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Detroit 8v71 recommended rpm

Discussion in 'Engines' started by wayneCCC, Jul 28, 2015.

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  1. wayneCCC

    wayneCCC New Member

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    I have found my new to me(1970) Chris Craft cruises nicely at 12 to 14 knots and about 1700 rpm. Great fuel economy and temperatures. I have the original sales literature and they all say the boat will do 25 knots. What is a maximum safe rpm for the detroits? Would be curious to discover the real max speed, if only for a few minutes.
  2. Capt Ralph

    Capt Ralph Senior Member

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    Naturals need to turn at least 1800. TIs need at least 2250.
  3. Capt J

    Capt J Senior Member

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    I'm not familiar with top rpm for the naturals. I thought the TI's were supposed to turn to 2350 rpms and cruise them at 1950-2000 rpms.
  4. Capt Ralph

    Capt Ralph Senior Member

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    I'll crack my books when I get back to my desk.
    That TI number may be 100 low.
  5. K1W1

    K1W1 Senior Member

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    From memory all that info is on a data plate on one of the Valve covers.
  6. Pascal

    Pascal Senior Member

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    Afaik both the N and TI are rated at 2300 rpm so a max cruise of 1900 to 1950 is reasonable.

    You didn't say what size both you have but running between hull speed and on plane is just about the absolute worst speed you can pick. You re pushing a big wake, loading up the engines and burning as much fuel as you would if you were on plane

    Hull speed is typical the sq root of waterline length times 1.3 so for a 50 footer you re looking at about 9kts.

    My 53 Hatt burns about 10gph (both 8V71Ns) at 9kts. If I push to 11kts, that jumps to around 18gpg, 12 kts is more like 25gph... Makes no sense.

    If the boat will get on plane then it is a much better speed. Make sure your trim tabs works and set them full down to help.

    Again not sure what size year Chris you have, but back when naturals were commonly used in the 60s and early 70s fuel was cheap so burning twice the fuel for a extra 2 or 3 knots wasn't a concern.

    So you re better off either running at hull speed or on plane, but never in between
  7. RER

    RER Senior Member

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    TI's spec is 2450 no load, 2350 loaded but I would not push a 40-year-old-plus motor. Old Detroits rarely turn up to spec at WOT. TI's are in the turbo boost at 1750. 1750-1850 is an ideal RPM for the 8V71 to cruise however in your case I doubt you plane at 12 ...maybe at 14 though. 1100 RPM should give you +/- 9 knots and way better fuel economy.

    Actually I had not seen Pascal's comments above when I posted. Basically saying the same thing except I would not cruise the old girl above 1850.
    Last edited: Jul 28, 2015
  8. wayneCCC

    wayneCCC New Member

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    Thank you all for the information. It is a 1970, 57 foot constellation.
    At the cruise speed I have been using, I am thrilled with the mileage (est 13 gph cruising Portland Or to Vancouver), but am encouraged that it could be even better. The engines are N's with about 3500 hrs on them. So with a waterline of about 53' , my hull speed would be around 10 knots? 1500 rpm usually gives me about 10 knots, but I have not had her long enough to do any comparisons on fuel consumption at different speeds.
  9. Capt Ralph

    Capt Ralph Senior Member

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    You need to see 1800 to 1850 WOT. 1450/1500 would be the long life throttle (approx 80%).
    I used to run straight 671Ns at 16/1700 all weekend per owners instructions on one boat.
    Many years later, Yep, it's still running with no problems.

    As K1W1 stated above, if you still have the tags on the valve covers, those are the ultimate numbers to go by.
  10. wayneCCC

    wayneCCC New Member

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    Well the tag on the valve covers state 2440 max rpm no load on starboard side and 2240 max rpm no load on the port side?
  11. Capt Ralph

    Capt Ralph Senior Member

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    Huummmm
    Would luv to get the serial numbers. Those numbers are no-load and similar to a T or TI.
  12. Pascal

    Pascal Senior Member

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    8V71s are rated at 2300, you need to see 2300rpm at WOT loaded. If you don't get that, or close to, it means you are over propped, something is wrong, the boat is dirty, etc...

    When I surveyed my 53, the engine surveyor noted the engines only turned 2200 under load and warned against running them hard until the problem was fixed but running at 1500 for hull speed woudl be fine.

    One thing I can't stress enough is how even 1/2 knot over hull speed will greatly affect your fuel burn. On your boat, your hull speed is about 9.5kts. Running at "around 10kts" may cost you 3 or 4gph or m ore if you are actaully running closer to 10.5. When trying yo find the best rpm for hull speed, take into account possible current and wind and obviously use GPS not paddle type instrument
  13. Capt Ralph

    Capt Ralph Senior Member

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    Your tags State over 2000 rpm. That kinda proved me incorrect. I maybe remembering old 2 valve heads before the '70s.
    Either way, you still have lots of drag or your over propped.
    Still would luv to look up the factory specs with your serial numbers.

    Have you preformed a no load high idle max test yet? This will tell you quickly what the governor is set for.

    Pascal is correct on any little throttle adjustment can change angle, mpg and speed greatly.

    Paddle wheel senders are not accurate but repeatable. Gps is accurate but over the ground, not thru the water.
    When measuring boat performance, make two passed with & against the current and average the speed recordings.
    If you have a paddle-wheel speed instrument, learning the errors and variations would be of benefit. also for other task when DR-ing.
  14. wayneCCC

    wayneCCC New Member

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    All Great information, and I will continue to experiment. I have not done any actual testing so far, have only had her for about a month.
    Port serial number 8VA94670, Starboard serial number 8VA93989
    Purchased from Original owner, so I do know the engines are original, with all service records, buildsheet etc.
  15. Capt J

    Capt J Senior Member

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    If the boat tops out at 25 knots, cruise speed you should see 20-21 knots generally in that type of boat and you'll probably need a lot of trim tab to keep her on plane. Running 1700 at 12-14 knots is a very poor speed and you're most likely just pushing a lot of water and high load factors on the engines. I would run them 1950 rpms and if you cruise at 17.5 or more on plane it would be an easier speed on the engines (less load factors and better fuel economy). I've had several D.D. dealers tell me that you can even run naturals at 2100 (if they achieve 2300 at WOT) if that's what it takes to keep the boat on plane and they sound like they're working less hard.
  16. rgsuspsa

    rgsuspsa Member

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    Attached are data sheet for the Detroit Diesel 8V-71 configurations available in year 1985. Per your engine serial numbers, they were manufactured prior to 1972, which is the earliest year I have data for. Suggest you determine your engine configuration based upon the fuel injector number installed, which can be determined by removing a valve cover and looking at the round, metal disc impressed into the cast iron body of each injector. The number on the disc is the injector number installed in your engine. Suggest you check each engine to confirm they have the same injector number, and all cylinders are fitted with the same injector number. Using the injector number installed, select the appropriate set of corresponding data from the data sheet. Determine the appropriate Rated RPM, which is the RPM at which the engine will produce maximum power. Your engines should be propped to enable them to reach between fifty and one-hundred RPM greater than Rated RPM. Maximum No Load RPM, also called High Idle Rpm, should be set at 150-200 RPM greater than Rated RPM.
    Any questions, please post.

    Attached Files:

    Last edited: Jul 30, 2015
  17. Capt Ralph

    Capt Ralph Senior Member

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    Sadly, build records before 1972 are not available to me. Sorry I can not deliver any DDC factory build notes.
    Do you have any info on 2 or 4 valve heads and injector size?
  18. rgsuspsa

    rgsuspsa Member

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    Hello Ralph: I do not have build information in my files earlier than that posted, and a serial number table for all Detroit Diesel Two-Stroke engine from 1972 thru 1995. Would you care for the serial number table?
  19. Capt Ralph

    Capt Ralph Senior Member

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    I'm sure we have the same thing.
    My DD shop can pull up better detail on lots of extra data. Sadly, they hit the '71/72 wall in the DD data base..
    ,rc

    Attached Files:

  20. rgsuspsa

    rgsuspsa Member

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    Yes, the serial number tables are the same. Forty year old paper data sheets and manuals are few and far between. Fortunately, replacement parts other than castings are not usually a problem, and the data sheets I do have from the early eighties and up are typically applicable to the 1970's engines, which were very mildly tuned.
    Last edited: Jul 31, 2015