Possible problems running oversize valves, 472 peanut relief pistons, MTS #15, Potter rockers.

Discussion in 'Big Block Cadillacs' started by DevilDeVille, Jun 13, 2018 at 7:05 AM.

  1. DevilDeVille

    DevilDeVille Member

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    Hi,

    Has anyone any ideas what might go wrong with such a combo? My main concern is valves interfering with pistons. Valve sizes are 2.25" and 1.88" and with 1.7:1 ratio, the valve lift should be .577"

    Thanks in advance,
    Martin
     
  2. Ted in Olympia

    Ted in Olympia Active Member

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    The peanut pistons are cut for high lift cams and big valves. Happened to me on my first build using 76cc heads and MT15 cam.

    Hit just enough to bend the valve but did not ruin them, they were able to regrind them and reused them.
     
  3. DevilDeVille

    DevilDeVille Member

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    Any guidelines or measurements on how much to cut and where?
     
  4. 8ad-f85

    8ad-f85 Well-Known Member

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    If you can turn the engine by hand with some clay on the piston, you'll see for yourself where if any interference is.
    You'll need to rig up a couple of solid lifters and maybe some very light springs with Play Doh .

    It's a pretty big diameter increase, so I'd for sure watch radial clearance too.
    If it needs very minor work, a die grinder and some care will do the trick otherwise an old valve with a lathe bit brazed on will work if need be.
     
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  5. DevilDeVille

    DevilDeVille Member

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    Main downside for me is loss of CR, when adding valve reliefs to badger peanut pistons. I was wondering if i go with my standard size valved 76cc heads, i can dodge the interference issue albeit using MTS#15 and Potter rockers? Sorry for stupid questions, but better safe than sorry i guess...
     
  6. 8ad-f85

    8ad-f85 Well-Known Member

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    Why worry about a tiny bit of compression loss from valve notches, if even needed?
    Even if you were on the low side of dynamic compression, a degree of cam advance would make that up and I'm sure you'd never feel it running.
    What would your compression be with the early heads?
    What fuel available?
     
  7. DevilDeVille

    DevilDeVille Member

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    10.5:1 as advertised, both engine block and the heads will be skimmed a bit in the machine shop. Best available fuel on the pump here is 98 RON which equals to 93 PON or 93 PUMP in U.S. In order to get two rigid lifters, i should probably let them be fabricated here - can anyone provide me with accurate measurements?
     
  8. 8ad-f85

    8ad-f85 Well-Known Member

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    10.5 as measured?
    I'd be nervous to do that with such a small cam unless the vehicle only weighs 1800lbs.
    I'd suspect 9.5 would be much safer without any penalty.
    Is that with large chamber heads?
    What does it calculate to with 76cc heads if considered?
    What is your intake closing point (in degrees) and trapped or dynamic compression ratio?

    A solid lifter could be made from an old hydraulic pulled apart or any other way that comes to mind.
    Any old chevy solid lifters in your area floating around?
    Sometimes really light springs one might obtain from a hardware store used as valve springs are sufficient.
     
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  9. DevilDeVille

    DevilDeVille Member

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    10.5 is a number factory gave for peanuts with 76cc heads for 1968 if i am not mistaken too badly. Thats the engine i am building. 472 +.040 over. Previous owner planted it in my 1978 DeVille but as it seems, at some point it has been "rebuilt" by bakers or gardeners or kindergarten teachers or shepherds so that one bank had original peanuts in it and the other one had one peanut and three brand new soap dishes. No wonder i had difficulties of obtaining smooth idle :D

    Only information about the cam is a card that came with it:
     

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  10. 8ad-f85

    8ad-f85 Well-Known Member

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    If your engine measures an actual 10.5 (and it would be imperative to really check) the following numbers are from a website used to calculate the trapped compression from when the valve actually closes.
    I used sea level for altitude.

    Static compression ratio of 10.5:1.
    Effective stroke is 3.62 inches.
    Your dynamic compression ratio is 9.47:1 .
    Your dynamic cranking pressure is 198.69 PSI.

    This is probably too high for your fuel.

    If 9.5 is truly measured...

    Static compression ratio of 9.5:1.
    Effective stroke is 3.62 inches.
    Your dynamic compression ratio is 8.58:1 .
    Your dynamic cranking pressure is 175.04 PSI.

    This is more reasonable.

     

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