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  1. #1
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    Default Compression test tester kit

    G'day,

    Been a while since coming in here.

    I'm looking to compression test my 304 and I'm asking for advice on where to grab a decent compression tester kit for it that'll fit out of the box. Any clues?

    Anyone know what plug size carby 304s are?


    I'm suspecting I have low compression in one or more cylinders (knock on wood).

    What numbers would I expect for cylinders running cam 286 10:1 comp?

    I may report numbers here later in and get so advice.


    Cheers
    Last edited by Megaman; 09-04-2019 at 06:06 PM.

  2. #2
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    If you are asking about the size of the spark plug adaptor you would need, most of the compression tester kits I have seen come with a selection of adaptors to suit all the popular plug sizes.

  3. #3
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    Quote Originally Posted by GtoGeoff View Post
    If you are asking about the size of the spark plug adaptor you would need, most of the compression tester kits I have seen come with a selection of adaptors to suit all the popular plug sizes.
    Sorry for the late response. Yes, size of plug.

    I ended up buying a kit off ebay. I started off hooking up the kit to the 308 but ended up leaving prematurely so did not end up testing the compression but will continue it on Tuesday. However, upon removing all spark plugs I noticed oil in the spark plug threads which is something I have always experienced with the 253 and now 308. Motor has only done 40,000 kilometres

    The electrodes were not covered in oil at all. Should this be a concern? Is this common in 308s? Motor idles and runs fine with the exception of some power loss hence the compression test. I haven't driven the car in months.
    Last edited by Megaman; 14-04-2019 at 09:16 PM.

  4. #4
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    Hi
    Compression tester by Ryco if there still available is a good unit . But all you really need is to have a quality gauge even with cheap adapters it does not matter really.
    Remember that oil rings can be worn more than compression. eg 165 psi with a little oil consumption


    prefer throttle open
    disconnect spark
    First test crank the same for each cylinder eg 8 times
    no more than 10% difference between cylinder readings

    Second test then apply 4 big squirts of oil to each cylinder done 1 at a time and retest
    if the reading increases means the oil is sealing the rings

    Leak down tester
    if a cylinder is between 5-10% leak down ,less than 5 great , closer to 10 % engine is worn , over 10% means trouble

  5. #5
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    Quote Originally Posted by swampy View Post
    Remember that oil rings can be worn more than compression. eg 165 psi with a little oil consumption

    Thanks. I had most of that already noted. Hopefully I don't need to go for the second test and all readings are within margin.

    However, with this bit here are you referring to the spark plug thread oil? Are you suggesting this is a common occurrence?

    Also, very high PSI readings generally means you should expect oil consumption?

    I'm hoping this oil in spark plug thread thing isn't a sign of compression/valve issues already.
    Last edited by Megaman; 14-04-2019 at 09:56 PM.

  6. #6
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    Hi
    Dry compression should be say around 165psi std engine .
    When u add oil it will go up . This shows if there is a difference between cylinders as well . Further investigation on the cylinder that goes up the most in psi .
    Then do a leak down on that cylinder to determine more accurately valves or rings etc

    Odd/ leaky cylinders should be easy to find .

    Compression canbe ok but oil rings can be worn . I suspect this happens just b4 the compression ring also starts to leak and lower its psi .
    Each engine has its own characteristics of compression leakage.
    Check ignition system on the low cylinder as well .

    Not uncommon for an oil leak to deposit on plugs , typically valve covers .

    Measured oil consumption ??? 1ltr used in 5000kms is max .

  7. #7
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    OK so the testing was done. Pretty much all chambers sit at 150 PSI. Lowest being just above 145 Psi and highest at 155 PSi so thatís good news.

    Only exception is I donít know what they should be as the motor is modified so potentially they should be sitting higher at 165 psi?? But I wonít know unless someone here knows more about my setup.

    At this stage things look good. Any thoughts?


    Iíll need to check oil consumption. I think it does eat some oil but too much.
    Last edited by Megaman; 16-04-2019 at 01:52 PM.

  8. #8
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    Quote Originally Posted by Megaman View Post
    OK so the testing was done. Pretty much all chambers sit at 150 PSI. Lowest being just above 145 Psi and highest at 155 PSi so that’s good news.

    Only exception is I don’t know what they should be as the motor is modified so potentially they should be sitting higher at 165 psi?? But I won’t know unless someone here knows more about my setup.

    At this stage things look good. Any thoughts?


    I’ll need to check oil consumption. I think it does eat some oil but too much.
    The cranking pressure depends on a few things, the most obvious is cranking speed of the starter motor and how much charge is in the battery this could affect it, you want to look at how quick it gets up to the 150psi if it does it in about 3-4 needle movements but if you have to keep cranking to get a number it could be a tell tale that the bore seal is a little off, this could also be because the engine isnt up to temp so best to run the engine a little while to get some heat in...

    Your camshaft will also affect cranking pressure, a later intake valve closing point as you get with a larger cam will reduce numbers if the compression ratio itself isnt high, as long as you are getting numbers that arent drastically different and one is 80psi while the others are 150 there is no need for concern regarding the health of the engine... A leakdown would tell you more, if the noise from the compressed air is from intake then its intake valves or seats, through exhaust then exhaust valves or seat, through the crank case then its ring seal... It gives you a percentage, and you wouldnt want more than 5-10%..

  9. #9
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    Tested the son in laws recently and it had 155-160psi, being factory stock, which should be fine.

    The 286 cam has a bit of overlap, so i would think 145-150 is expected if no compression enhancing mods were performed, prior to the cam install.

  10. #10
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    Compression enhancing mods? From memory (err.... 12 years ago) there were none that I could remember. I watched the motor go through the building stages with the exception of all the machining which was done off site so unless I'm missing some compression enhancing technique (none that I can think of) it shouldn't have had any done.

    The motor was last built 12 years ago with only just under 40,000 kays on the clock since. Its had its fair share of 6200 rpm wheel spinning shifts into 2nd so a compression test was in order

    Main reason for the check was because it was dynoed at a healthy 195 kilowatts at the wheels but since lost power and at the same time started running extremely rich and fumy to the point me clothes wreak and nature around it is dying.

    I think my next step is to re-dyno her and see what is up with it. I honestly don't see how a dynoed carby can de-tune itself but then again what do I know. It's my first completely modified projected and so naturally lack the knowledge and experience most of you here have.
    Last edited by Megaman; 17-04-2019 at 04:04 PM.

  11. #11
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    Have you checked the timing? maybe distributor wasnt tight or its slipped..

  12. #12
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    Quote Originally Posted by paulzig View Post
    Have you checked the timing? maybe distributor wasnt tight or its slipped..
    Yeah timing's good. 18 degrees

  13. #13
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    Quote Originally Posted by Megaman View Post
    Yeah timing's good. 18 degrees
    18* initial?
    What is total?

  14. #14
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    Quote Originally Posted by EH179 View Post
    18* initial?
    What is total?
    yep initial. Think last I checked...I think 34 total.

    I had a performance tester confirm 18 degrees is where it is the happiest but can't confirm total is 34.
    Last edited by Megaman; 17-04-2019 at 05:47 PM.

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