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Magnum head question

Discussion in 'Performance' started by PCRMike, May 28, 2017.

  1. PCRMike

    PCRMike Well-Known Member

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    i am building (retrofitting) a set of Magnum heads onto a 1967 318. I am doing all the custom work on them, and have a question about the rockers. I am maxing out at .500 lift and no more than 6500 rpm (planned shift point is 6000-6200). Are the factory rockers up to that? I never had a question about the shaft mounted ones. A conversion kit for the serious racers is about 400 bucks plus tapping for bigger studs. I am wondering if it is needed for my somewhat mild setup. Anyone have experience with these in that regard?
     
  2. RalphP

    Level 2 Supporter

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    While figuring, remember that the stock ratio for LA heads is 1.5, for Magnum heads 1.6 .

    RwP
     
  3. PCRMike

    PCRMike Well-Known Member

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    Yup. Figured in. The cam will be a .470 lift which equates to a .500 in a Magnum, while retaining the shorter duration for low end. Fast bleed lifters will ensure the torque stays put. I am just leery of an unfamiliar (to me) rocker setup.
     
  4. GLHS60

    GLHS60 Well-Known Member

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    Engine Masters built several 318's in the 400-425 H.P. range and made some rocker comparisons.

    I'll try and find the issue with Magnum comments and reply back.

    As noted the factory shaft setup is very stable.

    Thanks
    Randy
     
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  5. PCRMike

    PCRMike Well-Known Member

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    Thank you sir.
     
  6. PCRMike

    PCRMike Well-Known Member

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    I just found it. Amazing how similar their build is to mine. I am going to run a little less cam, but with deshrouded valves and bowl work. Same crank, pistons, and head choice. I won't be decking the block, as I am already booking on a 9.75 comp ratio. My cranking compression will be between 190-200 psi, so still more than enough to lay respectable torque numbers while revving out. They mentioned the different flange on the early 273 crank. I have a couple flexplates from the Barracudas my pair of cranks came from. Good thing I kept em.
     
  7. dana44

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    From what I recall about the swap, the pushrod holes through the Magnum heads need to be opened up to make up for the offset of the LA lifter bore location to the Magnum rocker arm positions. At the same time, I haven't heard of any problems or issues with the angle of the pushrods or any weakness associated with the rocker arms themselves, so I don't think you will have a problem. I would figure that by now if there was a problem or weakness we would have heard about it by now.
     
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  8. GLHS60

    GLHS60 Well-Known Member

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    Engine Masters Steve Dulcich has a real soft spot for 318/360's and has done a few.

    Which issue are you looking at, this one (Cheap and Mean) looks similar to yours??

    The "Cheap and Mean" 318 winter 2008 runs replacement Magnum heads on an early 318, XE 275 HL cam.

    Stock Magnum rockers performed well to the tested 6400 RPM.

    An earlier "Cheap as Dirt" 318 build ran stock shaft rockers to 6700 rpm and 1.6 roller's were done at 6100.

    Ironically, the forged 273 crank eventually broke after much usage, a fluke I'm sure but no cast cranks failed.

    Personally I would definitely deck the block, tight squish is a great detonation suppressor.

    I've built a few RB Engines and machined the heads closed chamber, much more detonation tolerant.

    The above 318 used 360 heads milled to 60 cc closed chamber, and was equal to the Magnum in H.P.

    Both used the same cams and made about 400 H.P., the Magnum increased to 425 H.P. after porting.

    I believe the Edelbrock heads are set up for SBC rockers.

    Wishing you great success!!!!

    Thanks
    Randy
     
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  9. PCRMike

    PCRMike Well-Known Member

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    Dirt cheap is what I was looking at. Also will be running the more expensive beehive springs with a lower seat pressure. Considerably easier on the studs than the dual coil.
     
  10. PCRMike

    PCRMike Well-Known Member

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    So..... the steel crank did not pan out. Pilot hole too small, so I am looking back at the cast crank. Wondering if it would be safe with 400 horses or so and spinning no faster than 5500 or so. I also have a Magnum crank. Which would be stronger, the LA cast or Magnum?
     
  11. dana44

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    Pilot hole can be redrilled at a machine shop easy enough, bushing made to fit ID/OD as needed, make it out of oilite bronze for durability.
    Otherwise, 400hp (500 is usually the limit safely) is no problem, up to 5500, actually 6000rpm is safe for a nodular cast crank
     
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  12. PCRMike

    PCRMike Well-Known Member

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    I can't find a shop to cut it out for me. I think I am within range to be able to use the cast, but would PREFER the steel since I have it.
     
  13. dana44

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    Is there a machine shop that can throw it in a lathe and if you have the proper bushing, cut the hole and fit the bushing? Doesn't have to be an auto shop, an actual machine shop can do it, it's a ten minute job, for a slow machinist.
     
  14. PCRMike

    PCRMike Well-Known Member

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    Nobody around would touch it. Just needed a slow feed End mill at 1.66". I ended up going with a freshly machines cast crank. Should handle the rpm loads fine at 6K
     
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