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305 heads on a 350, should I do it

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by TJS, Apr 30, 2002.

  1. TJS

    TJS 1/2 ton status

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    I have an 85 K-10 that I bought in Oct last year. I got it for a
    good price knowing it would need work. So far I have rebuilt the 700r4 did a lot of re-wiring of the hack job plow lite wiring that was there. It has a 305. Here is the prob. Now it is making noise. Sounds like wrist pin noise at idle. I have good oil pressure though. Here is what I plan to do. I have a 79 k-5 rustbucket with a good 350 in it with a holley 600 and E-bock intake. I plan on swapping this motor out to the k-10. Less than 5k miles ago I put a pre-emission cam in the K-5 blazer.
    The edlebrock performer cam that was in there wiped a lobe out in #6 Intake. When I swapped the cam I did not take the oil pan completly off which I should have to put a fresh gasket on and clean the contamination in the pan from the
    wiped out cam. So I think I am going to pull the 350 out of the K-5 and put a new pan gasket on it and maybe a bew oil pump and pickup. I also have a set of fresh 305 heads in my Dad's garage. They have been there for years and have not been bolted on anything since they came back from the machine shop years ago. These 305 heads are off of a 77
    Firebird with a 305 Chevy in it. I also just one a set of headers on E-bay for 40.00 and just ordered the dual header exh. kit from Summit. The heads that are now on the
    350 have some miles on them. Would it be adviseable to put the 305 heads on the 350. My buddy also has a comp cam 268 laying around that he never used and said he would
    give it to me. I already have tons of gaskets(all chevy guys have sets of gaskets they collect over the years..haha) and all I need are set of lifters. Any opinions would be appreciated. I figure it this way while I have the 350 out do all I can to it for what I can afford. I also have a 350 4bolt in my garage and I am acquirng parts for it. I am going to make it a 383. I already have the crank and misc. stuff but right now I
    do not have the time nor money to build that right away. Also the 79 K-5 has a fresh TH-350 and a good NP-205 in it. Thanks for your help.
    T.J.
     
  2. Z3PR

    Z3PR Banned

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    From my understanding, the 305 heads normally have 58 cc. while 350's are normally 76 cc or 64 cc if you're lucky. By installing the 58 cc heads, you'll raise your compression (usely a good thing if ya don't over do it) But the 305 heads have smaller valves. Check into it, I may be wrong about some things.
     
  3. Sparky

    Sparky 1/2 ton status

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    Personally, from what I know and what I have heard the 305 heads arent very good. Yes they are a little bit higher compression than the stock 350 heads, but they are a somewhat poor design. You will have to run 93 octane more than likly, and they arent very efficent when it comes to fuel economy and they certainly dont make much power for their compression ratio. However, if they are in that much better shape than your 350 heads go for it. You will probably gain a little bit of power. But IMO its not worth doing unless the 350 heads are in real bad shape.

    Sparky
     
  4. Quack_Addict

    Quack_Addict 1/2 ton status

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    A 350 with 305 heads will run but the 58cc 305 heads and smaller valves might cause issues (350 heads are normally 64-76cc). The higher compression from the 58cc's might cause preignition... so you'll have to run Midgrade or Premium. The small valves will restrict airflow into and out of the engine. I've never done it but have heard of people who have. If you are only doing it to get by with an engine for now until you can put something else in, go for it... but I wouldn't run a 350 with 58cc heads on it for too long personally.
     
  5. bigmack

    bigmack 1/2 ton status

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    Like the other people said, they are probably 58cc combustion chamber heads. It will depend on what piston you are running to figure closely on the compression you will have. If you are running stock type pistons (flat tops with 4 valve reliefs), the compression will probably be too much. The smaller valves in the heads will hurt performance too. But it will all depend on what the heads and pistons actually are.
     
  6. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    Like everyone else mentioned, the 305 heads have a 58cc combustion chamber versus a 350's 76cc chambers. Using 305 heads on a 350 with flat top pistons will net about 12.5:1 compression ratio. Way too much compression for any fuel that you would be able to buy at the pump.
     
  7. BorregoK5

    BorregoK5 1/2 ton status

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    If you take a stock 350 with a stock 8:1 compression ratio using 76cc heads and replace the heads with 58cc 305 heads, your compression ratio will jump up to around 9.5:1 which is quite doable. If you leave your ignition stock then you will have to run mid grade or premium to combat preignition as stated above. If you recurve your ignition you could just barely get it to run regular unleaded but at the sacrifice of quite a bit of advance. The small valves and runners will make for a smooth idle and high vacuum but you'll loose a lot of mid/top end and could experience poor overall gas mileage as stated above.
     

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