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Upgrading the fuel system question

Old 08-24-2022, 01:39 PM
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Default Upgrading the fuel system question

Hey guys, so Im new to the world of building a more high performance boat, here is a little history I have a 1996 Baja 252 with a (not original to the boat 502 Mag MPI) and I am looking to do some upgrades. The boat is in great shape but the engine is just down on power and it has been consuming some oil so I figure it is time to freshen it up over the winter. The boat was born with a 454 MPI in it, not sure if t was a Magnum or not but it still has what I believe to be original MEFI2 computer now adapted to the 502 Mag. After speaking to someone on this topic I could be running out of fuel on the top end causing it to just stop pulling after 4K RPM and nose over.

Regardless of the current situation this is my plan:
  • bore the stock 502 out to a 509Cid
  • Rods/pistons for a 10.0:1 comp
  • aluminum heads ( open cooling put fresh water only)
  • Not sure of cam profile yet but would like to get slightly aggressive
  • 525 EFI intake
  • Eddie marine thunder manifolds with dry tip risers
  • Holley HP stand alone EFI system
  • Sieman Deka injector

To get back on point with this topic, what type fuel pumps are the more high performance boats running? I am very up to date with the fuel injection world in cars but I do not know if it also adapts to the world of boats.

Option A
  • Good quality inline fuel pump (400LPH)
  • Good quality fuel pressure reg with 1 to1 boost reference incase I ever all a blower
  • Feed the pump with a -10 AN line from the tank and to a Y-block then from the Y-block -8 to each fuel rail.
  • Out of each rail with a -8 to the regulator
  • -8 line from the reg back to the tank for a good return system

Option B
  • Good quality inline fuel pump (400LPH)
  • Good quality fuel pressure reg with 1 to1 boost reference incase I ever all a blower
  • -10 from the tank to pump
  • -8 from pump to first rail
  • -6 to loop to 2nd rail
  • -6 out of the 2nd rail and to the reg
  • -6 back to the tank

Am I trending in the right direction here? My goal is 600 Crank HP
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Old 09-06-2022, 12:38 AM
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Before you mess with your stuff, it would definitely be helpful to know if your fuel flow rate is part of your problem.
Your fuel rails have Schrader valves for pressure checking. Get a mechanical fuel pressure gauge and have somebody watch the gauge when you do your speed run. If your pressure is stable up to wide open throttle and stays there, then you don't have a fuel flow/ line size problem. If it starts out stable but starts dropping as your motor falls on its face, then you DO.
But your problem could be a dirty/restrictive filter (either the one in the cool fuel unit or any other that might have been added), a cracked or leaking hose between the tank pickup and the fuel pump (cool fuel) unit allowing air into the lines, causing pressure loss.
A sticking anti siphon fitting at your tank (spring-loaded valve at the tank that keeps fuel from pouring into the bilge if you cut the supply hose).

Last edited by mcollinstn; 09-06-2022 at 12:58 AM.
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Old 09-06-2022, 12:24 PM
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Originally Posted by Joeyboost View Post
Hey guys, so Im new to the world of building a more high performance boat, here is a little history I have a 1996 Baja 252 with a (not original to the boat 502 Mag MPI) and I am looking to do some upgrades. The boat is in great shape but the engine is just down on power and it has been consuming some oil so I figure it is time to freshen it up over the winter. The boat was born with a 454 MPI in it, not sure if t was a Magnum or not but it still has what I believe to be original MEFI2 computer now adapted to the 502 Mag. After speaking to someone on this topic I could be running out of fuel on the top end causing it to just stop pulling after 4K RPM and nose over.

Regardless of the current situation this is my plan:
  • bore the stock 502 out to a 509Cid
  • Rods/pistons for a 10.0:1 comp
  • aluminum heads ( open cooling put fresh water only)
  • Not sure of cam profile yet but would like to get slightly aggressive
  • 525 EFI intake
  • Eddie marine thunder manifolds with dry tip risers
  • Holley HP stand alone EFI system
  • Sieman Deka injector

To get back on point with this topic, what type fuel pumps are the more high performance boats running? I am very up to date with the fuel injection world in cars but I do not know if it also adapts to the world of boats.

Option A
  • Good quality inline fuel pump (400LPH)
  • Good quality fuel pressure reg with 1 to1 boost reference incase I ever all a blower
  • Feed the pump with a -10 AN line from the tank and to a Y-block then from the Y-block -8 to each fuel rail.
  • Out of each rail with a -8 to the regulator
  • -8 line from the reg back to the tank for a good return system

Option B
  • Good quality inline fuel pump (400LPH)
  • Good quality fuel pressure reg with 1 to1 boost reference incase I ever all a blower
  • -10 from the tank to pump
  • -8 from pump to first rail
  • -6 to loop to 2nd rail
  • -6 out of the 2nd rail and to the reg
  • -6 back to the tank

Am I trending in the right direction here? My goal is 600 Crank HP
Your fuel pump at 190 lph IF its working like new isnt keeping your stock 502 from going over 4000, now, if your regulator is bad or pump is weak, yes.
Now, as far as your planned upgrades, either will easily support 600 hp as I have dynoed stuff with stock 190 pump and single line on a mpi 502 and gotten to 580, 600 (corrected depending on BSFC) as limit with slight drop in pressure so youll be plenty big.
I DO have some bad news for you, how ever, a 525 intake does NOT use normal injectors and will take some fab and custom machining to put any 'normal" injector in, including the sieman dekas as it uses the same honda keihan style bastard injectors like a gen V 454/502 with ,mefi1. They are shorter then the actual honda keihan injectors also so it takes work to even put japanese import injectors in (last time I did that I machined custom spacers) so IF you have already purchased a 525 intake you will need to address this in your build. Now, a 500 intake and rail DOES use normal injectors, you could use a 500 lower and rail with 525 plenum and tb, FA. Smitty
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Old 09-19-2022, 01:14 PM
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Well thank you for shedding the light on the injector information, I paid for a 525 intake but it hasn't been shipped yet. I was going to buy a 500 intake but I was talked out of it due to it not handling the CFM necessary to support 600hp. Now you have me thinking here!!!!!! This bit of information does suck, the person I am purchasing the 525 from said it runs a typical 63Lb siemen Deka injector, I had never heard of a 63lb only a 60lb but again I'm talking car stuff here that I normally play with.

Do I back out of this purchase?
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Old 09-22-2022, 06:02 AM
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Originally Posted by Joeyboost View Post
Well thank you for shedding the light on the injector information, I paid for a 525 intake but it hasn't been shipped yet. I was going to buy a 500 intake but I was talked out of it due to it not handling the CFM necessary to support 600hp. Now you have me thinking here!!!!!! This bit of information does suck, the person I am purchasing the 525 from said it runs a typical 63Lb siemen Deka injector, I had never heard of a 63lb only a 60lb but again I'm talking car stuff here that I normally play with.

Do I back out of this purchase?
sieman dekas are often referred to as a 60 or 63 lbs, they are one and the same. Since a 525 intake and rails uses a honda kiehan style injector (at least on ones Ive seen) and rail has 11.5 mm holes for top of injector vs 14.5 mm like most "normal" stuff (AND 500 EFIS) , has somone replaced the rails with 500 efi rails and modified the lower for the simean dekas? Yes, the 525 TB and Flame arrestor are much better than the stock 500, a 500 lower offers no performance advantage or disadvantage over a 525, the upper on 500 can have front modded and bigger tb put on.
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Old 09-22-2022, 06:29 AM
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Thank you for this info, I'm sure it will save me a lot of head scratching. I have seen these intakes listed new on the internet, are they cheap knock offs? I would buy a new 525 upper and a 500 lower?
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