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  • 5.7 carbed vs 5.7 MPI

    I have a 24 Albemarle Jackshaft CC. a year ago August I replaced my worn out 5.0 GI with the 5.7 Carbed motor. I am considering changing the motor to MPI. I am at 270 horsepower with the carb. I am turning Duoprop D4's. I burn about 16 gallons per hour at 3600 rpm's. I am cruising at 21 knots. That translates to about 1.5 mpg
    If I modify my motor to MPI, I am told my horsepower would increase to 320. Does anyone have any data on this motor in a jackshaft center console? I don't want to use numbers from an express because they sit higher on a plane than I do. The express has it's motor at the transom. Mine is amidships so I think that I am pushing more water. Any ideas would be a great help.
    US Submarine Force Veteran
    USS Corporal SS346

  • #2
    Your HP would not increase (EFI does not improve power, infact on a given engine, the peak power slightly drops). where the EFI motor would have more power, is that the combustion mix is changed (compression, timing, fueling and cam) to allow the motor to run closer to the edge of detonation than a carb

    your fuel economy would not increase (EFI does not offer noticeable improvements in fuel consumption) Spark ignited motors have a BSFC of 0.4#/HP/Hr

    you will notice easier starts and mild manors around the dock

    you can get your current motor up over 300hp fairly easily with a few component changes
    Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

    1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://forums.iboats.com/forum/owner...988-rogue-2420

    Past Boats
    1970 Wooster Hellion - Merc 9.8
    2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
    1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

    What’s behind you doesn’t matter.Enzo Ferrari

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    • #3
      Thanks for the reply. What type of component changes. I am running a Quadrajet carb and a Davis HEI distributor
      US Submarine Force Veteran
      USS Corporal SS346

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      • #4
        Originally posted by BarryTurano View Post
        Thanks for the reply. What type of component changes. I am running a Quadrajet carb and a Davis HEI distributor
        The q'jet is an outstanding carb. Make sure the air valve tension spring is tight enough so the secondary air valve is closed at cruise. This also keeps the manifold vacuum higher and keeps the power piston closed.

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        • #5
          Originally posted by BarryTurano View Post
          Thanks for the reply. What type of component changes. I am running a Quadrajet carb and a Davis HEI distributor
          cam change, 2" carb spacer, and possibly a re-tune of the carb.
          Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

          1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://forums.iboats.com/forum/owner...988-rogue-2420

          Past Boats
          1970 Wooster Hellion - Merc 9.8
          2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
          1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

          What’s behind you doesn’t matter.Enzo Ferrari

          Comment


          • #6
            Ayuh,..... I agree with Scott,.... MPI will never pay for itself,.... not in a million years,.....

            A bigger cam, 'n carb retune can get ya darn close to 350 hp,.....

            Fuel economy may improve, but probably not,......
            as Scott says, it takes X number of fuel Btus to move a boat,.... regardless the motor used,......

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            • #7


              Thinking I may have been looking at this problem from the wrong end. When I put this motor in I was still running D5's.I could not get above 4000 rpms. I dropped down to I4's. My rpm's got up to 4400 if I was going with the tide and wind at my stern (everything going in my direction short of hoisting a sail) I spoke with Ken at Prop Gods he put me in touch with Pablo at General Propeller. He suggested going down another number of pitch. He says that I should get up on plane quicker and hold plane at a lower rpm. And should bring wot to close to 4800 which would bring my cruise up to 3600 (@75% of wot) As of now 3600 rpms is 82.5% of wot. And it is possible my secondaries are slightly open. I am thinking I can probably cruise at 3400 where I am burning about 12gph. If I can get 21 knots at 3400 rpm that is the same speed I am getting at 3600 but 4gph less is a big deal. Especially on a 150 mile stream trip. I will be hauling in about 2 weeks for annual drive service. I will put the new wheels on then and report back. Stand by and thank you all.
              Barry
              US Submarine Force Veteran
              USS Corporal SS346

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              • #8
                Pablo in the guys at General Propeller know me by name. Regarding your particular issue dropping down in Pitch will help bring the engine RPM up. However cruise is a function of hull length and hull weight (power required to move the load) However you might be in the secondaries if the engine needs it.

                A carb spacer will help with low to mid-range torque which might help. Overall you're going to need a cam change to move your power or most importantly you might need to build a stroker to have the torque to turn a set of B5s. Not sure what your boat weighs, however the rogue in my sig weighed 3600# and I was spinning B6s at 4800 rpm with my 5.7 running vortec heads and a cam
                Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

                1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://forums.iboats.com/forum/owner...988-rogue-2420

                Past Boats
                1970 Wooster Hellion - Merc 9.8
                2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
                1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

                What’s behind you doesn’t matter.Enzo Ferrari

                Comment


                • #9
                  I knew of two guys who ran a speed shop in Atlanta who documented a study they did on various engine improvements they planned to offer. This was in 2001 as I remember. They took a stock carbed 5.0L engine from an 85(?) Trans Am and converted it to MPI using only the GM stock FI components. The dyno tests showed an increase in maximum power from 162 to 201 BHP. Those numbers closely aligned with the advertised power of the two available 5.0L engines in 85, 165 and 200. How do you guys explain that?

                  By the way, the engine dyno tests were done at the flywheel.
                  Last edited by DouglasW; December 22nd, 2018, 08:01 PM.
                  Mariah Shabah Z202 5.7L Mercruiser B3
                  Jensen Marine 1972 Cal 21 cabin sloop
                  Retired EE of 45 years

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                  • #10
                    It is an interesting question. I think if someone just took a standard GM marine engine and converted it from a carb to MPI, they might not realize the full benefit of EFI. That is, an EFI engine due to better fuel atomization and fuel distribution, can tolerate both higher compression and more spark advance. It is those factors that make more power. If you go back and look at some of the changes made to popular engines that were in use before the EFI change over in the 80s, you find out that in order to get the full benefits, other changes were made too and for sure they found more HP in the same engine after those changes were made. There are differences in Marine use due to the constant load, so some of the changes may not be as dramatic.
                    1988 Four Winns 200 Horizon
                    4.3 OMC Cobra

                    98 Jeep Grand Cherokee 4.0 Selectrac
                    07 Jeep Grand Cherokee Limited 5.7 Quadradrive II

                    "While air doesn't freeze....rust never sleeps"

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                    • #11
                      Originally posted by DouglasW View Post
                      I knew of two guys who ran a speed shop in Atlanta who documented a study they did on various engine improvements they planned to offer. This was in 2001 as I remember. They took a stock carbed 5.0L engine from an 85(?) Trans Am and converted it to MPI using only the GM stock FI components. The dyno tests showed an increase in maximum power from 162 to 201 BHP. Those numbers closely aligned with the advertised power of the two available 5.0L engines in 85, 165 and 200. How do you guys explain that?

                      By the way, the engine dyno tests were done at the flywheel.
                      The same base 5.0 with a proper carb and a properly designed manifold with divorced - long runners (similar to the RPM air-gap) would have put out nearly the same power (unless you swapped cams as well - see below). remember, the carb that was on the car was a smog carb with a million vacuum ports and actuators, on a badly designed cast-iron intake which had the base of the carb sitting low in the valley, where the TPI manifold used had the large divorced plenum and long-runners to increase velocity which in turn makes mid-range torque

                      The engine builds in the F-bodies from GM were different for each of the offerings the TPI was the LB9, the base 5.0 at 160hp was the LG4 and the HO carb version was the L69. While the rotating assemblies were common, some of those changes were the camshaft. EFI needed a different lobe center as well as less peak lift and a bit longer duration to work. install a knock sensor controller on the HO carb version and increase the timing a bit and you can exceed the TPI power output

                      When I seriously thought of converting one of my hot-rod builds in the early 90's, the peaky little 8000 RPM grenade I built would have lost about 10% (30hp) as the cam required to get EFI to function would have reduced output noticeably. Not to mention the conversion at the time cost almost as much as the motor cost to build.
                      Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

                      1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://forums.iboats.com/forum/owner...988-rogue-2420

                      Past Boats
                      1970 Wooster Hellion - Merc 9.8
                      2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
                      1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

                      What’s behind you doesn’t matter.Enzo Ferrari

                      Comment


                      • #12
                        The Atlanta guys did not change the cam, although they may have changed heads. The point of their test was to make a validated benchmark for further modifications. The question remains, how did they get that gain from the SAME engine.
                        Mariah Shabah Z202 5.7L Mercruiser B3
                        Jensen Marine 1972 Cal 21 cabin sloop
                        Retired EE of 45 years

                        Comment


                        • #13
                          Originally posted by DouglasW View Post
                          The Atlanta guys did not change the cam, although they may have changed heads. The point of their test was to make a validated benchmark for further modifications. The question remains, how did they get that gain from the SAME engine.
                          intake alone is a large step. the only "magic" in the TPI setup used in the 80's was the large plenum and the long runners - it basically tripled the velocity of the air entering the system at 4000 RPM and below, therefore pushing more charge into the combustion chamber. does same thing a 2" carb spacer on a marginal carb manifold does. or a properly designed medium rise carb manifold - increase plenum volume.

                          they would have had to change cam to get the EFI to function as the carb cam didnt generate enough vacuum
                          Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

                          1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://forums.iboats.com/forum/owner...988-rogue-2420

                          Past Boats
                          1970 Wooster Hellion - Merc 9.8
                          2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
                          1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

                          What’s behind you doesn’t matter.Enzo Ferrari

                          Comment


                          • #14
                            Originally posted by DouglasW View Post
                            The Atlanta guys did not change the cam, although they may have changed heads. The point of their test was to make a validated benchmark for further modifications. The question remains, how did they get that gain from the SAME engine.
                            At the same rpm? Same a/f ratio? Same timing?

                            Horsepower is a function of torque and rpm. Horsepower is work, torque is a force, rpm is a distance. THIS is the difference, not just EFI. All EFI does is spray fuel directly, rather than airflow sucking fuel out of a venturi. More rpm's, more fuel sucked in.

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                            • #15
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                              Originally posted by Scott Danforth View Post
                              Your HP would not increase (EFI does not improve power, infact on a given engine, the peak power slightly drops). where the EFI motor would have more power, is that the combustion mix is changed (compression, timing, fueling and cam) to allow the motor to run closer to the edge of detonation than a carb

                              your fuel economy would not increase (EFI does not offer noticeable improvements in fuel consumption) Spark ignited motors have a BSFC of 0.4#/HP/Hr

                              you will notice easier starts and mild manors around the dock

                              you can get your current motor up over 300hp fairly easily with a few component changes
                              Think he is talking about MPI, not efi. Mpi has massive power gains and efficiency over the Carb Scott
                              dont think this case it would be worth while though. Be as well buying a good used complete engine.

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