compression test, comp ratio? & leakdown. | FerrariChat
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compression test, comp ratio? & leakdown.

Discussion in '308/328' started by smg2, Sep 17, 2006.

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  1. smg2

    smg2 F1 World Champ
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    #1 smg2, Sep 17, 2006
    last night i did the leakdown and comp tests. the comp numbers are higher than the stock figures of 160~170. so based on that any ideas as to what the ratio is based off the compression test?

    i've searched and can't find a direct answer. yes cam timing has an effect on compression, so the dynamic ratio is what i'm after. static is never seen as it only accounts for near impossible open/close values on the valves.

    leak down numbers came back real good. i couldn't track were it was leaking to as it was to quiet. so i'd assume the crankcase, past the rings.

    -re-post-
    compression test: Psi
    #1 : 190
    #2 : 190
    #3 : 185
    #4 : 195
    #5 : 190
    #6 : 195
    #7 : 195
    #8 : 185

    Leak down test: %lost
    #1 : 5%
    #2 : 5%
    #3 : 5%
    #4 : 3%
    #5 : 5%
    #6 : 4%
    #7 : 5%
    #8 : 5%
     
  2. GTHill

    GTHill F1 World Champ
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    #2 GTHill, Sep 17, 2006
    I don't know if it helps, but when I had my 328 PPI'd, my compression was almost identical to yours.

    Gene
     
  3. Artvonne

    Artvonne F1 Veteran

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    #3 Artvonne, Sep 18, 2006
    Ya know, lumpy cams will get you lower compression.
     
  4. Sheldon Rubin

    Sheldon Rubin Rookie

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    #4 Sheldon Rubin, Sep 19, 2006
    I seem to recall that a 9:1 C.R. should yield about 155 psi. Like the Manuals say, the absolute reading is less important than the consistency from cylinder to cylinder. But another important indicator is just how many compression strokes does it take to reach maximum psi. If you not maxed-out at 6 strokes, then your ring seal may be weak even though the C.R. readings meet specification at more strokes. And the first two strokes are really important in assessing engine condition. I seem to recall reading somewhere that the first stroke should give a reading of at least 75 psi for typical high compression engines.

    I found that really accurate Leak Down testing requires the use of a twin gauage tester, i.e. one that reads both the inlet pressure as well as the leak down from the cylinder. This enables one to first set the leak down gauge at the reference point, e.g. 100. Then you note the incoming pressure, say 20 psi. Next connect the hose to the cylinder and readjust the setting to maintain the same 20 psi. Only then will one get an accurate leak-down reading; especially if you have leaky cylinders. I use a a modified physician's stethoscope to listen for leak-down sounds by probing down the dipstick tube or the throttle body or inlet manifold.
     
  5. smg2

    smg2 F1 World Champ
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    #5 smg2, Sep 19, 2006
    the leak down tester i have is a twin gauge MAC tool unit. it's set to 100psi and the second gauge reads the pressure loss in the cylinder. the noise from the valve between the gauges was louder than the escaping air in the cylinder.
     
  6. Rifledriver

    Rifledriver Three Time F1 World Champ

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    #6 Rifledriver, Sep 19, 2006
    Far too many variables to determine compression from a compression test. Cranking speed and cams are two. One of the reasons that a cranking compression test should never stand on its own to determine engine condition. It is really only good for a cylinder to cylinder comparison.
     
  7. smg2

    smg2 F1 World Champ
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    #7 smg2, Sep 19, 2006
    brian do you knwo of anyway to calculate the compression ratio based off the compression numbers?
    seems that if you have the cam timing events, cylinder volume, head cc's and deck hight you should be able to work backwords.

    i'll see what i can do in excell by working backwords. if we can calculate dynamic compression off engine data and cam events then it seems that getting a resonable compression number should be a matter of math.
     
  8. Rifledriver

    Rifledriver Three Time F1 World Champ

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    #8 Rifledriver, Sep 19, 2006
    Like proving a bumble bee can fly. Sure it can be done but you need to know what you need to know for the formula. Precise cranking speed and the volumetric effinciency at that speed for starters, and all for what? You got what you got.

    Much easier to just tip the motor up and cc it.
     

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