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How low of a compression should I remove engine and do overhaul?

Discussion in 'Technical Q&A' started by mahlerfree, Apr 5, 2021.

  1. mahlerfree

    mahlerfree Rookie

    Dec 19, 2019
    41
    do we have a rough figure when I am supposed to open up the engine?
    100 psi?
    80 psi?
     
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  3. Tegethoff

    Tegethoff Karting
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    Jul 19, 2014
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    Adam
    Compression numbers are widely variable based on engine compression ratio, valve timing, battery voltage, starter age/condition, plugs in or out, throttle open or closed, etc.

    Compression numbers are better compared cylinder to cylinder, to gauge one cylinder against the rest.

    Cylinder leakage test gives a better view to overall combustion chamber sealing, but even that isn't really the litmus test some may think it to be.

    An engine that has perfectly balanced compression numbers, and very low % cylinder leakage may still be in need of overhaul, and yet an engine with poor compression and high % leak may be fine mechanically but have other factors that are causing poor results.

    What are you seeing that leads you to think you may need overhauling?
    Power loss, poor running, exhaust smoke, etc?
     
  4. mahlerfree

    mahlerfree Rookie

    Dec 19, 2019
    41
    The car is misfiring. Initially my mechanics replaced all possible regular items, plugs, coils, etc, and a few sensors. The misfire became worse recently.
    We did compression and leak down. Only problem is that, when doing leak down, the gauge went dead so we only had an esimate by hearing the sound coming out of the oil dipstick hole. 2 cylinders are having a lot greater leakage than the other 6. (forgot to mention , its a 360 modena).
    When warm, compression is ranging from 90-110psi, all cylinders. When cold, compression is ranging from 95-105psi. Funny a warm engine has lower compression than a cold one.
    The car's been extensively raced on track before, so I am guessing that the engine had huge wear&tear.
    so I was thinking if I should keep driving it as is or do an overhaul.


     
  5. Tegethoff

    Tegethoff Karting
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    Jul 19, 2014
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    Are you consuming oil?
    When are the misfires occuring? Idle, heavy load, light load?

    It sounds like you are are looking in the right direction, I would want to know the cause of misfire before condemning the engine.

    Warm engine difference could possibly be fuel thinning out oil on the cylinder wall from a leaking injector, which may be a source of misfire, but that's guessing.
     
  6. mahlerfree

    mahlerfree Rookie

    Dec 19, 2019
    41
    About engine oil. Its a point of confusion for me until this day.
    About 500 miles ago, I spotted oil leaking from the plenum area. Upon checking oil level, I saw the oil is way way over the normal level. ( 180f degree oil temp, idling). I thought I overfilled the oil, alas, I called the mechanic. Answer is no, he specically remember the oil level, he even send me a pic with oil on dipstick saying he kept a record. Then I was thinking of head gasket. Again, that's a no, because my coolant tank is at normal level.
    So I drain 1.5 liters of oil. Funny thing is, when I check again, the level is low(180f, idling). And I added back 1.5 liters, guess what, the oil level is just spot on. check the coolant tank again, same level, no coolant loss.
    So that means, I don't really burn any oil, nor am I getting anything into the oiling system. Something must bump the oil level to a very high position at one point.
    That, I've never heard of.




     
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  8. wmuno

    wmuno Karting
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    The general rule-of-thumb is that the minimum for any one cylinder is 90 psi with a warm engine, all plugs out, and a fully charged battery. Plus, all the readings should be within 10% of each other.
     
  9. Ferrari Tech

    Ferrari Tech Formula Junior

    Mar 5, 2010
    681
    Georgia
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    Wade Williams
    You need to find the misfire issue before condemning the engine itself. I was given an engine to rebuild because of a run condition. It was not diagnosed by me, I just had the skills for the rebuild. Found nothing wrong. After getting it back together and running, I was tasked with finding the issue. As it turned out, the ECUs fire the injectors through a ground. For whatever reason the ground was poor and had the injectors firing lean. The fix was adding ground wires to the ECUs to allow for proper injector firing. All this is just to say, make sure you know why it is doing what it is doing before spending big money.
    The compression number don't sound bad, they are even and not likely to be the issue.
    Can you tell which cylinder is misfiring? With 360s, I have found that any faults for misfire rarely have the correct cylinder errored. Thorough diagnostics can be time consuming but will pay off in the end.
     
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  10. mahlerfree

    mahlerfree Rookie

    Dec 19, 2019
    41
    To ferrari tech and Tegethoff

    My mechanic, not a ferrari one, but in the business for 20+ years, considering the car had 5000 miles of track time, said it is engine mechanical wear&tear after ruling out all other issues, including ecus.
    I had my doubt, but after comprehensive list, I'm begining to believe it. The car only misfires when engine is cold, on random cylinders. Each time the engine was started cold, a random number of cylinders misfire, and there's no pattern to be recognized.


     
  11. Tegethoff

    Tegethoff Karting
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    The misfire issue would be my initial concern, at this point diagnostic time from someone more familiar with Ferraris would probably be less than the sales tax on an engine rebuild, and I would want to know why it's misfiring before paying the expense of an engine rebuild.
     
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  13. Ferrari Tech

    Ferrari Tech Formula Junior

    Mar 5, 2010
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    Wade Williams
    Having been in the Challenge series with 360s, I can say we never rebuilt one because of track time. These engines are well built. Random misfire on start up does not sound like a bad engine to me. It would be worth money to ship it to someone that knows the cars. It could literally be a 100 different things.
     
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  14. GTUnit

    GTUnit Karting

    May 25, 2014
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    Socal
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    Jack
    Misfires can be tough to diagnose. If your mechanic is pointing at the engine then go to another mechanic for a second and third opinion.
    These cars can have specific and unique issues that a seasoned shop who sees many of this model should be able to identify faster and more reliably then a shop that doesnt see these often.

    The most difficult misfire diagnosis I had to make involved flaky injectors. 3 were bad, not just one. Found this using a 2 way OBD2 scanner that had the ability to cycle the injectors while engine off.

    What did they check? How experienced are these mechanics?
    If its a miss then its probably a fuel issue or an ignition issue.

    Did they check spark strength and consistency on all cylinders? If its a ignition issue you should see it in the spark. Unless its a timing problem.
    What injector tests did they perform? Was Fuel pressure checked? OBD temperature sensor readings show correct values?

    Im not a fan of drawing definitive conclusions from compression tests. My F355 had poor compression on one cylinder but when I pulled the heads and did a leak down, the leak down readings were exceptional. Head had carbon deposits on the seats preventing good seating and one valve was slightly bent.
    Cylinder liners were perfect as can be. 40k car.
     

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