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  1. #41
    TechnicalTom
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    A general guide would be , the more radical the camshaft the more likely to show low ish figures, on a standard ish engine with mild camshaft (or factory )+ modest compression ratio 130 would be fine,but the variation between cylinders is important as is a good battery for consistent checking.

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  3. #42
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    That's quite odd techtom as my sbc uses a solid roller cam [email protected] thou and pumps at 190 psi,,, it's a matter of matching your static compression with your cam,,, and mine is a street motor

  4. #43
    TechnicalTom
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    Thats a healthy figure , as you say matching the static and dynamic compression can yield good results,especially on a rebuilt SBC using modern alloy big valve heads, but I still stick with my " generalisation" quite often when using radical camshafts the compression figures will be lower than expected , all due to the valve closing event ,Also on a SBC the the cylinder head chamber / head gasket can be optimised to achive good results, that may not be the case with with dished pistons a thick head gasket and in the case of an early SBC a 76 CC cylinder chamber ( cast iron and small valve) , which could be found on " Smog era" American Chevy engines.
    As an example of the variations ,I just did a compression check using my Snap On tester earlier this evening on an ancient MA 10 engined Nissan Micra and read 190 ISH, on all 4 cylinders, (all standard.)

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  6. #44
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    Generalisation should not be used in engine building. If you put a healthy cam into an engine without regard to the static compression then the cranking compression will obviously go down. A good engine build is the combination of compatible parts all working in harmony. If its worth doing its worth doing right. Guys often pick engine components based on nothing more than friends advice and then convince themselves that its working wonderfully. Joe Sherman once wrote about cranking compression in street and race engines. Very long read. But it said that up to 200 psi would run on pump gas and the higher numbers used progressively better race fuel. 130 is in my opinion a very low figure and the engine isn't sealing well or it has been fitted with a cam to big for the rest of the combination. Both situations can run but neither is right

  7. #45
    Official RnS Addict beach57's Avatar
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    The engine is a standard camaro 1979 350.

  8. #46
    TechnicalTom
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    Quote Originally Posted by zephyrc View Post
    Generalisation should not be used in engine building. If you put a healthy cam into an engine without regard to the static compression then the cranking compression will obviously go down. A good engine build is the combination of compatible parts all working in harmony. If its worth doing its worth doing right. Guys often pick engine components based on nothing more than friends advice and then convince themselves that its working wonderfully. Joe Sherman once wrote about cranking compression in street and race engines. Very long read. But it said that up to 200 psi would run on pump gas and the higher numbers used progressively better race fuel. 130 is in my opinion a very low figure and the engine isn't sealing well or it has been fitted with a cam to big for the rest of the combination. Both situations can run but neither is right
    Lets be quite clear here I am not recommending using " Generalisation" when building an engine,If you read my posting I am making an Observation,Now back to the topic in hand , 130 PSI cranking pressure , though not ideal if ok on an old SBC , and the engine would run ok .What "BIG JOE"Sherman ( A well respected engine builder in CA and prolific poster on many USA engine forums) says is fine if you are dealing with a well thought out combination of parts , mainly aftermarket ( read very expensive) .but the OP is stating he has a different set up.IE and old SBC . Another element here is the accuracy of the compression checking, is the compression tester good ?, is the engine up to the best temprature ?is the battery cranking the engine fast enough? and so on.The most accurate way to know for sure is by the correct use of a quality leak down tester ,and an experienced person to carry out that test,a test that is usually quite difficult in many "hot rods" due to Headers ETC , being in the way, I am sure the OP knows this by now anyway. I Personally have 4 different compression testers Snap On ETC . and each tester gives different readings , so I would say compression tester readings should be used as a guide only , a rough guide at that ,

  9. #47
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    I agree entirely Tom

  10. #48
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    Pistonpusher is offline
    Compression test figures will also vary considerably depending on cranking speed.

  11. #49
    Official RnS Addict beach57's Avatar
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    Well update removed heads and found a small bolt in cyl 6 ? used to be a rattle but could never figure it out as it would go away,knacked seat and bent push rod and looks like intake valve bent as well,found water staining in number 7 looks like head gasket gone, anybody ideas of a cost of machining a new seat in a cyl head ?.

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