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What is normal compression for a 350 SB?

Discussion in 'The Garage' started by az88k5, Jul 19, 2006.

  1. az88k5

    az88k5 1/2 ton status

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    Who knows what the normal compression is for a GM crate 350 small block? Let me know what is considered low, normal, and high compression. Thanks :dunno:
     
  2. ryan22re

    ryan22re 1/2 ton status

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    Low is less than 8.5

    Normal is 8.5-9.5

    High is 9.5 and up.

    GM tbi/vortec 350's run around 9.3:1.
     
  3. az88k5

    az88k5 1/2 ton status

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    I do not understand these numbers 8.5-9.5 :dunno:
    A friend ran a compression test and he said it was 110 in some and higher in other cylinders. Does this make sense to you?
     
  4. BadBob

    BadBob 1/2 ton status

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    That's cranking compression/pressure. The ZZ4 was in the 190 ball park, but that's on the high end. 110psi is flat out sad, there's something worn out in there. You should have probably somewhere in the 150-170 psi range.
     
  5. theperfectgarage

    theperfectgarage 1/2 ton status

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    Even at your altitude, 110 is low. Time for head and or block work.
     
  6. 76zimmer

    76zimmer Flyin Rat Premium Member GMOTM Winner

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    How do the cylinders compare with one another? I think that if your at 110 psi or better (although its low) on all cylinders you should be ok. Did he take out all the plugs when doing the test, open the choke, and crank for 5 or more revolutions per cylinder? What do you plan on doing with it?
     
  7. ryoken

    ryoken Puppy Fabricator Premium Member GMOTM Winner

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    most sb's will be in the 135 to 155 range on a compression test.. cylinders need to be within 10% of each other...
     
  8. adamforsythe

    adamforsythe 1/2 ton status

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    How would I check my compression on my 350?
    Thanks
    Adam
     
  9. ryoken

    ryoken Puppy Fabricator Premium Member GMOTM Winner

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    Run the engine until it reaches normal operating temperature. Tests done on a cold engine usually show lower readings. Remove the plug wires and take out all the spark plugs. The ignition system MUST then be disabled. If this isn't done it will continue to generate high tension voltages into the HT leads which will have nowhere to go with the plugs out. These high voltages will find another route to earth and can damage the ignition system or even the car's ECU. Unplug the low tension connections to the coil or to the distributor.

    Screw the gauge into cylinder 1 and rest it somewhere you can see the dial while you crank the engine. Open the throttle fully either by pressing the accelerator or wedging the linkage open under the hood. If the throttle isn't open then air can't get into the cylinder and the readings will be far too low. Crank the engine until the gauge stops rising and count the revolutions while you do so. It should normally take no more than 10 engine revolutions (5 compression cycles) to get a full reading. You can count the cycles by watching the gauge too - each jump of the needle is one compression stroke. Write down the final reading and also make a mental note of how quickly the gauge rose on the first few cycles. Then just repeat for the other cylinders. Make sure that each cylinder reaches its highest reading after the same number of engine revolutions.

    If all readings are good then the test can end there. If any cylinders are low then a "wet" test can be done. This involves squirting a few ccs of oil into the cylinder and repeating the test. The oil will help seal bad rings and increase the reading but won't affect it if the problem lies in the valves or head gasket.
     
  10. beater_k20

    beater_k20 Banned

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    a compression test wont confirm engine work that needs to be done. the size of the camshaft in relation to the compression ratio of the engine could very well create 110 psi in an otherwise healthy engine, with the only problem is that the cam is entirely too big.

    what you need is a cylinder leakdown test to tell you exactly what is wrong with it. not only will it tell you what is wrong, it will tell you how bad it is. the only thing a compression test will tell you is the difference between the cylinders. generally +/- 5% is considered acceptable.
     
  11. adamforsythe

    adamforsythe 1/2 ton status

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    Cool, ok now how can I tell what my ratio is like ( 8.5.1, etc)?
    Thanks
    Adam

     
  12. ryoken

    ryoken Puppy Fabricator Premium Member GMOTM Winner

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    Well, there is no real way to do it with the engine assembled. But genereally effective compression ratio's are the compression test reading divided by 17 to 20...

    so, 150 divided by 17 is roughly 8.8... 150 by 20 is 7.7, so somewhere in that range...

    real compression ratios are done by measure cc volumes of the combustion chamber, head gasket volume and effective dome volume using a burrett, plate, etc... I do have the formula around somewhere....
     

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