346 vs 516 heads on 72 400

Engine, Transmission & Driveline

  1. TroyCo

    TroyCo Member

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    So here's the drill: I need to rebuild the 346 heads on the 400 in my Polara, and I have access to a set of 516 heads I can put on the engine as well. I have also added a Performer RPM intake, Holley 600 cfm carb, and a comp 262 lift cam recently. I understand the 516s are closed chamber and I would get a slight compression bump with those heads, but are they worth adding to this engine, or should I stay with the originals?
     
  2. Davea Lux

    Davea Lux Old Man with a Hat FCBO Gold Member

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    The 516 heads do give a boost in compression and have a better quench factor.. Downside is that these heads were designed for an era where leaded fuels were the norm. For any type of a performance build, the non-hardened seats in the 516's will erode and the valves will start to sink. You would want to install hardened seats and bronze valve guides as part of any rebuild of those heads. The 346 heads come with hardened seats, so you are ahead of the game in that respect. You might want to consider doing a bowl job on the 346 heads and investing in a set of headers. The downfall of the 400 engine is a low compression ratio to start with, so to get decent performance, you need to get the compression ratio up to about 9.5-1, that will mean a set of aftermarket pistons as part of an engine rebuild. The 400 had a factory compression ratio of 8.2-1 but a lot of those engines ran more in the neighborhood of 7.8-1 to 8.0-1 because of variations in machining. Running modern composite head gaskets is only going to get you to about 8.2-1 to 8.5-1 with the 516 head and that is still well below an optimum performance number.

    Dave
     
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  3. TroyCo

    TroyCo Member

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    Thanks for the reply. A little more background, I'm just looking to wake it up a little bit, around 300 or so hp. I plan to do a rebuild soon, next year or two perhaps. The 516s I'm looking at have been redone, so those mods are more than likely done to it. The 346s need to be gone through, couple of jacked threads and a broken manifold stud. I'm thinking the cost to re-do those heads will end up close to the cost of the 516s. Whichever head I choose will get the same gasket matching accordingly.
     
  4. detmatt

    detmatt Old Man with a Hat FCBO Gold Member

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    The 516’s have bigger exhaust valves installed when they were redone?
    I’d use the 516’s personally and put 1.74 exhaust valves in them.
     
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  5. TroyCo

    TroyCo Member

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    Just spoke to the owner of those heads, the valves are stock size, but they had a 3 angle job done, bronze guides and hardened seats installed. For what I'm looking to get out of this engine, it isn't a factor for me to install larger valves at this time.
     
  6. CBODY67

    CBODY67 Old Man with a Hat

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    DO consider getting the Mopar Performance porting templates and using them on either set of heads. The smaller 1.60 valves can be somewhat compensated for with a bit longer exhaust duration on the cam.

    Just some thoughts,
    CBODY67
     
  7. furious70

    furious70 Senior Member

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    The pros and cons of each for that engine are a wash probably. Go with the most cost effective one
     
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  8. Mike66Chryslers

    Mike66Chryslers Senior Member

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    Depends what performance level the OP is going for. Ported heads have better top-end power (high RPM) but you sacrifice power at the bottom-end of the power band. Larger ports means the air is travelling slower so you lose the scavenging effect. 516 heads don't have a good port shape vs the later heads to begin with, so probably not worth the effort of porting because you don't gain that much from what I've read.
     
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  9. CBODY67

    CBODY67 Old Man with a Hat

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    Granted, "size" is not everything in head ports, but "shape" can be more important. BTAIM

    I suspect that in the heads orginally designed in the 1950s, that much was done "by sight" and how they looked and were suspected to perform. Getting the right balance of low-lift flow vs high-lift flow was not a real consideration, I suspect, as it came to be in more current times. It was more about total flow and .500" lift, it seemed. Huge valves were supposed to be better. Just "fill or empty the chamber" seemed to be the main orientation. But some began to notice less lower-rpm power with huge ports, which then generated the need for lower rear axle gears to reap the benefits of those huge ports.

    The dynamics of the incoming air into the cyl was not a real consideration, just get it in. Similar on the exhaust side, I suspect. MANY theories of valve sizing had not been fully researched or computer-modeled. And THEN how that all related to rod length-to-stroke ratios, too! Which also relates to "piston dwell time at TDC".

    Generally, even for a hp motor, intake gasket port matching, with casting clean-up in the ports, and possibly some work in the bowl area is good enough, by observation. Past that, LOTS of theories out there of what might work best, by observation.

    Enjoy!
    CBODY67
     
  10. furious70

    furious70 Senior Member

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    Go to the moparts tech archives to read what Dwayne tested 10-15yrs ago with these heads. He goes by fast68plymouth there.
     
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  11. MONC440

    MONC440 Member

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    If you truly plan on a full rebuild in a few years use the 516 heads and a shim head gasket that will give you a good increase in compression. I have done this many times on mid to late 440 motorhome engines and had great results.

    I have also had 346 heads milled down to a closed chamber and had good results also. Just don't use a composite head gasket because you will loose any benefit you made by going close chamber. The shim is .020 and composites are 0.039 to 0.042. The only drawback to the shim is you need to retorque the head bolts.
     
  12. TroyCo

    TroyCo Member

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    How long after installation would that need to be done? Also, can you recommend a decent set?
     
  13. furious70

    furious70 Senior Member

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    I used shims on my turbo engine for 8 years and never retorqued.
    Which ones? In today's shortage world, any ones you can get!
     
  14. Gerald Morris

    Gerald Morris Senior Member

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    This thread WOULD HAVE BEEN very interesting to me, as I have 516, 915, 906 and 452 heads and a 400 I wished to rebuild, BUT A SOCIOPATH BITCH DESTROYED MATHILDA MONDAY MORNING, so I'm a bit at a loss for enthusiasm for this topic, interesting as it otherwise would be to me. I'm facing serious repairs, IFF I can make her damned insurance pay!
     
  15. MONC440

    MONC440 Member

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    CHRYSLER 7.2L/440 Mopar big block RB Head Gaskets - Shim steel Gasket Material Parts & Accessories | Summit Racing

    Good ol’ Mr Gasket from summit.

    BTW I always paint mine with spray on copper gasket maker.

    Permatex 80697 Permatex Copper Spray-A-Gasket Sealer | Summit Racing

    Also head bolt re torque is recommended but not required after 500 miles. The last 440 I built was 10.5:1 compression and now has close to 10,000 miles on it and the guy has never retorqued them.
     
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  16. furious70

    furious70 Senior Member

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    Also, the MP templates, correctly followed will not increase the port size too much for anyone. 440's should have all had max wedge sized heads from the factory :thumbsup: