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  1. #1
    Registered articfriends's Avatar
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    Modifying a stock 454/502 mpi intake

    This winter i have been freshening/upgrading my 540/procharger motor and have debated pros and cons of modifying my stock 502 efi intake, plenum and throttlebody that i am blowing thru w/blower or just buying a good aftermarket intake and billet throttle body. When i built 540 2 years ago it dynoed kinda weak because of various reasons(too small of injectors,merlin heads w/only 65 % intake to exhaust ratio,8 rib drive that slipped on dyno slightly etc). The 540 only made 629hp and 697 ft lbs of torque. The boat went SLOWER (76 on gps) than it did w/502 w/stock cam and merlin heads. I was able to get boat to run 85 on gps by raising base timing,turning up fuel pressure,changed to 12 rib drive etc. The general consensous on this board was the 540 was making between 725-750hp at the edge of melt down. This thread is going to detail how i modified the stock intake and fuel rails without spending a ton of money just alot of sweat equity. I chose to modify the stock intake because i am over budget on my cylinder heads,cam,new pistons,thermal coatings etc. If the motor doesnt make at least 825 hp on the dyno the intake will go in the trash and i will just buy a good intake,billet throttle body and new plumbing to get boost to the manifold. Hopefully this thread will help a few of you guys(like sutphen 30) who are trying to do the same thing.
    To start with i took the stock throttle body to our local head porter/flow guy and had it flowed with and without the screen procharger has you put in it when you convert your system. The throttle body w/screen only flowed 520 cfm at 28" water column,w/out screen it flowed just over 600. Although those numbers seem really low it is deceiving because throttle bodys are not rated by flowing them like we did AND a blown motor will push air thru it at a much higher rate.

  2. #2
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    Re: Modifying a stock 454/502 mpi intake

    The next thing i did was have the flow guy calculate the flow of the stock 502 intake. Based on the square inches of the smallest cross sectional area of a runner it is capable of flowing over 400 cfm un-modifed (my head intake ports only flow 360-370 cfm-AFR 315 cn'cs).Armed w/this info i started modifying every thing. There is no aftermarket company that sells a high flow throttle body for the 454/502 mpi intake and no one capable that i could find to build or modify one either so i chose to run it as is. Intake manifold-being the stock intake ports already ouflowed the head intake ports i decided having it professionally ported by a company like azsm would be a waste of money so i port matched it myself. I milled the intake runners down so that they matched the floor of the plenum(in stock form they jut above the floor more than 3/4's of a inch). While i was milling it i also cut 1 inch out of the runner divders because every one else always does so it must be good . Next i blended the edges of the runner tops and the center dividers so they wouldn't disrupt flow and would have a good transition when the air entered runners.
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  3. #3
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    Re: Modifying a stock 454/502 mpi intake

    The second pic is of the transition fron the intake plenum to runners.Next i port matched the intake runners as far in as i could reach(about 2 1/2 -3 inches in. I have never ported a intake or head before so its not beautiful but hopefully it works!
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  4. #4
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    Re: Modifying a stock 454/502 mpi intake

    The next thing i did was address the stock fuel rail as i am switching to 63 lb injectors from the stock 42 or 43 lb (whatever they are). I was told when i ordered them they would be .030 longer than the stock ones so they should be close to stock length. The stock rail has around a 7/16 id and supplys 8 injectors. Mine being a 2000 it didn't have the restricted # 7 feed hole like you always hear about. I was supplying stock rail with (2) -8 hoses since i put procharger on but i was still worried that it would never supply enough fuel.I decide to build twin rails to feed 4 injectors on each rail.
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    Last edited by articfriends; 05-23-2005 at 06:01 PM.

  5. #5
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    Re: Modifying a stock 454/502 mpi intake

    After receiving the rail i quickly realized the -10 physically would never work under the stock plenum so i started working on the -6. I cut it to the estimated length that i thought i could work with,the first problem i realized was that when i stuck the stock injectors in the manifold the rail would not even touch them before it hit the outside of the intake ports so i started milling the bottom side of the rails down to get more clearence so i could get them closer to the injectors. I also cut them off shorter so i only had to clear one set of ports,not two and figured i'd just run the fuel lines supplying them under the intake.Well,that was a waste of time because once i got the injectors i found out they were more like 3/4 inch longer than stock !
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  6. #6
    GPM
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    Re: Modifying a stock 454/502 mpi intake

    You should talk to Kennywould about all the mods he tried on his 502 EFI manifold. He removed a couple pounds of aluminum from the runners. He drilled the manifold for standard size injectors, made his own plenum with 2, 105 mm throttle bodies, never ran for s#%t. You might want to consider the Holley marine efi manifold and the Accufab 4 barrel throttle body. Good Luck.
    Last edited by GPM; 05-23-2005 at 06:37 PM.

  7. #7
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    Re: Modifying a stock 454/502 mpi intake

    So i threw those rails in the BIG pile of other parts that i have bought/modied thru out the years that never fit !Another problem with the -6 rails is they had a .500 i.d and they had to have the ends drilled to .575-.580 so they could be threaded w/3/8's npt so i could have un-restricted flow from my -8 adapters.After checking around i found that azsm sells fuel rail stock with .575 i.d so i ordered another 3 ft and started over!The 1st set of rails really looked hacked up from clearencing them too so i wasn't heart broke about starting over!I cut the rails close in length to the stock rail and borrowed a msd brand injector pocket cutting tool from Tyler Crockett. I practiced drilling some injector pockets w/the tool on the extra pieces of rail stock and the holes looked like garbage and weren't smooth so i would recomend AGAINST buying a msd injector pocket cutter,it's 159$ and the hole is .552 in dia,the stock rail hole is around .538 in dia so even if you could get tool to cut a smooth hole it really feels loose when you stick the injector in it.I took the rails to a local guy that can perform miracles on his mill and he cut the 8 holes with a mirror finish with the stock dia!
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  8. #8
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    Re: Modifying a stock 454/502 mpi intake

    The next thing i had to do was build something to hold the injectors down on the intake and plumb them. I made various brackets out of 1" wide flat plate and bent them over in a vice using a hammer. I also milled slots in them and mocked it up about 25 times. I ended up using brackets shaped like a horseshoe that i intended to put on top on the bottom.
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  9. #9
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    Re: Modifying a stock 454/502 mpi intake

    The rails when they are in place are sandwhiched together,the top bracket slides down on top. The rails had to be fed from the front because there is no room in back for the lines. Because the lines are nestled together the supply lines had to be cut to length and tightened down before putting the clamps on top. I also had to relocate the temp sender in top of the stat housing from procharger (it was directly in the way of the fuel lines). I plugged the hole and drilled and tapped a 3/8's hole in the side of the stat housing for the sensor. I had to unwrap the factory harness to get the wires to reach then re-taped it.
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  10. #10
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    Re: Modifying a stock 454/502 mpi intake

    I also used button head screws to hold the brackets down,they still touched the rails and interfeared with the rails sitting down right so i had to notch the back side of the rails with a die grinder in a couple of spots.The rails have small cooling fins in them that sorta lock together when they are in place but i ended up having 3 fins above 2 fins so rails are slightly crooked(not enough to be a problem). Because of that i had to use some thin rubber under neath the top brackets to hold down the slightly lower/off center rail (which i wrapped in foil tape).
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