HP to cubic inch ratio - Is it right?

[Miss Lisa]

I have read that the ratio of hp to cubic inch ratio can determine when an engine will need an overhaul. For example:

CAT 3208 with 375 hp with 636 cu in overhaul would be about 5896 hours

DD 6-71 TA with 450 hp with 426 cu in overhaul would be about every 1056 hours

DD 6-71 N with 260 hp with 426 cu in overhaul would be about 6100 hours. A broker once told me that 6-71 N is the best engine ever made - they are bullet proof!

Any opinions on this method? I have read online that heat is the big enemy of any engine. The more hp the more heat produced, and the sooner major overhaul comes around.

I have also read that the cost of an overhaul can be around $3,000 per cylinder! If an overhaul is required every 1000 hours on a DD 6-71TA - I don't think I want DDs in my boat.

AM I WRONG IN MY ASSUMPTIONS?

 

[Boatsb]

Don't assume.

 

[Glory]

Yes!!

 

[saltshaker]

The only thing correct in your post is that burning fuel is what wears an engine. There is no magical formula. While the 671Ns will last longer than 671TIs, the TIs should get more like 3000 hours on average. Sure they'll be tired sooner if you run them hard all the time and don't take care of them but that's the same for any engine. 3K per hole is about right for a Detroit, closer to 4K from a Detroit dealer.

 

[Scott & Malina]

I have read that the ratio of hp to cubic inch ratio can determine when an engine will need an overhaul.

Miss Lisa,
Please put down whatever you are reading. HP and CU have absolutely no correlation on how often any engine requires rebuilding. How well one takes care of them has everything to do with how often engines require overhauling.


Scott & Malina

 

[rwappleton]

My 6-71TI's (390 HP)with 3000 hours are running strong with no issues. I expect at least another 3000. I spool them up once a day just to clean the turbos. I agree that the 6-71 was the best one ever. But not at 450HP

 

[jim rosenthal]

It's kind of right in part, if that can be said to be "right". Lots of things contribute to TTBO: use pattern, maintenance, engine design. The amount of fuel burned is a pretty good indicator of TTBO, because it takes into account a lot of those things- if I am not mistaken, fleet operators base overhaul intervals on total fuel burn, among other things.

All other things being equal, you could argue that, for example, a 6-71 putting out 250 hp will require less frequent overhauls than a 6-71 putting out 485 hp... but all other things are seldom equal.

You could also argue that getting more hp out of fewer cubic inches means that the engine is working harder, and that squeezing more power out of fewer cubic inches is going to mean more frequent overhauls....possibly. Maybe, maybe not.

Modern common-rail diesels squeeze more power out of four-stroke diesels than I thought was possible. And yet they seem to be lasting okay... for now. I think the jury is still way out on those engines. I know that CR engines are much more efficient in using fuel to make power than older mechanically injected ones were. Whether that translates to increased longevity.. who knows?

There is no formula, not that I've ever seen, for predicting engine useful life on the basis of displacement and horsepower. What you want for that is an engine survey done by someone who knows that particular model engine- and has looked at a lot of them.

 

[Boatsb]

Miss Lisa,
Please put down whatever you are reading. HP and CU have absolutely no correlation on how often any engine requires rebuilding. How well one takes care of them has everything to do with how often engines require overhauling.


Scott & Malina

Put down the bottle. Even a overly maintained engine will wear.

 

[Miss Lisa]

I have a friend who lives in Destin, and he recommended that I talk to his mechanic. Long story short the mechanic said that the hp/cu in ratio is BS! He said that total fuel flow thru the engine determines overhaul times. 30,000 gallons is usually when an overhaul is needed. Of course, the maintenance of the engines has a great deal to do with it - he said!

Sounds like he agrees with some of you guys.

Thanks for helping me learn.

 

[doc g]

So an engine that burns 150 GPh will only last 200 hours but an engine that burns 10gph will last 3000 hours ?! Wrong!

 

[luckydave215]

Common rail engines meter fuel in several small sprays instead of one big spurt, this results in smoother rise of combustion pressure - there is less of a hammering effect on the internal parts.
In theory, this should make life easier for the reciprocating parts. It absolutely makes the engine quieter in operation, less hammering on the parts must be better.
Long term reliability? Time will tell, as the new engines are also made much lighter weight. Maybe the engineers can get away with it, they're not stupid.

 

[Glory]

Common rail engines meter fuel in several small sprays instead of one big spurt, this results in smoother rise of combustion pressure - there is less of a hammering effect on the internal parts.
In theory, this should make life easier for the reciprocating parts. It absolutely makes the engine quieter in operation, less hammering on the parts must be better.
Long term reliability? Time will tell, as the new engines are also made much lighter weight. Maybe the engineers can get away with it, they're not stupid.

"spurt", it sounds so low tech.

I'm looking forward to spurting!!

 

[Miss Lisa]

What is a common rail engine?

 

[Top Hatt Craig]

I'm curious what does "need overhaul" mean to you, or anyone? From what I've been reading two stroke Detroit Diesels eventually misfire and smoke excessively at cold start, but still run well after warming when they have a huge number of hours on them. Some owners may find it acceptable, others definitely not. The guy who surveyed our boat said he's seen 6-71Ns in commercial boats still in service at twenty thousand hours, and the same age engine in a seldom used yacht failing miserably a cold start test with seemingly low hours. He also said in his experience the DDs that did need overhaul had been overheated, usually caused by clogged screens or failed impellers.

 

[dottieshusband]

You're all wrong...It's about vibration analysis...that's the true predictor of metal fatigue and system failure...yeh, that's the ticket..vibration analysis !

 

[Glory]

You're all wrong...It's about vibration analysis...that's the true predictor of metal fatigue and system failure...yeh, that's the ticket..vibration analysis !

I sent a way for 2 vibration analysis kits today! I hope they come soon!!

 

[Pearl Island]

I am not a expert but think it can be explained like this.

Take two engines same brand and spec., two boats same model, - Both boats propped right, (meaning you get max rpm at WOT), both maintained according to the books.
You run one on average at 50% of max fuel consumption according to the fuel curve and the other one at on average at 80% of max fuel consumption according to the fuel curve. The one at 50% should last longer than the one at 80%.

Same as above, two different engines, one with higher hp/cu, both runned at let's say 50% fuel according to the fuel curve, the one with lower hp/cu should last longer.

Fuel consumption=hp=heat. As a general guideline a diesel engine have a output of 20hp/gl fuel.

More hp/cu=less iron to disperse the heat.

For engine life it is more important how many hp is taken out of the engines (=fuel consumption) than the than the rated hp.

 

[jim rosenthal]

Common rail diesels use electronically governed high-pressure injection systems to atomize and inject fuel into the combustion chambers. This is much more efficient both in fuel dispersal and fuel metering. The spray pattern is more controllable and the amount of fuel delivered is very consistent from one cylinder to the next, so the engine runs more smoothly. All new modern diesels are CR, I think. Cummins, though, still sells their old designs through their reman program.

The top hp available through the reman program is 370hp from 5.9 liters. I think the CR B series engines, which, to be fair, are 6.7 liters now, is 540hp- maybe even more. The difference is impressive.

 

[Boatsb]

I owned both a 5.9 and a 6.7. Their outside dimensions are similar. Their fuel systems are night and day. I believe the 5.9 with a common rail system was 425 or so hp. The 6.7 list 550.

It's quieter, smoother and less efficient thanks to the pollution bullshit but it's far better than the old 5.9.

 

[SEVEN]

No need to over think this. They need to be rebuilt when they need to be rebuilt. Take care of them they will last you a life time. Don't and they won't. Simple.