Effects of Running Diesel at Low RPM

[kelpy]

The statement actually refers to your apparent lack of understanding of how Detroit Diesels work.
I'm done with this thread. I wish everyone a great Labor Day and enjoy yourselves!!

[Pete]

I really appreciate your sensitivity in not exposing my lack of understanding on how Detroit Diesels operate. And there is no doubt in my mind that you could thoroughly embarrass me with you vast and absolutely comprehensive mastery of the subject. Too bad you appear to be a guy who tosses one in the punch bowl, and runs out the back door, leaving us all to wonder what was that all about.

BTW, you are current with the meds?????

Pete

The statement actually refers to your lack of understanding of how Detroit Diesels work.

[Seapig100]

I know I am just an uneducated idiot, however one thing I have learned is not to take advice without first researching said advice. In the process of doing this research some very interesting points have been learned. First we get somewhere between 18 and 20 hp per gallon of fuel burned second Detroit 8-71's were put in generators governed at 1200 rpm's. There have been a few points made as to why I should not run my engines at 1200 rpm's. I listened and researched and asked questions. The questions are based on these two facts. If a person has no ability to answer my questions without taking their ball and going home then I have to wave the bs Flagg on their whole argument. My research tells me I am producing about 80 hp per motor (hardly lugging a 325 hp engine) and I am running the motors at almost peak torque. I know and it's not disputable that the ideal place to operate an engine is the rpm that makes peak torque. So there is zero doubt to whether this is doing damage to the engines it is not. Now are the blowers operating with enough cfm to scavenge the spent exhaust? I would be very surprised if Detroit has multiple blowers for the engines. I looked and find no such evidence. What I did find was that the generators were considered marine engines. In other words the generators governed at 1200 rpm are the exact same engines as I have in my boat. So this blows up the scavenging air problem. We can then look at all the boats and what boats are getting the most hours from their engines. Up here in Boston the lobster guys take the prize. They have little problem getting 20,000 hours from their Detroits. These guys run their motors at or just above idle for hours and hours at a time. Show me a boat that's been run at 1800 to 2000 rpm's with 20,000 hours on them. So I think I am in my right to question anyone who makes claim that running these engines at 1200 rpm's is causing excessive wear.

[Pete]

Let me add one more research point. As I already noted DD engines are kit engines with many interchangeable parts that look identical and but are speced differentially, including scavenger blower volume ratings. The 71's you mention are basically the same engines but I do know the parts hung off the blocks vary with the application. To make this short, 8V71's came in recreation/pleasure boat setups and on the other end continuous duty/commercial duty setups. I would think the constant RPM setup for generators is also different but have no facts.

As an example 671s came rated at about 180 HP continuous duty NA. Recreational duty got up to 390 HP still NA. Add a turbo and they got up to 485 HP. Same basic block, same 671 designation, big differences.

Those 20,000 hours engines I would wager are continuous duty ones. Not what you have in your Hatteras. Check me out on this.

Pete

[Seapig100]

Thanks Pete I will try to find out if the blower is different then maybe the best solution is to change the blower. Our engines are most certainly 100% duty cycle engines. For those not understanding what a duty cycle is, it is how long a piece of equipment can run before being shutdown down for a designated shutdown period. Our diesels can no doubt run indefinitely without being shutdown making the 100% rated. Now is the scavenger pump different? I will attempt to get the answer to the question.

[kelpy]

Well Pete, you lured me back with your post. I think you are doing this on purpose.

A 390 HP 6-71 NA? Good luck showing us one of those!
And I don't believe that there are different displacement blowers either. Why would there be? At any given RPM, there is always excess air for scavanging. 426 CI displacement (6-71) is the same regardless of power output. The blower neither knows nor cares how much power is being produced. But a turbo does.
And "kit" engines? All manufacturers supply engines that are used in various applications. They simply are engineered and built to perform their designed task. Nothing "kit" about them.
In our applications, the Detroits are refined (for their time), precisly engineered marine propulsion engines. If you leave them as designed, prop them correctly to reach full RPM at WOT, and tune them up once a year or so, they are able to chug along for thousands of hours with proper care.

Here is a link that will help define & describe marine Diesel engine duty ratings.
https://www.sbmar.com/articles/conti...t-perspective/

[Avenger]

I knew you guys could do it with the help of the internet. Who needs to listen to a guy with an engineering degree?

BTW, there are different blower drive ratios, but you're barking up the wrong tree. Engine life is measured by fuel burn. That 210 HP 6-71 has the same life expectancy as the 485 HP 6-71TIB.

[Pete]

Kelpy, and here I thought your word was golden and you were bowing out. On the 390 HP 671, you got me, I should have typed 310 HP as produced by J&T in the late 70's, used in a few Hatteras but mostly by the Jersey builders. I can see a 79 Viking out my window as I type with a pair installed.

As for the blowers, I will leave that to you to check out. Also, you may not want to call DD kit engines, but I suspect you are in the minority when talking to people who know something about them, including me who you believe knows so little. An engine that you turnaround the block and bolt on some different parts to make it rotate in the opposite direction is a kit to me.

For my money, DD have many attributes, but refined and precision are not included.

And as for your overall summary, for NA engines I believe we are close on our opinions. Prop for WOT, run them hard enough to get good combustion heat and scavenger air flow, regular service and I will add there are longevity benefits of slightly underproping. Those torque curves everyone loves are limit curves, and running engines at their limits of output or close to it is not how you get extra long hours to rebuild. But all DD are not created equal. Remember the 671TIB/485 HP, 1000 hours with luck engines.

Where I suspect we vary in opinion is the turbo variations of DD. While the scavenger blower is linear with speed, turbo are not. Running turbo engines slow, especially over propped produces rich fuel mixtures and is absolutely unhealthy for the engine.

Welcome back.

Pete

[kelpy]

Well, after seeing some errors and inconsistancies, I had to jump back in. But, with Irma quickly approaching and me being located in Fort Lauderdale, I will have to postpone further debate. Two yachts to get up the New River to Rosciolli tomorrow and my boat Thursday before lift bridges close will keep me busy.
See you all next week. Best of luck to the others here in south Florida and to those along the storm track. Feel free to PM me if you need some local assistance. I will be more than happy to help if I can.

[Pete]

And good luck to you too. Stay safe, boats are easier to fix than people. No new and very few used parts available in the human marketplace. Unlike DD, humans are not constructed from a catalog of kit parts.

As for this thread, I have concluded that this thread is making no contribution to the group, at least our exchanges. So, why continue?

When you return to this thread, and I hope Irma permits that in short order, you will find like Elvis, I have left the building. I will be moving on to more productive endeavours, including other HOF threads.

My word is golden, so enjoy, over and out.

Pete