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305 heads on 350

Discussion in 'The Garage' started by Goose, Apr 20, 2006.

  1. Goose

    Goose 1/2 ton status

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    I just replaced my valve cover gaskets and looked up the head numbers.
    Surprise! The PO put 305 heads on a 350. (confirmed 350 block by numbers also. Heads are 14022801 and block is 14010207.

    I've head of this in the past to supposedly increase compression ratio.
    What are the pros and cons of this combo? Does it flow as well??

    thanks,
     
  2. 1977k5

    1977k5 3/4 ton status Vendor

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    A search would do you well, but failing that...

    305 heads have smaller valves, do not fit the 350's 4.000" bore correctly, have very low flow numbers and will pretty much choke a 350. It will raise your compression but it is making less power with 305 heads than with pretty much any head that came on a 350.
     
  3. Goose

    Goose 1/2 ton status

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    crap, and thanks,
    I'll search also.
     
  4. BranndonC

    BranndonC 3/4 ton status

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  5. Slapperbar

    Slapperbar Retired Navy NDT Examiner Premium Member

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    I have a set of 305 heads on my 350 in my '91 S-10. Ported out and punched with 2.02 and 1.60 valves.I don't know what the numbers are because when I went to run it at the track I spun a rod bearing.:doah: It ran like a raped ape thou.
     
  6. beater_k20

    beater_k20 Banned

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    the smaller ports, and smaller valves have already been discussed, and both are known to nix any possible gains. installing larger valve in the 58cc chambers also does the same. it puts the valve head too close to the combustion chamber wall, impeding flow on the affected side of the valve. Slapperbar, you may have been better off with a set of stock 1.84/1.50 valves with a good 3 angle valve job.
     
  7. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    I replied to another post about this. There are 305 heads that have the small 1.72" intake valves and also 305 heads with 1.94" intake valves from the factory. In fact some 305 and 350 heads have the same casting number so you can't always go by the casting number. I'm an automotive machinist by trade and have seen my fair share of heads and also oddities that GM did, this being one of them. There are also pistons made now that allow the use of 305 heads on a 350 without raising the compression ratio way up.
     
  8. beater_k20

    beater_k20 Banned

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    at which point you just killed the slight possibility of a performance gain. now not only do you have low compression, but you now also have (in most cases) inadequate valve size, and inadequate runner size.
     
  9. 4xcrazy

    4xcrazy 3/4 ton status

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    BASICALLY,,,, as you already know,,,they work if you need the motor running, but do nothing for the performance that a 350 is capable of.

    I have a set of 305 heads on a 350 block here, i installed them, because it's all i had rebuilt at the time, and i really needed the motor running, to be used.....and with the intention of replacing them in the future with the correct units.....just need to get around to it:doah: :D
     

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