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76cc to 64cc compression ratio

Discussion in 'The Garage' started by b454rat, Mar 22, 2006.

  1. b454rat

    b454rat 1/2 ton status Premium Member

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    I used to know how to figure it out, but it's been a long time since I've messed with engines. I have a stock 400 with the awesome 76cc heads, guessing comp ratio is around 8:1. If I put the Vortecs with 64cc's on it, what would the compression be on it?? Piston kits used to tell you what it would be with different CC heads, but can't find that info anymore.....
     
  2. mountainexplorer

    mountainexplorer 1/2 ton status

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    The SB400 that I had was about 10.5:1 with flattop pistons and 041 casting 64cc chamber heads. If you still have the factory style dished pistons my guess may be like 9.5:1. But I'm no expert at calculations.
     
  3. onlychevy6

    onlychevy6 1/2 ton status

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    You are pretty much dead on with that assumption. The piston will dictate what your true compression will be.
     
  4. b454rat

    b454rat 1/2 ton status Premium Member

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    The motor is 100% stock. CR is around 8:1 on these, looked at a couple pistons and listed the ratios above. Now I need an intake. Anybody have one?? (cheap)
     
  5. 76zimmer

    76zimmer Flyin Rat Premium Member GMOTM Winner

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    8:1 seems low for stock should be 8.5 at least.........I would believe it would raise about .8:1 for the 12cc in combustion chamber difference. Should give about 9.3 for the 4 valve relief style flattops
     
  6. b454rat

    b454rat 1/2 ton status Premium Member

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    Not sure on the ratio, but I know in the 70s pretty much everything got low compression for emissions and crap. As long as it's under 10:1 so I can run pump gas with it.
     
  7. 76zimmer

    76zimmer Flyin Rat Premium Member GMOTM Winner

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    should be good to go. I run 9.5cr with my Vortecs, and run unleaded 89 w/ 8 degrees initial and 34 total in by 2500rpm; no knock even while pulling a trailer.
     
  8. Beast388

    Beast388 1/2 ton status

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    Just to give you an idea, if you have 9:1 comp. with 76cc heads, then you will have 10.25:1 with 64cc heads....all other variables remaining the same.

    With 8.5:1 it would jump to 9.4:1 with 64cc heads.:D

    There are quite a few more determining factors than piston shape when figuring compression ratio.:deal: :D
     
  9. R77K10

    R77K10 Banned

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    as low as 8.0 Cr was very common, just look at the pontiac v-8s in the 70s alot of them were 8.0CR stock

    im not sure on the 400 chevy engines, but they were probably not much more, if at all

    make sure you srill the steam holes in the vortechs or whatever heads you use

    good luck
     

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