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Piston/Compression reccommendations?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by BayouBlazer88, Aug 4, 2003.

  1. BayouBlazer88

    BayouBlazer88 1/2 ton status

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    I'll be boring out my 88' 350 .03 over and therefore installing new pistons and rings. I don't want too high of compression because I want it to run on either 87 or 89 octane gas. I've also got the vortec heads with 64cc combustion cambers. With that in mind, what kind of compression should I shoot for? I've been looking at pistons on Jeg's and Summit and the lowest compression I can find is 9.35:1 with the Federal-Mogul hypereutectic pistons (which seems a little high) and are $21 each. They sell cheaper pistons but the compression for those is just over 10:1. Also, would I be ok just going with pistons from Autozone or O'reilly? I don't drive hard so I don't need some kind of high performace racing piston.

    -one more thing, When you increase the volume of the combustion chamber such as when you bore out the cylinders, does compression go up or down? I thought it was down but the guy at Jeg's told me otherwise.
     
  2. 88Silverado

    88Silverado 1/2 ton status

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    Save your money and get standard cast pistons and cast rings. You can run flat-tops with the vortec heads and about a .042 head gasket and be around 9-9.2:1 compression.

    The deck height is where compression estimates gets messed up (how far down below the top of the block deck the piston is when its fully up.

    Compression Calculator

    Northern Auto has a nice rebuild kit for around $150
     
  3. Thunder

    Thunder 3/4 ton status

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    Yup. Assuming you are keeping the TBI system. Just go with some stock pistons. You dont want to go with higher comp ratio pistons because of the vortec heads. With their smaller combustion chamber that will raise your comp ratio to around 9 to 1 + with stock style pistons.
    The stock chip in the ECM wont handle much over a 9 to 1 comp ratio. Getting your comp ratio up too high will mean getting a new chip burned and a diferent cam. Try to stay around 9 to 1 maby 9.5 max. for best all around driveability. If you are going with a carb you can go higher if you want but anything over 10 to 1 will require preimum fuel. Higher comp ratios also require a cam ground for the comp ratio you are running.
    If this is your first engine build just try to keep it as close to stock specs as possible until you learn the hows and whys of performance engine building. Mistakes are expensive.
    Dont be afrade to ask questions here. Thats what these boards are for. /forums/images/graemlins/thumb.gif
     
  4. 89Jimmy

    89Jimmy 1/2 ton status

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  5. BayouBlazer88

    BayouBlazer88 1/2 ton status

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    Yeah I'm definitely keeping the TBI, wouldn't want any worse gas mileage than I'm already getting. Anyway, good to hear that I can use the cheap stuff. And don't worry, I won't hesitate to ask questions on here, I never have. I got my memberships worth the first time I posted on here.

    By the way, how many cc's do 4-valve reliefs take up? I need to know in order to use that compression calculator. thanks guys.
     
  6. 88Silverado

    88Silverado 1/2 ton status

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    [ QUOTE ]
    how many cc's do 4-valve reliefs take up?

    [/ QUOTE ]
    Good question. You have to ask the manufacturer.
    My flat-top 2 valve reliefs are 4cc.
    ALSO...most aftermarket piston manufacturers move the rod connecting pin up .020 to accomodate for a milled block during a rebuild. This is to keep the compression around the same as stock. This can also mess up your calculations.
     
  7. Beast388

    Beast388 1/2 ton status

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    [ QUOTE ]

    By the way, how many cc's do 4-valve reliefs take up?

    [/ QUOTE ]

    It does depend on the piston brand, but generally, the volume of the 4 valve reliefs is about 7-8 cc's. /forums/images/graemlins/thumb.gif

    And yes, increasing the cubic inches (bore or stroke or both) increases the compresion ratio if using the same head. You are pulling more volume into the cylinder and then compressing it to the same combustion chamber as you had before. /forums/images/graemlins/thumb.gif
     
  8. dyeager535

    dyeager535 1 ton status Premium Member

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    Dunno is speed-pro is even around anymore (division of fed-mogul?) but less than 2 years ago, hyper pistons for the SBC were just under $100.

    Hypers are better pistons than cast, and if they cost the same or less, it's foolish to not use them. They *probably* aren't as forgiving as cast pistons (neither is anywhere near forged piston strength) but if the engine is in good tune, you won't need to worry. If it's not, the knock sensor will do it's best to save the engine. /forums/images/graemlins/smile.gif

    Speed Pro pistons I've got (.030 over) run right at 9.2:1 compression with the standard head gasket... .031" or something like that? That is well within the range for Vortec heads and injection on 87 octane. (the above linked article stated 9.4:1 on vortec heads, .030 over and 87 octane with carb...edit, that was with different heads. 9.5:1 should still be possible with vortecs and 87)

    Don't forget the balance.
     
  9. BayouBlazer88

    BayouBlazer88 1/2 ton status

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    What would the deck height be for a block that has been decked? I'm again trying to calculate my compression ratio but don't know what to put in for the deck height. And I'm not sure if the pistons that the machine shop put in sit lower to copensate for a decked block. Just for the hell of it, I typed in the same thickness as the gasket (.042") for the deck height and it gave me a CR of about 9.1:1 which is fine with me.
     
  10. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    Deck height is a measurement from the crankshaft centerline to the cylinder head surface of the block. It is best to (square) the decks and not just deck the block. Squaring the decks means to make the deck height the same from front to back as well as 90* to the crankshaft centerline. Everyones deck height will be different since they may have had more or less material removed when the block was decked/square decked.
     

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