Can't maintain 175 psi in cylinder

Discussion in 'Classic Mustang Specific Tech' started by my66coupe, Dec 2, 2003.


  1. my66coupe

    my66coupe Founding Member

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    Ok, I recently did a compression check on my car, and it turned out i was making only 125 psi all around. I loosened up the rockers and acheived a max of 170-175 all around. When I went to tighten up the valves, I couldnt keep it at 175. With slightly turning in the nut by hand like 1 turn, (rocker arm still really loose) I did a compression check just to be sure, and it would lower to about 125 again. I am doing all of this at work with 3 mechanics helping out. Each suspeced push rod length. I went home and got my older push rods which are 1/8'' shorter and it improved to 150. I called a local speed shop for a even shorter push rod, and the have a push rod that is 1/8'' shorter than my shortest. Before I go pick up some push rods, what else do you think my problem is. Heres what my valve train is looking like...

    Cam.....unknown (sounds like performance cam)
    Push rods......6 7/8'' hardened pushrods old ones are 6 6/8''
    Comp cam 1.6 roller rocker arms
    Comp cam dual valve springs
    Manley guide plates

    thanks alot guys

    Mike
     
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  2. 2nd Mustang

    2nd Mustang Founding Member

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    I read somewhere that "lobe separation" has an effect on compression readings. I have a Crane Hydraulic roller cam with 1.6 roller rockers, 10.5 - 1 pistons and my compression in all 8 cylinders (early 302) is 130 lbs. per cylinder. Some of the more experienced gearheads here can probably answer this better.
     
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  3. none67

    none67 New Member

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    honestly, i don't really see a problem as long as there all between 135-175 your in good shape, and if u add a table spoon of greese into the cyl. the psi will jump about 10..
     
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  4. mustangracer

    mustangracer Founding Member

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    Yikes, don't add grease to the cylinder, just use some 10W30 or similar. If the rings are worn your cranking pressure should increase.

    On a performance cam the valve overlap can reduce the cranking compression. Some of your intake charge can sneak out the exhaust valve before it closes. If all the cylinders are the same (within 10% or so), then you should be fine. By loosening the valve lash you're reducing that overlap, and it's keeping the pressure up. As long as your valves are set right, I wouldn't be messing with them. If it sounds like a performance cam when idling, then that's probably what is happening.
     
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  5. Ozsum67

    Ozsum67 Too much thin air Founding Member

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    YIKES!!! I wouldn't advise wd40. It is mostly perfumed kerosene which would tend to thin out the protective coating on the cylinder walls. Regular oil is just fine. Grease if you have to. :D
     
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  6. SuperDave

    SuperDave Early-Model Mentor Founding Member

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    Has this block been "decked" (machined down)?
     
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  7. my66coupe

    my66coupe Founding Member

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    Not sure if the block has been machined down at all. Im pretty sure it hasnt. Its a recent rebuilt mexican 302.

    Mike
     
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  8. slapper

    slapper Founding Member

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    Everything written above is good advise.
    I don't think you have a problem. I would suspect that you're losing cranking pressure during valve overlap, as Mustangracer said.
    Actually, the cranking compression test is not very good.
    In the future, do a differential compression test, or leak -down as some call it, if you really want to determine engine condition through compression testing.
    It will be readily apparent if there are issues with rings, intake or exhaust valves,
    or head gaskets, etc.
     
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  9. crushnut

    crushnut New Member

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    Care to explain? Please :) Thanks
     
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  10. SadbutTrue

    SadbutTrue Founding Member

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    alot of engines run excellent without a single cylinder at 175...
     
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  11. 351LX

    351LX Founding Member

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    In a leak down test you use an air compressor and a leakage tester to put a metered amount of compressed air into the cylinder when it is at TDC.It tells you right how well the cylinder is holding compression but also lets you determine where you are losing compression.If you are losing past a valve you can usually hear it escaping through the carb or exhaust.If it is the rings you can feel it out the breather.On a car I am working on right now a compression test indicated a low cylinder.I used my leakdown tester and could see bubbles coming out the rad indicating a blown head gasket or possibly a crack.
     
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  12. SuperDave

    SuperDave Early-Model Mentor Founding Member

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    Are you trying to ACHIEVE 175 PSI or MAINTAIN 175 PSI? There is a difference and the terminology is important. A compression leak-down test evaluates how well the engine HOLDS pressure.
     
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  13. my66coupe

    my66coupe Founding Member

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    I loosened my valves to see what max, psi i can acheive. 175 was the max, I would like to maintain that psi. Today I tried both sizes of push rods and got 125 yet again. The guys at work think 125 is really low. The car has flat top pistons and stock size combustion chambers is this normal or what? I also thought that i may have loosened the valve too far and when I did a compression check, it wasnt letting all of the compression out at exhaust stroke, and just built up to 175.... any ideas?

    Mike
     
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  14. SuperDave

    SuperDave Early-Model Mentor Founding Member

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    FYI: A standard compression test is based on using the starter to crank the engine over (with all plugs removed) and testing each cylinder with an accurate (many aren't) gauge. It is important to get a good seal on each cylinder when being tested to get accurate results. It is also important to get a fairly equal readings than it is to get high readings. Since the reading is based TEMPORARY pressure achieved, it says nothing about the ability to hold that pressure. Reading should NOT vary for than 15-20 PSI from each other.

    The "leak down" method is explained above. It is considered a more useful diagnostic assessment.
     
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  15. mustangracer

    mustangracer Founding Member

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    hey Ozsum....I said 10W30...not wd40....as in motor oil...you read too fast... :D
     
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  16. pabear89

    pabear89 Active Member

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    Frist are you doing the test correctly?
    Have you prop open the carb to allow air in?
    Many do the compression testing incorrectly by no holding the carb throttle plate FULLY OPEN, and this can give false readings.
    Are ALL the plugs out?
    it makes the engine spin freely.

    What does the spec say you should have for that motor?

    PB
     
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  17. my66coupe

    my66coupe Founding Member

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    I did not remove all the plugs, I did have the carb fully open though. Tommorow I will try the method you described. Does anyone know the psi range that a fresh mexican 302 should be running?

    Mike
     
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  18. pabear89

    pabear89 Active Member

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    Ruff guess about 160
     
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  19. INDSKYS

    INDSKYS New Member

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    Wow Looks Like I Lucked Ot Then My Temp Motor A 302 .i Just Put In From A 85 Merc Cougar ,tested At 165 Lowest And A Flyer At 185. Avg 170and It Had A Few Hours On It. Got It For A Hundred Bucks ---the Whole Frickin Car Lol .it Did Run Like Crap When I First Put It In Tho.and That Stumped Me Into Comp Checking Then Vaccum Checkingall Which Were Great. Then Found Out The Firing Ordwer I Had Set It For Was An Ho 302. And I Wired Em Ass Backwards Ala Chevy Clockwise Amazing What The Correct Order And Rotation Can Do For A Motor .
     
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  20. my66coupe

    my66coupe Founding Member

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    Ok so what can I do to maintain the 175?

    Mike
     
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