DD 671 Horse Power Ratings

[don_robertson]

I'm looking at a boat that has a Detroit 671 engine. The listing states that this engine is rated at 135 hp. I've seen many listings for boats with 671s rated at from 135 to over 700 hp.

I'm new to diesels in general and Detroit diesels in particular. Can someone explain what governs the horsepower rating of these engines and how they can be rated over such a large range? Is there a book or web site somewhere that can help me learn more about them before I commit to buying the boat? I'm also looking for a source opf performance specs for this engine - including a fuel consumption curve.

Thanks in advance for your help.

[jgmo]

"Howdy,
I'm no 6 71 guru, just an old dog mechanic that has worked on a few so these are my ramblings and not much here in specifications. You don't say what year an engine we're talking about or if it is an inline 6 or a V-6. That would have something to do with horsepower ratings and the ability to alter them. But we'll proceed anyway.

Although it might be possible to obtain 700hp from a 6-71 I don't know that you would want to own one. It would be a finicky, delicate little fuel gobbler that would eat you right out of your boating budget. I would term a 700 hp 6-71 a "race" engine. Exotic.

With that said, there is a great range of power ratings for most diesel engines depending on need and preference and the 6-71 is no different.

The basic ability of any engine to produce horsepower comes down to fire. How much fire can you make in a given amount of time? The fire triangle consists of: Fuel-oxygen-ignition source.

We all know that the ignition source is compression and, although higher compression in a "performance" engine DOES contribute to increased horsepower rating, for this discussion we'll concentrate on the other two factors of the fire.

First and foremost-FUEL. Pour in more fuel than the next guy and you COULD make a bigger fire. The 6-71 utilizes unit injectors to deliver fuel to the cylinders and you can buy them in a variety of fuel delivery rates. Of course you need to feed the higher delivery units with an equally larger pump and piping. That's about it. Pretty simple. Well, not quite but almost.

Equally as important as fuel delivery is OXYGEN. Without increased oxygen content in the combustion chambers, the increased fuel volume will not burn. Thus, we start adding things to facilitate getting more air into the cylinders.

First, you start with a higher volume blower. All the 2 stroke diesels, like the 6-71, use a Roots type positive blower to force air into the cylinders. Without this blower, the engine will not start because the 2 stroke has no intake valves to close off the cylinder to create a vacuum to "suck" in air. You can increase the air volume from the blower by specifying larger housings with larger vanes and, through drive gear ratios, increase the speed at which this contraption turns to "blow" more air into the "charge" box so that it will rush into the cylinders through the intake ports located in the cylinder liners. Of course, these ports need to be sized for the increase in fuel and air volume or you've wasted your money on the other stuff.

Now, there's a limit to all this but that's not the limit! I'll explain;

When you reach the point that a mechanical blower can no longer add additional air volume to the cylinders, you may still increase the air volume to the cylinders! How? Well, you may have already guessed...a turbocharger.

You can add a turbocharger to provide additional (too much actually) air for increased volumes of fuel delivery. The turbo works on the principle of exhaust gas leaving the engine turning a vaned wheel which is connected, with a shaft, to another vaned wheel that "sucks" in fresh air and "blows" it into the cylinders. These are amazing and powerful devices for horsepower increases. They too can be had in many different volumetric sizes and, where clearance space might be an issue, you could run two smaller ones in parallel to feed the air to the engine. These things are so powerful that much of the air produced by them is "valved" away from the engine to prevent damage! They produce so much air flow that it was discovered that the Roots blower that they were pushing the air through to get it into the blower air charge box was actually getting in the way even as it turned at high speed and tried to pump air!

Ok, that's about all I have time for and it is a pretty rudimentary explaination to your question at best. But, I hope that I may have helped you understand what you're looking at.

Just one more thing. And they don't teach this in physics class. There truely is another "leg" to the fire triangle...MONEY! How much you got? Because in this horsepower business, there ain't no free lunch. You said you were interested in fuel consumption. I am too when on the water. It's a really big deal running out of fuel offshore. But, you ain't gettin' offshore if you can't even afford to "feed the beast". And, I don't mean just the cost of diesel. As you increase horsepower you also increase maintenance costs and associated wear costs. The more HP you use, the quicker all that iron is going to wear out. You can slow it down by doing good maintenance but it still happens. Those turbochargers I mentioned can routinely operate at well over 100,000 (that's one hundred thousand) RPM at full boost. They will eat you out of house and home if you don't keep them serviced and protected. Frequent oil and filter changes cannot be ignored, AT ALL, if you want this stuff to last. And, if you are paying someone else to take care of this for you, a bad service could cost you your life on the water.

Me, personally, I like the old "naturally aspirated" (no turbo) 6-71. She is a bullet proof workhorse that will get you there and back time and time again. Not a thoroughbred by any definition. Noisy, leaky, slow down the backstretch but reliable as all get out. Reliable is a life saving word 50 miles offshore.
Gday"

[don_robertson]

"Jimmy,

Sorry for my delay in following up. I've been away from my e-mail for a while.

My question didn't arise from a desire to increase the power from the DD 6-71 in the boat I'm looking at, but rather from a desire to understand how the same basic engine could provide such a wide range of power ratings. Your mini tutorial did a good job of helping me do that. Thanks for your detailed reply."

[edington60]

"Hi Guys, Look I am new to marine diesels, and I am interested in buying a 6-71N (Inline 6) to power a 55ft wooden trawler. Could somebody tell me which year model I should be looking for. Thanks."

[cobe_marine]

"Jimmy,great job breaking down the whole thing in laymen terms-you sound like a teacher!?! I too am a big fan of the ol' dinosaur diesels. Give me a 2 stroke Detroit any time over all these new fangaled,computer controlled,aluminum block high reving ones (see Yanmar)."

[nbcbob]

I can add that J and T Marinized the 6-71. I have a 6-71 TIB and its rated at 485 HP fuel at WOT is 16.5 GPH. I run mine at 1900 to 2050 and born about 12GPH in easy seas.

[Barkspud]

jgmo that was a most interesting read im new to desiel and detroits
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