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Hot Rods compression guess on chevy 396

Discussion in 'The Hokey Ass Message Board' started by Ghost28, Feb 23, 2021.

  1. Ghost28
    Joined: Nov 23, 2008
    Posts: 3,200

    Ghost28
    Member

    I just picked up a 396 375 horsepower short block with a set of 66 to 67 427 close chamber heads. what is a good guess on the compression , and will it have to run on the street with aviation fuel??
     
    Last edited: Feb 23, 2021
  2. BigDogSS
    Joined: Jan 8, 2009
    Posts: 979

    BigDogSS
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    from SoCal

    The L78 396 375HP engine had a factory compression ratio rating of 11:1. Are your heads oval or rectangular ports?
     
  3. Ghost28
    Joined: Nov 23, 2008
    Posts: 3,200

    Ghost28
    Member

    oval, and would a set of 454 open chamber heads work on this engine to keep the compression down
     
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  4. 1971BB427
    Joined: Mar 6, 2010
    Posts: 8,719

    1971BB427
    Member
    from Oregon

    What head numbers on your heads?
     
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  5. Ghost28
    Joined: Nov 23, 2008
    Posts: 3,200

    Ghost28
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    both heads are stamped 3904390,, GM4T,, one is marked h16 67 and the other is marked g26
     
  6. 1971BB427
    Joined: Mar 6, 2010
    Posts: 8,719

    1971BB427
    Member
    from Oregon

    Those are the closed chamber 98cc heads. Valves should be 2.19"/1.72" and they will give you fairly high compression with almost any piston of the type used in 360-375HP 396 engines.
    Yes the 454 open chamber heads will work, and I think would be perfect too reduce compression to a workable level with today's pump gas. And if you find some 454 heads with the larger 1.88 valves they will flow better than the 1.72 valves will. This larger valve can be used if your block is notched at the top like some 396 engines were.
     
  7. Ghost28
    Joined: Nov 23, 2008
    Posts: 3,200

    Ghost28
    Member

    thanks for the info. the block is notched, and I think I have a set of the big valve 454 heads. I will look up there numbers
     
  8. Ghost28
    Joined: Nov 23, 2008
    Posts: 3,200

    Ghost28
    Member

    I have a couple of big block head sets, one is 3933148 and the other is 353049 in your opinions which would be a better set to use.
    Any opinion on a good roller cam and lifers for a bit of lump to the idle
     
  9. 1971BB427
    Joined: Mar 6, 2010
    Posts: 8,719

    1971BB427
    Member
    from Oregon

    The 049 heads are I believe 120-122 cc? Which with 375HP domed pistons should bring you to around 9.5-10.0 compression. Those 148 heads have smaller 112 cc chambers and will still be fairly high compression. Likely in the 10.5 range. I'd use the 049 heads myself.
    The 781 and 049 heads are considered to be some of the best 454 heads.
     
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  10. Ghost28
    Joined: Nov 23, 2008
    Posts: 3,200

    Ghost28
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    Thanks again. I am gonna to some research on what roller cam, lifters and rockers I want.
    I am putting all this in a 56 chevy 150 2 door sedan, with a tremec tko 600 5 speed with 370 posi gears
     
    Last edited: Feb 23, 2021
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  11. 1971BB427
    Joined: Mar 6, 2010
    Posts: 8,719

    1971BB427
    Member
    from Oregon

    What intake, and carbs will be on top? And 4 speed, or automatic?
    I've had really great results with Howards Camshafts recently, and running my first roller camshaft in a 350 SBC in my '39 Chev coupe. I personally prefer steel roller tip rockers, and don't usually go full roller rockers. The roller tip rockers fit under most valve covers. I just like the simplicity of using roller tip rockers. I've used Crower steel roller tip rockers on my last 3 engines.
    Howards has a 243/235 duration @ .50" with .600" lift, and a 109 LSA that's a great street/strip cam. it's a 2000-5900 rpm rating, and would work well. They also have this slightly smaller cam that is a great choice for a somewhat smaller displacement engine like yours, and might be an even better choice for a mostly street car.
    225/231 duration, with .567/.578 " lift, and 110 duration. A little less top end rpm, but better low end. #CL120245-10
     
  12. Ghost28
    Joined: Nov 23, 2008
    Posts: 3,200

    Ghost28
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    Good info, I don't have a intake manifold or carb picked out yet, but I do have a holley, ooops I believe 780 double pumper in my inventory. I will be a tko 600 tremec manual 5 speed
     
    Last edited: Feb 23, 2021
  13. Truckedup
    Joined: Jul 25, 2006
    Posts: 4,661

    Truckedup
    Member

    In my opinion for a street car is best to build for pump gas and avoid the pain in the ass of of special fuel needs...
     
  14. squirrel
    Joined: Sep 23, 2004
    Posts: 55,942

    squirrel
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    can you post pictures of the pistons? or measure the dome height?
     
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  15. sdluck
    Joined: Sep 19, 2006
    Posts: 3,186

    sdluck
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    To my knowledge there is no 780 double Pumper 3310 was the early 780 later changed to a 750 and the 750 would be a

    Sent from my SM-J737T using The H.A.M.B. mobile app
     
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  16. 1971BB427
    Joined: Mar 6, 2010
    Posts: 8,719

    1971BB427
    Member
    from Oregon

    Yep, a 780 was a vacuum secondary carb. If it's a double pumper it's not a 780, unless somebody totally reworked one custom into a double pumper.
     
  17. Deuces
    Joined: Nov 3, 2009
    Posts: 23,757

    Deuces

    If it a List# of 4779 on the choke horn, it's a 780 double pumper....;)
     
  18. 1971BB427
    Joined: Mar 6, 2010
    Posts: 8,719

    1971BB427
    Member
    from Oregon

    But I thought that made it a 779 double pumper???
     
  19. Fordors
    Joined: Sep 22, 2016
    Posts: 5,373

    Fordors
    Member

    The 4779 is a 750 CFM double pumper.
    Holley center bowl DP’s were made in 50 CFM increments.
     
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  20. 1971BB427
    Joined: Mar 6, 2010
    Posts: 8,719

    1971BB427
    Member
    from Oregon

    You do realize we're joking, right?
     
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  21. Ghost28
    Joined: Nov 23, 2008
    Posts: 3,200

    Ghost28
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    IMG_20210223_151048.jpg IMG_20210223_151101.jpg
     
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  22. 1971BB427
    Joined: Mar 6, 2010
    Posts: 8,719

    1971BB427
    Member
    from Oregon

    Hard for me say without seeing the whole piston, but those don't appear to be factory pistons? Look similar to what Sealed Power sold as replacement pistons for the 396 engines. Depending on the piston, these may no longer be a 11:1 piston. If the wrist pin location adjusts the height, the Sealed Power were rated 9.5:1 with a 100cc chamber. So be sure what you truly have before changing heads, or you might end up way low!
     
  23. squirrel
    Joined: Sep 23, 2004
    Posts: 55,942

    squirrel
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    cast pistons?

    375 hp were forged, weren't they?

    anyways, can you measure the dome height?
     
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  24. Deuces
    Joined: Nov 3, 2009
    Posts: 23,757

    Deuces

    The original 4779 dp came with the down leg venturi boosters... And the smaller diameter throttle shafts.... Which made it a 780....
    L@@k it up.... ;)
     
  25. 1971BB427
    Joined: Mar 6, 2010
    Posts: 8,719

    1971BB427
    Member
    from Oregon

    Yes, but I believe these are replacement pistons, so non factory anyway. Even some cast were milled tops, so tough to tell. But Sealed Power used this shape, and the as cast finish, which leads me to think they are cast Sealed Power, and might be lower compression than original 375 HP.

    He needs to check the bore size to see how it compares to stock also.
     
  26. Truckedup
    Joined: Jul 25, 2006
    Posts: 4,661

    Truckedup
    Member

    those pistons may be showing a bit of detonation erosion?
     
  27. squirrel
    Joined: Sep 23, 2004
    Posts: 55,942

    squirrel
    Member

    these work fine with closed chamber heads...but they're not 375 hp pistons. Forged replacement 325 hp.

    396 piston.jpg dome height.jpg

    stamp.jpg
     
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  28. 1971BB427
    Joined: Mar 6, 2010
    Posts: 8,719

    1971BB427
    Member
    from Oregon

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  29. squirrel
    Joined: Sep 23, 2004
    Posts: 55,942

    squirrel
    Member

    or maybe they're the 11:1 pistons? they also sell some with a .110 dome

    yeah, they're 60 over, like my tired old 396 block...and they were in a 30 over 402, before it spun a main and a couple rod bearings.
     
  30. Ghost28
    Joined: Nov 23, 2008
    Posts: 3,200

    Ghost28
    Member

    Laying a flat edge across the dome it measures pretty close if not exact to 3/16th of an inch. I am gonna have it taken apart and checked out for new bearings and rings, If they are the 325 horse sealed power, I am thinking on using the closed chamber heads I got with it, if I can keep the compression usable for the street and possible some strip. Should I be concerned with detonation corrosion lookin at them? The pistons on this engine don't show any over size, and the bore with out getting too technical is 4 inches plus a tad over 1/16th inch
     
    Last edited: Feb 23, 2021

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