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How much is too much boost?

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    How much is too much boost?

    I got two new turbo's from ATC and did sea trials yesterday after my engine restoration from a fire this summer. The new turbo's at WOT boosted 18psi, which to me seems pretty high for these engines. My EGT's were not exceeding 1000 *f pre-turbo, which should be fine. This is a 1989 3870 with the Hino EH700 engines and after market turbo and intercooler.

    #2
    any engine with a turbo(s) should be able to withstand a minimum of 35psi of boost pressure.... the boost pressure rises with the amount of "pull" that the engine is creating with any given model/size of turbo... if the engine has little stress being placed on it, the boost pressure will be minimal, and so the engine can run at the upper end of its rpm range and see no boost pressure until a load is placed on it...

    there are only 2 real dangers of having high boost pressure, and one is from damage to the turbo itself, as high boost means the turbo is turning at high speed, which may be higher than its designed to spin...and it can come apart
    two.. if the boost is high, it means their is back pressure being created (which drives the exhaust impeller) and if this back pressure gets too high in relation to the boost, it can cause a melt down, with not only the turbo being at risk, but also the pistons and rings...
    Last edited by Centerline2; Yesterday, 09:42 AM.


    NU LIBERTE'
    Salem, OR

    1989 Bayliner 2556 Convertible
    5.7 OMC Cobra - 15.5x11 prop
    N2K equipped throughout..
    2014 Ram 3500 crew cab, 6.7 Cummins
    2007 M-3705 SLC weekend warrior, 5th wheel
    '04 Polaris Sportsman 700 -- '05 Polaris Sportsman 500 HO
    Heavy Equipment Repair and Specialty Welding

    Comment


      #3
      But the CR of the original NA engine is not ready for that much boost and with that much more oxygen it will be a lean burn condition.

      I think the upfit turbo was originally about 5-6psi.

      Personally I am not a fan of this mod for only marginally improved cruise speed of about 1kn.
      1999 Sandpiper Pilothouse - Current
      1989 3888 - 2011-2019, 1985 Contessa - 2005-2011, 1986 21' Trophy 1998-2005
      Nobody gets out alive.

      Comment


      • Centerline2
        Centerline2 commented
        Editing a comment
        lean burn in a diesel?.... thats the way diesels are designed. if its too lean, give it more throttle and it will go rich and smoke a bit until burns the excess fuel that is causing the rich condition..... a diesel can only ingest as much air as the cylinder volume/size will let it, and the turbos job to supply pressurized air for the engine when it needs it....the reason for the desired pressure is so the air can be quickly pushed into the engine, and fill the cylinders to their limit.
        the turbo is driven by the amount of throttle the engine is getting. as long as the air and fuel filters are clean, and the system is working, a diesel engine is self regulating/balancing within the parameters of the way its set up and tuned.
        an engine without a turbo is extremely under powered in relation to the same engine that has a turbo, and the one without the turbo will smoke more because it isnt getting the air it needs to have an efficient burn in the cylinders, but there is no down side to having more air than the engine needs to run, with the exception that too much boost can blow seals and gaskets in the air intake system... ANY engine that is designed to work with a turbo will be able to take 35psi of boost, but very few are set up to give more than about 25-30.... but by opening the fuel delivery too much above spec with a turbo that can create the pressure and the skys the limit on boost and power, until the engine has a melt down or comes apart catastrophically...

        an engine fitted with a turbo that is able to create a higher boost, will be able to deliver more power to its very limit of its fuel setting, but if the turbo cant create boost, it wont run any better than a naturally aspirated engine.. a little bit of boost and it will only run a little bit better than a naturally aspirated engine...

        installing a different turbo on an engine than the system was designed for, will cause the optimum "working" rpm/range of the engine to move up or down in the rpm range (the cam selection can do this too, but its not as simple to change the cam) and a turbo that is too small will NEVER be able to deliver the boost that the engine needs to perform optimally, and one that is too big cannot be driven to a high enough speed to create the boost needed for the engine.
        but there is very little chance that a turbo will ever give too much boost to any diesel engine, as the boost is only created by the engine being ran at a higher rpm (to create a high abundance of exhaust gas to drive the turbo), and this cant happen unless the fuel valve is opened.... close the fuel valve and you lose boost pressure immediately....

        with all this said, I do agree there is very little reason for a boat engine to create more than about 20psi of boost... unless there have been other changes made to create a high performance boat, which then starts creeping away from dependability...

      #4
      Balanced being the keyword. If the pump was setup for 5-6psi and then you triple it there is too much air and it will lean out and run egt that will be much hotter than what is sustainable.

      This engine started life as a NA engine compare CR with a engine designed for turbos and you will see the turbo engines will typically be lower because of additional compression coming along from the turbo.
      1999 Sandpiper Pilothouse - Current
      1989 3888 - 2011-2019, 1985 Contessa - 2005-2011, 1986 21' Trophy 1998-2005
      Nobody gets out alive.

      Comment


        #5
        Forewarned, I know Jack crap about diesels but......Standard pressure at SL is 29.92”Hg which equates to approximately 14.6 psi. Considering the volumetric inefficiency of intake manifolds and the air filter restriction on the NA EH700 I’d guess the highest intake manifold pressure the engine could see would be somewhere around 28”Hg or 13.7 psi. To suddenly overboost the manifold pressure to 18 psi, which equates to approximately 36.6”Hg, will be quite a shock to that EH700! I’d be very concerned the fuel mapping would be incorrect and the cetane rating possibly insufficient considering the higher CR of the NA EH700. I’d be very careful about pushing a NA, now turboed, EH700 to hard.
        Jim Gandee
        1989 3888
        Hino 175's
        Fire Escape
        [email protected]

        Comment


          #6
          Jim, thinking about what US Marine did to the EH700’s over time and adding turbos and aftercoolers, I looked to see if the part numbers of the intake, cam, etc changed from the NA. I found that there are really only minor differences between the two, and those differences appear to be plumbing for air, oil or water as would be expected. I expected to find different part numbers for the manifolds.
          Torski was in an odd situation because the PO had installed the turbo kits then failed to maintain them. Whatever adjustments to fuel pressure and that sort of thing were presumably already done.
          P/C Pete
          Edmonds Yacht Club (Commodore 1993)
          1988 3818 "GLAUBEN”
          Hino EH700 175 Onan MDKD Genset
          1980 Encounter Sunbridge "Misty Blue" (Sold)
          MMSI 367770440
          1972 Chevrolet Nova Frame off Resto-mod in the garage
          Boating on the Salish Sea since 1948

          Comment


            #7
            Just a small correction to what Centerline2 had to say. Diesels don't have a throttle - their intake is always fully open (not controlled by a throttle butterfly as a petrol motor is). The amount of fuel being injected is the variable that supplies power and that is controlled by your accelerator linkage. A turbo supplies more air, but not through a throttle. Obviously, the more oxygen supplied by the turbo enables more fuel to be injected to give more power.
            OK, I know I'm being pedantic.
            Peter Walter-Smith
            Riverport of Goolwa, South Australia
            2014 Bayliner Element XL
            115HP Mercury EFI

            Comment


            • Centerline2
              Centerline2 commented
              Editing a comment
              you are correct..... but notice i didnt say throttle plate or butterfly. the engine does have a throttle lever, https://www.merriam-webster.com/dictionary/throttle ...and by increasing the position of this lever the engine gets more fuel... so saying give it more "throttle", is correct (if the engine needs it) no matter if its a diesel or a gasser... to be more precise, I suppose one could say, "open the metering valve in the fuel pump more", or maybe, increase the voltage a little to open the fuel rail in the injectors more.... but as precise as this is, I have only heard these words used when a pump is on a injector/pump test bench being modified or during problem solving...

            #8
            Originally posted by Pcpete View Post
            Jim, thinking about what US Marine did to the EH700’s over time and adding turbos and aftercoolers, I looked to see if the part numbers of the intake, cam, etc changed from the NA. I found that there are really only minor differences between the two, and those differences appear to be plumbing for air, oil or water as would be expected. I expected to find different part numbers for the manifolds.
            Torski was in an odd situation because the PO had installed the turbo kits then failed to maintain them. Whatever adjustments to fuel pressure and that sort of thing were presumably already done.
            Pete, I’m always Leary of aftermarket add ons, especially turbos because the OEM obviously didn’t engineer the motor for boost from the start. Even though Hino made the EH 700 TI I suspect the fuel mapping, timing and other important aspects were designed specifically for the boost, which is different than just bolting a turbo to a NA engine. I don’t know how much difference it makes but my Hino manual indicates the CR for the EH700 is 17.9:1 vs the TI version which is 17:1.
            Torski has what he has so that’s what he has to go with. Glad to see he has the knowledge to measure the EGT and be mindful not to over temp. Personally, except for an emergency, I wouldn’t subject those boosted engines to excessive load for longevity sake.
            Jim Gandee
            1989 3888
            Hino 175's
            Fire Escape
            [email protected]

            Comment


            • Centerline2
              Centerline2 commented
              Editing a comment
              adding a turbo by itself it going to do very little for the engine, as it will need more fuel to drive the turbo to create proper boost pressure... this is the reason turbo motors have larger injector nozzles than non turbo engines (the difference in timing is not usually a factor)... a turbo by itself can actually hinder an engines performance if not fitted to the exact specs of the engine fuel delivery system, as the wrong size turbo can cause an exhaust restriction and not deliver enough boost to overcome the loss of power caused by the restriction, UNLESS more fuel is added... but there is no fear of getting too much boost if the fuel delivery is not increased. the exhaust restriction CAN cause damage to the engine/pistons/rings if the restriction is too severe for the rpm its being driven at for any length of time....

              it IS also true that the later electronic engines can self-adjust the timing to optimize the performance of the engine as necessary within its operating range, but by adding a turbo to a NON turbo engine of this type (without modifications being made elsewhere), its going to upset the proper readings being seen by the computer and it will throw codes and not perform well at all...

            #9
            A bit of a back story about me, I own a custom shop that regularly works on Toyota Diesel engines, and they're essentially the same as the Hino EH700's just in a smaller package (4.0 liter). These engines can handle about 7psi of boost, but will fail prematurely because they lack piston skirt oilers. From reading the EH700 repair manual I do not see these oilers listed in the lubrication system, but that doesn't mean they aren't there. Does anyone know?

            At cruising speed (2000 rpm 10 knots) my boost is about 7psi which I agree is more appropriate for this engine. I do wonder if anyone is more familiar with this particular engine and appropriate level of boost. Lots of opinions here which I respect, it scares me to run that much boost on a 30 year old engine ;-)

            Comment


              #10
              How about contacting Earl, the Hino guru? Turbo kits for your engine have been around for years and Earl is bound to have worked on an engine with one. He can also answer the question piston skirt oilers.

              I don't have Earl's number but someone here will pipe up and provide it.
              1999 3788, Cummins 270 "Freedom"
              2013 Boston Whaler 130 SS
              Anacortes, WA

              Comment


                #11
                Torski- My own opinion is that 18psi is very high for a new engine, let alone an older engine. But the best advice is to cal Earl. You can reach him at 425-691-8516 or email him at [email protected].
                Good luck on your project- Mike
                Gibraltar, Mi.
                1986- 3870- Hino 175's - Just purchased May ,2017
                34' Tollycraft- sold
                88 26' Shamrock/ Diesel
                14' Zodiac Bay Runner

                Comment


                  #12
                  after following this thread, which is titled "how much boost is too much?" which has been adequately answered, I think there is another question that needs to be answered here, so I am adding to what I have written previously to be a bit more specific.... while 8-10psi of boost is enough for the engine to see a good benefit from having a turbo, and with most smaller engines, having 8-10psi boost will give all the power output that the engine can dependably develop and handle.....

                  there are a couple factors that play into the safety of a turbo that is able to create the high boost pressures, (no one seems to monitor the back pressure on the exhaust side, which is the biggest danger) but the "engine" itself can handle the higher pressures as long as the PCV system is working as it should, AND as long as the engine doesnt develop so much power it becomes in danger of its own destruction, OR, when the turbo spins so high developing the high boost, that the turbo comes apart and takes the engine with it as collateral damage..

                  PCV system...higher boost pressure will cause more gasses to enter the crankcase , but as long as the crankcase ventilation is open, the gasses can escape and not build to a pressure that would blow seals or cause oil leaks.

                  larger engines have been creating 25-35psi+ turbo boost pressures since the 1960's, and the small performance turbo engines are developing at least that much these days.... so the question may be better asked, how much power, for how long, can the small stock engine develop before it damages itself?.... I dont have an answer for this question.


                  NU LIBERTE'
                  Salem, OR

                  1989 Bayliner 2556 Convertible
                  5.7 OMC Cobra - 15.5x11 prop
                  N2K equipped throughout..
                  2014 Ram 3500 crew cab, 6.7 Cummins
                  2007 M-3705 SLC weekend warrior, 5th wheel
                  '04 Polaris Sportsman 700 -- '05 Polaris Sportsman 500 HO
                  Heavy Equipment Repair and Specialty Welding

                  Comment


                    #13
                    Isn’t there something like a waste gate that acts like a relief valve?
                    P/C Pete
                    Edmonds Yacht Club (Commodore 1993)
                    1988 3818 "GLAUBEN”
                    Hino EH700 175 Onan MDKD Genset
                    1980 Encounter Sunbridge "Misty Blue" (Sold)
                    MMSI 367770440
                    1972 Chevrolet Nova Frame off Resto-mod in the garage
                    Boating on the Salish Sea since 1948

                    Comment


                    • Centerline2
                      Centerline2 commented
                      Editing a comment
                      on some turbos, but not all of them..... for about every turbo made that doesnt have a wastegate, there is an equivalent available that is equipped with the wastegate... more money....
                      a wastegate is just a spring loaded flapper door.... it is equipped with a calibrated spring that when the exhaust pressure gets too high, it pushes open as much as needed to dump the excess exhaust gasses so the turbo doesnt spin to high and cause problems.....

                    #14
                    I thought the purpose of the waste gate was to release pressure for momentary off throttle events like shifting gears in a car. The turbo can't unspool quick enough to the pressure has to go somewhere. Not to prevent excess boost. We don't need them in boats since we don't shift.
                    Tony Bacon,
                    Washougal, WA
                    Caspian
                    1997 3788 Twin Cummins 250hp

                    Comment


                    • Centerline2
                      Centerline2 commented
                      Editing a comment
                      The wastegate has many good reasons, but the boost pressure will be suddenly lost as soon as there is a reduction in fuel delivery... it takes fuel and rpm to make boost pressure... remove either one and the boost will go with it..
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