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How are these compression #s for a 355?

Discussion in '348/355' started by Sandy Eggo, Nov 20, 2009.

  1. Sandy Eggo

    Sandy Eggo F1 Rookie
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    Jun 4, 2009
    3,275
    Encinitas, CA
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    Rick
    Hello

    I'm in the middle of a PPI on a 96 355 that I am pretty serious about (obviously) and just got the compression #s. The shop says they are very good - looking for some additional opinions. Car has about 22,000 miles. Cut-n-paste from the shop's service manager below...

    Compression numbers as follows:
    full throttle, 10 cranks, on a battery charger (the right way)
    #1 220
    #2 208
    #3 212
    #4 220
    #5 232
    #6 230
    #7 218
    #8 223

    Thanks.
     
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  3. saw1998

    saw1998 F1 Veteran

    Jun 8, 2008
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    Very good numbers, with very reasonable percentage of variance! Best of luck with the car.
     
  4. FullChat

    FullChat Formula Junior

    Jan 1, 2007
    339
    San Antonio, Texas
    Right at 10% variance, but 208 is the weakest hole. Strong numbers!
    Why is #2 always/usually the lowest???
     
  5. AceMaster

    AceMaster Three Time F1 World Champ

    Feb 6, 2009
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    About a week or so, there was someone else asking about compression #'s and Rifldriver explained something about how and why #2 and #7 were the odd ones....I don't want to misquote him so I won't even try to explain what he said, and unfortunately I don't recall the thread, but it was not that long ago, certainly within the last week.
     
  6. 348SStb

    348SStb F1 Rookie
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    #5 348SStb, Nov 21, 2009
    Last edited: Nov 21, 2009
    Just a side comment here...

    Ten cranks seems a lot. I'd be interested on how the #s look on a 6-crank test and say an 8-crank test.

    The question is not how much compression a cylinder can generate after infinite cranks, but how quickly it can generate good compression.

    Testing methodology of course matters. There is no single way to do a compression test -- but it is a test that is either done in the proper manner or in a for-the-sake-of-it manner. Results are spurious and indicative of really nothing unless an understanding as to the proper methodology exists. The first element to such understanding is an acknowledgement of exactly which variables are involved in arriving at the resulting psi #s. How those #s are obtained is more important than what the #s are in terms of their largeness or smallness. Folks not familiar with the science involved in a compression test must be very cautious about treating it like an easy thing like, for example, getting on a scale to measure body weight. Measuring body weight is as easy as making a quantitative measurement with a scale. Testing an engine for compression is a thing achieved successfully by careful analysis and employment of the scientific method.

    Original poster -- these comments are not necessarily applicable to you... I just thought I'd give my $.02 so others who may be unfamiliar might benefit from an understanding of what your thread is about...
     
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  8. saw1998

    saw1998 F1 Veteran

    Jun 8, 2008
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    Mike:

    If I remember correctly (I don't want to piss-off our experts), I think Brian mentioned that it was due to possible cylinder wash by gasoline from: (i) the injector positions on the fuel rail; and (ii) dirty injectors staying open after engine shut-off and allow gas to pool in the cylinder. I'm probably wrong. If so, sorry Brian.
     
  9. Rifledriver

    Rifledriver Two Time F1 World Champ

    Apr 29, 2004
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    Those can all cause problems but are a result ofsomething wrong.


    Most normal wear will happen in the hottest cylinders. No motor ever built that has more than one cylinder has even temperture on all cylinders, it just never happens. Air cooled VW's were well known to have #3 run hotter than all the rest. In a 355 it is #2 and 7. Those are the two who usually lose compression first and the two who burn out headers first.
     
  10. Rifledriver

    Rifledriver Two Time F1 World Champ

    Apr 29, 2004
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    Nothing in the world wrong with cranking over 10 compression strokes. I was taught that way over 40 years ago.
     
  11. 348SStb

    348SStb F1 Rookie
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    #9 348SStb, Nov 21, 2009
    Last edited: Nov 21, 2009
    Right, nothing wrong per se with 10 strokes... but what I was saying is that attention is typically paid *also to how quickly (e.g. in how many strokes) "good" compression is achieved... I imagine when a skilled technician such as yourself is conducting the test, he reads the pressure gauge and watches the increase in compression with each stroke and makes a mental note of how the compression is increasing... Isn't it important to observe how many cranks it takes to achieve "acceptable" compression?
     
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  13. fatbillybob

    fatbillybob Two Time F1 World Champ
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    Aug 10, 2002
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    did they leak it down?
     
  14. rbellezza

    rbellezza F1 Rookie
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    Jun 18, 2008
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    Yep, they sound like strong numbers.
     
  15. Rifledriver

    Rifledriver Two Time F1 World Champ

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    #12 Rifledriver, Nov 22, 2009
    Last edited: Nov 22, 2009
    Many things should be observed. The rise in pressure should be consistantly lower with each stroke. If it suddenly jumps up after reaching a particular pressure it can indicate sticking rings. There are many things to watch for. It is far more than just the ultimate pressure achieved. It is also far more of a diagnostic tool than a test of engine wear or condition. A leak down test is far more indicative of that and is one of the reasons the aviation industry relies far more on leak down than cranking compression. I always say if I was going to do just one it would be a leak down test.
     
  16. 348SStb

    348SStb F1 Rookie
    Owner

    Thank you.
     

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