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Detroit Diesel 4-71 Engines

Discussion in 'Engines' started by brian eiland, Dec 22, 2013.

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  1. brian eiland

    brian eiland Senior Member

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    A vessel has been brought to my attention that is equipped with twin 4-71 DD engines' It quoted as have 660 hp total,....330hp each.

    Is that even possible with these size engines??
  2. Capt J

    Capt J Senior Member

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    I'm not very familiar with 4-71's, but 6-71 TIs make 485hp....divide that by 6 cylinders and it's 81hp a cylinder.....multiply by 4 and it seems very reasonable.
  3. SeaEric

    SeaEric YF Historian

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    In my experience GM 4-71ti's are typically rated @ 300 hp in recreational boats.
  4. dennismc

    dennismc Senior Member

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    That seems high, 6-71's come out of the factory at 280 hp, then if required they can be goosed from there, mine 310 out of J&T, still naturals..
  5. Capt Ralph

    Capt Ralph Senior Member

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    In 1996, Detroit Diesel rated their 4-71 TI with blower bypass at 325 hp/2600rpm.
    Any of the after market marinizers could have tweaked a few more ponies.
  6. PacBlue

    PacBlue Senior Member

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    As stated, 6-71's maxed out at 485 hp which works out to 80.8 hp / cylinder. This was not their "long life" version. A 4-71 would be 323 hp (call it 325 hp)at that same cylinder output.

    I always thought the 410 hp version of the 6-71 was the best compromise between power / longevity, this would make it 68.3 hp / cylinder. This same output in a 4-71 would give you 273 hp.
  7. 993RSR

    993RSR Senior Member

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    6-71TIB 485's or 4-71TIB high output both have a history of early failure or cyl wear (800 hours +/-) unless you de-tune or run them at 75% of wot
  8. Capt J

    Capt J Senior Member

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    Which brings about the point of why have them in the first place. The TI's are a much better engine. It's like the short life of the 92 series......There's nothing wrong with running 71 TI's 2000 rpms, if they're propped right so they achieve WOT rpm's. Which is probably the same speed as running TIB's at 75%.
  9. 993RSR

    993RSR Senior Member

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    Agreed.
    Parts are cheap for the old Detroits
    Service is cheap compared to todays fully electronic motors
  10. brian eiland

    brian eiland Senior Member

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    I'm a little confused. What is the difference between 4-71TI and 4-71TIB??

    Aren't most 71 series engines 2 cyc diesels? Don't 2 cyc diesels require forced air input (blowers or turbos) ??

    And how about compression ratios. Whether an engine is naturally aspirated or supercharged (turbo or blower) doesn't the compression RATIO remain the same for the same style engine?

    Of course the pressures involved with the supercharged engine are much greater, and require that much more of a robust engine??
  11. 993RSR

    993RSR Senior Member

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    Back in the late 80's & 1990 Detroit offered the motors but they were marinized by aftermarket distributors.
    Competition between Johnson & Towers, Covington & Stewart & Stevenson to outdo each other and win boat manufacturers business was active.
    I do not know the exact answer to your question other than the injectors were larger 105-120mm if memory serves and the engines were tweeked for more air (blower speed?) and higher wot rpm. 2600 I think which is 300 over DD
  12. Capt J

    Capt J Senior Member

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    Yeah, I've seen 6-71s that turned 2600 rpms, and I think some even turned 2800 rpms if I'm not mistaken (J+Ts). I don't know what difference there was in the long block on DD's, but a lot of the marinizers played with bigger turbo's and bigger injectors. J+T tended to hot rod them the most with the biggest turbo's and biggest injectors of all the marinizers, but at the expense of longevity. They didn't seem to mess with top rpm much on the 8v71's and 12v71's, just turbo's and injectors. I've also seen some weird things, like I ran a set of Covington 12v71 TI's and it used a generator setting for the rack injector height of 1.46", and if you tuned the rack to the marine setting for injector height 1.96" in the DD book, it would black smoke like crazy.

    Once Penske got involved with DD and the DDEC's in the late 90's, I think all of the marinizers stopped playing with turbo's and injectors and just marinized them.
  13. 993RSR

    993RSR Senior Member

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    All DD of this era were 2 strokes except the 8.2L 3208 Cat copy that did not work out to well.
  14. brian eiland

    brian eiland Senior Member

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    I think I might shy away from a vessel that was trying to pump that much HP out of each cyclinder (~80),...from what I am reading on several forums.

    It would also appear to be a problem that to get these bigger output figures that big injectors are being utilized. Now if someone planned on running these engines at a more sedate level (trawler speeds) it sounds as though dilution of the oil would be a serious problem??
  15. 993RSR

    993RSR Senior Member

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    Someone planned on running these engines at a more sedate level (trawler speeds) it sounds as though dilution of the oil would be a serious problem??

    Pull the throttles back on motors of that era & output and you will be pumping more fuel out your exhaust than you are using. Also the cyl temps cool down & motors are not happy campers.
    The fully electronic engines on the market today allow you to run slow without loading up the engines.
  16. brian eiland

    brian eiland Senior Member

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    Thanks for that dbltime, I've made a mental note of that.

    Now if I can just keep my memory on track :eek:;)
  17. brian eiland

    brian eiland Senior Member

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    Interesting submissions from a trawler forum discussion....


  18. PacBlue

    PacBlue Senior Member

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    The 4-71TI was really a "bridge" engine for that time period. The small block 300hp Cummins and Yanmars were just beginning to appear or where on the near horizon. It was a difficult market segment, as the manufacturer needed a significant market share for on-highway (trucking) applications before the financial justification for marinization could take place. Fortunately, there are many choices now in the 300 - 350hp range, so it is definitely easier for the architect/designer/builder/owner.

    Taking a high-output model of any diesel and forcing it to "loaf" in a trawler/displacement application is not advisable. You wouldn't have a Thoroughbred pulling a plow now, would you?

    Carbon build-up, coking on valves, etc., . I am more in favor of selecting the right engine for the application....
  19. Capt Ralph

    Capt Ralph Senior Member

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    Detroit 2 stroke always have the blower (supercharger) for scavenging the spent cylinder of exhaust gasses. This same blower will ad some air pressure the the cylinder helping the combustion.
    Inline, 4 cylinder, .71 liter per cylinder, natural; 4-71

    Throw a turbo on top of that and now your helping the blower throw more air into the cylinder. Insert an air cooler before the blower (TI) or after the blower (TA) and your now shoveling in dense, cool air into the cylinder. The bigger the turbo, the more air gets shoved into the air box path & cylinder.
    More cool air, more power.
    Inline, 4 cylinder, .71 liter per cylinder, turbo charged with intercooling; 4-71ti

    DA "B".
    There is a point the blower starts to restrict the air flow from the turbo. It just gets in the way and will not allow any more air to get force thru it. That's where the BY-Pass (B) comes in. Extra plumbing allows the turbo air to go-around the blower into the air box and cylinders.
    It's usually not utilized till the upper rpm range, last 15% of throttle. Gives an engine around 5% more power.
    Inline, 4 cylinder, .71 liter per cylinder, turbo charged with intercooling, blower by-pass option; 4-71tib

    On V blocks, remove the "-" and insert the letter V.

    6-71 vs 6v71. same options as above.

    Hope this helps,
    rc
  20. Capt J

    Capt J Senior Member

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    It depends. Some DD's like 12v71TI's run pretty clean at 1000 rpms. Whereas JT 12v92 TI's don't run clean at lower rpm's. Also if the motors run at operating temps there it seems to help a lot. I ran a set of 12v71 TI's on the great loop (5300 NM's) at 1000 rpm's 90% of the time and we had no oil dilution issues they would get a little carboned up. Every 6 hours, I would run them up until the turbo's started building boost (1350-1400 rpms) and cook off the soot for about 5 minutes, then run them 1950 rpms for about 25 minutes and then back down to 1000 and there were no ill-effects. If you're stuck taking the inside on a 75' North to South, many yachts have run slow speeds for a week with DD's with no ill effects. Many places on the Great Loop, there are very few places you can bring them up to cruise for a couple of days, like the Erie Canal west of Lake Sylvan. We burned 13gph at 1000rpms and did 10.3 knots, and burned 85gph at 16.5 knots at 1950rpms. Do the math of 1.5 gpm with the generator, or 5.5 GPM, spread over 5300NM's. 7500 gallons (actual that we used) versus 29,000 gallons of fuel. We saved $107,500.00 on that one trip in fuel, and the trip was in 2008 and the motors are still running perfectly.

    I spoke to several DD factory engineers about it at the time, and they had mixed emotions. Some recommended running at cruise all of the time, some said running at cruise all of the time would shorten the life over a 50/50 mix of slow speeds/cruise speed (typical yacht usage vs' commercial usage). All of them said that running at 1000 rpms for 4hrs (if they run very dirty at slow speeds JT's) to 6 hours, and then running them at cruise for 30 mins should be no problem for them at all. They said at 1000, they're still seeing a decent load 40% (guestimated by them), and as long as operating temps were 160F or more, they should be fine and not severely effect the lifespan if you clean them out every 4-6 hours. They said running dead idle, is a totally different story. Besides the fuel saved over 1000 hours, could pay to rebuild them 3x over.

    All of the charter SF here in South Florida NEVER, EVER, run above 1000 rpms and they're getting 5500-6500 hours out of 8v71's and 12v71's (TI's)