355 - Confused about my recent cylinder compression numbers | FerrariChat

355 Confused about my recent cylinder compression numbers

Discussion in '348/355' started by ewn355, Jan 24, 2022.

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  1. ewn355

    ewn355 Rookie

    Jan 19, 2014
    16
    I have carried out a biennial cylinder compression test of the F129B engine in my F355 M2.7 since I bought the car. The compression test procedure was established based on information that I have found in this forum, and I have been using the same procedure since it first was established. In short, the procedure is:
    1. Fully charge the battery
    2. Take the car for a 30 min drive
    3. Connect battery to battery tender/charger
    4. Remove relays for fuel pumps and injector ECUs
    5. Jam throttle fully open
    6. Remove all spark plugs
    7. Install pressure gauge into a cylinder
    8. Crank for 8-10 revolutions using a remote starter button
    9. Record the pressure gauge reading
    10. Remove pressure gauge
    11. Move to the next cylinder and repeat step 7-10
    The full procedure can be found in the attached pdf file.

    I have carried out a total of 5 compression tests. The first 4 tests were very consistent, with hardly any drift in the numbers (for all 8 cylinders) over time. However, the latest compression test that was carried out this summer was very different: All 8 cylinders showed higher compression, and the average compression was approximately 15 psi higher compared to that for the previous tests.

    The test conditions were similar for all the tests: Same location (in my garage), same time of year (July or August), same pressure gauge, roughly the same mileage between the tests (~1000 km per year), same type of engine oil. The pressure gauge was not calibrated prior to any of the tests.

    Between test 4 and 5 the following changes were made to the car:
    • A new starter battery was installed (700 CCA (old) vs. 720 CCA (new)). The old one was basically worn out. I don’t know its age, since it came with the car.
    • A Capristo (Sound 3) muffler was installed
    • The catalytic converters in the by-pass Y-pipe were deleted
    • A remote control operation of the exhaust by-pass valve was installed. Now, around 90 % of the driving is done with the by-pass valve open.
    What can cause such a large increase (~8%) in the cylinder compression? Can a new (and fresher) starter battery affect the compression numbers this much? Can a faulty pressure gauge suddenly show higher compression numbers? Any input from you in the form of theories or experience will be appreciated.
     

    Attached Files:

  2. Ferrarium

    Ferrarium F1 Veteran
    Rossa Subscribed

    Jul 28, 2018
    5,624
    Central NJ
    Full Name:
    Eric
    With compression tests the absolute number is not super relevant. It is the difference between cylinders that's key. Battery does not affect cylinder sealing. Neither does exhaust system. Valves and rings sealing is about it. Timing can affect it by changing the sealing of valves as well as carbon build up changing the sealing of valves etc but its usually only on a few cylinders not all. Engine temp will affect it, so how long after shut down tests were done is one possibility.
     
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  3. Targatime

    Targatime Formula 3

    Feb 22, 2014
    1,211
    Los Angeles
    If the numbers remain consistent across all cylinders and have simply increased by the same amount, don't sweat it. It's clearly a testing variance, not a compression problem.
     
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  4. WATSON

    WATSON Two Time F1 World Champ
    Owner Rossa Subscribed

    Sep 9, 2010
    21,610
    WI
    Timing is a big part.

    My car tested 15 PSI lower on the R/H side than the left prior to my to last engine out.
    After the engine was out the R/H side timing was checked and off over 3 degrees.

    Resetting the timing completely balanced r/h and l/h sides within a couple of PSI.

    Key isn't the actual compression numbers per se. As mentioned it is the consistency across the all the cylinders.

    Having said that, if they are all at 100, you might want to regroup. :)
     
  5. m.stojanovic

    m.stojanovic F1 Rookie
    Silver Subscribed

    Dec 22, 2011
    2,743
    Malaysia - KL
    Full Name:
    Miroljub Stojanovic
    Higher engine cranking speed will most likely produce somewhat higher compression readings. Perhaps the new battery turned the engine faster.
     
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  6. Gialllo uno

    Gialllo uno Formula Junior

    Jul 5, 2014
    342
    Reisterstown, Maryland
    Full Name:
    Mark
    If I were to guess a bit I would think maybe a few things possibly contributed, 1. maybe there was a slightly lower ambient temperature that day, 2. The new starter combined with the lower restriction exhaust probably caused the engine to turn over slightly quicker, 3. Maybe a slight carbon buildup which I doubt as it sounds like you are very attentive with your car. You could check with an inspection camera and look into the cylinder with the plug removed. You can get an inexpensive one at Home Depot or Lowes.
     
  7. Ferrarista98

    Ferrarista98 Formula Junior
    Rossa Subscribed

    Jan 7, 2018
    257
    Full Name:
    Jason L
    With the exhaust changes you made and the open exhaust valve, wondering if the engine temp was lower after your 30 mins drive this test than after your last test with stock exhaust. Other would have to weigh in on whether a cooler engine would result in higher compression numbers and your 8% difference.


    Sent from my iPhone using FerrariChat.com mobile app
     
  8. ewn355

    ewn355 Rookie

    Jan 19, 2014
    16
    Thanks for the input. Some feedback to that:
    • I have fairly good control with the engine temperature (via oil/coolant temperature gauges on the dashboard) and the exhaust temperature (I inspect the exhaust (at idle) regularly with an infrared thermometer)
    • The increase in compression is different between cylinders: The average was 15 psi, but varies from 5 psi to 22 psi.
    • Cam timing could play in, but I don’t see such trend (between the banks) in the readings
    • I did a cylinder leak down test at the same time as the first cylinder compression test. Thus, I have the baseline numbers, so performing a new cylinder leak down test could reveal whether the cylinder sealing has changed or not. Will probably do that this summer.
    My top three theories so far:
    1. The “newer” battery delivers more power to the starter, increasing the cranking rpm which results in a higher cylinder compression
    2. Running the engine with the modified exhaust configuration has somehow removed carbon buildup on the exhaust valves. Maybe the open exhaust (cat-less) could initiate a process similar to that of an Italian tune-up.
    3. I’m too confident in the established methodology, overlooking a (unknown) factor which is affecting the measurement and consequently the cylinder compression numbers are inaccurate or invalid.
    Please feel free to debunk.
     
  9. Ferrarium

    Ferrarium F1 Veteran
    Rossa Subscribed

    Jul 28, 2018
    5,624
    Central NJ
    Full Name:
    Eric
    I am not clear, is this something your worried about? What is the worry specifically? I'd be more concerned about the 5 psi to 22 variance than the absolute number.
     
  10. SoCal1

    SoCal1 F1 Veteran
    Owner

    Jun 14, 2011
    8,562
    SoCal LA/OC/New Mexico
    Full Name:
    Tim Dee
    Temperature, barometric pressure, engine temp, battery voltage
     
  11. 26street

    26street Formula Junior

    Jan 30, 2021
    343
    Westchester New York
    Full Name:
    Mark k
    Just wondering are you planning to do another test to see if anything changes— try to keep the same conditions or as close as possible

    The numbers you getting are good and leak down test from what you are saying yielded nothing wrong so good again

    by doing all these test again you can the average it out which will be a good numbers because the testing is showing that already

    Would like to know what you get if you do


    Sent from my iPhone using FerrariChat.com mobile app
     
  12. taz355

    taz355 F1 Veteran
    Owner Silver Subscribed

    Feb 18, 2008
    5,942
    Indio Ca/ Alberta
    Full Name:
    Grant
    From what I have seen the battery makes a huge difference
    You could put an rpm gauge on it this time and next to see what the rpm is while cranking.
     

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