant in them, not seawater cooling. But if you have a seawater aftercooler don't stress too much, just have it cleaned on a regular maintenance sched.
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Old 04-15-2017, 01:25 AM   #4
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but I have a turbo-charged Cummins QSB 5.9L engine chipped to 380hp.
Amazing difference in Cummins engines.
We have a turbo charged nt855 variant.

It is a whopping 14 litres vs your 5.9l
Supposedly 360hp @ 2100rpm - but I have never gone past 1800
240hp at 1800rpm and where we run at 1250rpm around 150hp.
That gives a pretty constant 8 knots @ approx 15lph
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Old 04-15-2017, 02:04 AM   #5
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If I can have enough power without a turbocharger, I`d stop there, and avoid adding another maintenance item. And I have.
But the turbocharger is ubiquitous these days, most (? all) new boat engines are turbo. I think owners will be rebuilding their beloved low rpm Gardeners for as long as they can, if and when they need it.
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Old 04-15-2017, 03:01 AM   #6
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The real reasons turbos came about was for increased hp in the same block. Later turbos were used in yachts to allow a smaller physical size engine have more power. Many engines have turbo and non-turbo models of the same block. The advantage is a smaller engine foot print. The disadvantages are another accessory to maintain, higher exhaust gas temperatures, shorter engine overhaul cycles.
Most of the disadvantages can be minimized by good maintenance and keeping the throttle below 80% of hp. High EGTs (exhaust gas temperatures) cause increased wear in valves, pistons and sleeves. My Detroit mains are naturals and can go 10,000 hours easily, many go double that. Turbo versions usually need overhaul a 3-5000 hours depending on the captain. Each engine brand is different.
I've run a lot of diesels and owned many diesels and usually get much longer life than other captains because I don't hot rod them and keep the oil and fuel clean.
When I have bought new engines, I buy non-turbo unless a turbo diesel is the only choice to meet the need.
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Old 04-15-2017, 03:03 AM   #7
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If I can have enough power without a turbocharger, I`d stop there, and avoid adding another maintenance item. And I have.
But the turbocharger is ubiquitous these days, most (? all) new boat engines are turbo. I think owners will be rebuilding their beloved low rpm Gardeners for as long as they can, if and when they need it.


Yep, sums it up pretty well.

Yes, it is better on emissions and makes a lot of sense for over the road applications.

For boats, that use most of running in a small rpm range, it's a whole another story.

Think about it, a turbo produces more hp per given volume.
Which one will wear out first?

The market, not physics, is the reason are that turbos in full displacement boats.
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Old 04-15-2017, 03:08 AM   #8
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Funnily enough a 6lxb gardner was what the cummins repower in ours was working towards without having a motor that was constantly running at full noise.

The sort of sister ship, the 55ft Santa Barbara has a Gardner and apparently ran hard to achieve 8's.
The engine manual and spec sheet on ours has the Gardner 6lx and 8lx specs written in beside the nt855 specs as a comparison and our lightly loaded 1250 rpm puts out about the same hp as a 6lxb gardner running at full noise.

Downside to Gardner's from what I can gather is finding someone to work on them.
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Old 04-15-2017, 05:38 AM   #9
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A turbo will be found on most boats that plane , where high fuel burn is required.

For a displacement boat a NA ,( natural aspired ) is frequently better.

The hassle is at displacement speeds boats are easy and cheap to push so the turbo may not get enough exhaust gas at high enough temperature to function.

Then its like a restriction in the exhaust , to no purpose .

If your fuel tab is 3-4 GPH you wont need a turbo, 30+ GPH , its probably required.

Any extra weight from an NA engine wont be noticed in a displacement boat
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Old 04-15-2017, 06:13 AM   #10
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As mentioned, with the exception of very small diesels, non turbo is no longer an option in the USA because of emission standards. In displacement mode at lower RPMs, the turbo is running but really isn't generating a lot of boost pressure. So it really doesn't change the wear aspects of the motor. In the planing mode at higher RPMs, there is more boost, heat, and wear. The turbo gives the engine the ability to generate more HP for the same size motor. For the Cummins B series motor, that difference can be over 250%. For that big a difference, there is going to be more wear, no free lunch. Modestly boosted turbos can have very long lives. The Cummins B 210 HP has 150% of the non turbo model. I have one in my Dodge pickup with 430,000+ miles on it. The block and turbo have only seen oil, coolant changes and valve adjustments.

If you're buying a used boat, previous owners maintenance and use will be more important than whether the engine has a turbo.

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Old 04-15-2017, 08:45 AM   #11
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Another way to think about it:

If the boat's hull is designed to go faster (on plane, or somewhat on plane) as well as slow (displacement speed), a turbo can make that flexibility more practical with a smaller engine.

Examples:

My 26-footer, with a 260hp turbocharged/supercharged/electronic high-RPM Volvo KAD44P diesel, happily goes 6 knots, or planes all day at 18. Top speed 25 knots. A non-turbo diesel big enough to plane it would not fit physically in that boat. I've put 6500 hours on it, mostly in remote country along the Inside Passage. Maybe only 1/4 of them at 18 knots. The flexibility has sometimes been very nice to have. Proper setup, reasonable operation, and regular maintenance are essential to make such an engine last.

My semi-planing 37 Nordic Tug, with a 330hp 5.9 Cummins turbo diesel, can happily do 7-8 knots, but can cruise at 12 knots or even more, if on occasion I want more speed and am willing to burn much more fuel. I generally don't want to travel that fast these days, but some folks might. I think my Cummins may outlast me.
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Old 04-15-2017, 09:33 AM   #12
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I agree with all of the comments above, re how turbos can produce more power out of a small block. But let me say a few words about modern turbo charged engine longevity.

The venerable Ford Lehman 120 hp or Perkins 135 hp that were used on trawlers built in the late 70s through early 90s were generally normally aspirated (although the older Mainship 34 used a mildly turbo charged version of the Perkins) and would last many thousands of hours if limited to producing 50 hp, which is all a trawler needs to make displacement speed.

Today you have common rail turbo charged Cummins engines producing four times that hp from nearly the same displacement, the QSB 480. Not often used in trawlers but the lower 330 hp version, which is just a chip change definitely is used as RCook notes above.

The QSB5.9 engine can probably make 200+ hp continuously and last as long as a Lehman or Perkins. That is 3-4 times the continuous hp of those older engines. So what gives? How can a modern engine last the same as a 50 YO engine making four times the power. The answer is lubrication, metallurgy, design and cooling.

Lube oils are much better today than they were 50 years ago. Lubricity hasn't changed much but stability (the ability to last without breaking down), acid tolerance (less of a problem with today's low sulfur diesel) and viscosity control are much better today.

Today's engines are made with better metallurgy: the cast iron block, the cylinder coatings on some engines, bearing materials, valve and valve seat materials are all better today than they were years ago.

Design: Engines are tested at high power to failure and that info leads to design changes to bearing supports, head bolts, etc.

Cooling: The cooling system of a 480 hp engine has four times the heat rejection capability of a 120 hp engine. Just as importantly the 480 hp engine has lube oil jets that keep the bottom of the piston cool. But keeping the cooling system in top shape is a real issue for the high output engine because if something goes wrong at high power, you don't have much time to react before severe damage occurs. So raw water flow or mixer temperature alarms are essential.

So all of these changes over the last 50 years allow the same block to produce 4 times the power and last just as long. But you have to maintain them and monitor them as the margins are smaller.

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Old 04-15-2017, 10:02 AM   #13
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A turbo will be found on most boats that plane , where high fuel burn is required.

For a displacement boat a NA ,( natural aspired ) is frequently better.

The hassle is at displacement speeds boats are easy and cheap to push so the turbo may not get enough exhaust gas at high enough temperature to function.

Then its like a restriction in the exhaust , to no purpose .

If your fuel tab is 3-4 GPH you wont need a turbo, 30+ GPH , its probably required.

Any extra weight from an NA engine wont be noticed in a displacement boat

I agree with you FF. I wish my boat had a NA version of the Cummins. I hear they have been made in the past. That is only because I normally run between 6-8 knots. I just don't need all the extra horse power. However, the builder put in a 380hp turbocharged engine, "upgraded" from the standard 330 hp turbocharged version at the request of the original owner. North Pacific is putting a Cummins QSB 6.7L 355hp engine in their current 45' (beautiful boat btw).
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Old 04-15-2017, 10:11 AM   #14
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I agree with all of the comments above, re how turbos can produce more power out of a small block. But let me say a few words about modern turbo charged engine longevity.

Thanks for the great post David. It helps for me to hear an informed perspective from all the knowledgable TF members instead of my own ignorance ramblings.

When I bought my boat I asked the builder about the 5.9L 380hp that the original had asked for. I was concerned (being simple minded) that maybe it wouldn't be as robust as its normal 330hp version. He told me that was't the case. Sure it is just a chip change that causes it to be able to generate that extra 50hp but along with that comes some upgrading of the supportive systems such as cooling. His thought, like you explained, is that for my use that simply will make the engine more reliable, not less.

You mentioned improvements in oil. This last oil change I was looking for API CJ4 oil but found that the oil that I have been using is now a newer CK version. Oil always confuses me a bit. In general I understand that the improvements in additives are generally a good thing but it always makes me nervous.
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Old 04-15-2017, 12:09 PM   #15
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I think what everyone is saying is correct, but that we are measuring using the wrong yard stick.

It's really about the service rating of the engine, not turbo or no turbo. Yes, there is some correlation, but as has been pointed out, it's loose at best.

If you want longevity, the metric should be the duty rating for the engine. Every manufacturer uses different terminology, so you need to read the exact definitions. But they all start with continuous duty, which means the engine can be run at full rated power 24x7x365. From there the duty cycle drops down to things like 16/24hrs at full power with the remaining 8hrs at 200 rpm below full rated rpm, on down to pleasure duty which is around 1hr/8hrs at full power, and someone posted 6/60 minutes as another example.

I would go much more based on the duty rating, and pay no attention to whether the engine has a turbo. There are lots of continuous duty engines with turbos that will run for tens of thousands of hours.
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Old 04-15-2017, 12:58 PM   #16
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"Cooling: The cooling system of a 480 hp engine has four times the heat rejection capability of a 120 hp engine. Just as importantly the 480 hp engine has lube oil jets that keep the bottom of the piston cool."


If this is regarding a Cummins 6b I would really like to be able to read the source on this information. Being somewhat familiar with that model as it moved from 250/300/330/370 etc hp the ability to reject 400% more heat is something I would like to read about.
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Old 04-15-2017, 01:11 PM   #17
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I agree with Twisted. I would not choose a boat, because it had a turbo or didn't. I don't know how much work the turbo on current boat is doing at the RPMS that I normally run, 1200-1800. It is a Cummings QSB 330hp. I also don't think that they are a maintenance issue either. Of course SH-- does happen. Also in regards to the Cummings, I wonder how much help the after cooler is. Seawater temps where I boat are in the 50 to 65 degree range and air temps, for the most part are the same. Every now and then higher air temps occur and engine room temps run close to 100 degrees. Other than the increase in temp of combustion air due to the turbo, I would think that there is not a lot of differential in those temps. So I would say a trawler without either or both, would also be acceptable
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Old 04-15-2017, 01:33 PM   #18
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Also in regards to the Cummings, I wonder how much help the after cooler is. Seawater temps where I boat are in the 50 to 65 degree range and air temps, for the most part are the same. Every now and then higher air temps occur and engine room temps run close to 100 degrees. Other than the increase in temp of combustion air due to the turbo, I would think that there is not a lot of differential in those temps. So I would say a trawler without either or both, would also be acceptable

Interesting idea. I also am not sure how much the turbine spins up at the rpms we run. Without the turnbine compressing the air and raising the temps, the aftercooler may be unnecessary. OTOH, since we do operate in 50 degree water most of the time, it is likely cooling the intake air on those rare hot days giving us a bit more power.

I wonder what would happen if the aftercooler were removed? Install an EGT gauge and keep a close eye on it?
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Old 04-15-2017, 02:07 PM   #19
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Dave:

Turbos on high output engine take the air at atmospheric pressure, about 15 psig and boost it 30 psi to 45 psig. That is a three fold compression ratio. That ratio, increases the temperature of the air by several hundred degrees. If that hot air were used by the engine, it would cause sky high EGTs and would melt pistons and scramble the valves.

So the air cooler is there to bring that temperature down to about 20 degrees above sea water temp (if it is a sea water after cooler). It has little to do with cooling ambient air down to sea water temps.

That turbo is only compressing the air to that ratio with lots of fuel at high power output. At low power loadings, the turbo produces little or no boost and the air cooler does minimal good as the air temperature entering the air cooler is essentially at ambient.

smitty477:

Well, my statement that the cooling system handles 4 times the load on a 480 hp engine as on a 120 hp engine is based on info I received from boatdiesel and on basic physics.

Any recreational diesel converts about 1/3 of the energy in the fuel into work, ie producing hp at the shaft, 1/3 goes out in the exhaust and 1/3 is absorbed by the cooling system and transferred to sea water. So a 480 hp engine releases the energy equivalent of 480 hp in btu to the sea water system. Same for a 120 hp engine. So the 480 hp engine has to have 4 times the cooling system capacity as the 120 hp engine.

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Old 04-15-2017, 02:32 PM   #20
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"Well, my statement that the cooling system handles 4 times the load on a 480 hp engine as on a 120 hp engine is based on info I received from boatdiesel and on basic physics.

Any recreational diesel converts about 1/3 of the energy in the fuel into work, ie producing hp at the shaft, 1/3 goes out in the exhaust and 1/3 is absorbed by the cooling system and transferred to sea water. So a 480 hp engine releases the energy equivalent of 480 hp in btu to the sea water system. Same for a 120 hp engine. So the 480 hp engine has to have 4 times the cooling system capacity as the 120 hp engine.
David"


OK - so you were just relaying some general guideline about diesel engines in general and how they need to reject more heat when they burn more fuel.
I mistakenly thought you might be referring to the 480 Hp version engine up higher in that same post and saying that it is equipped to deal with up to 4X the heat load of its 330 hp predecessor.
My mistake - thanks
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