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S14 2.5 - "High" Cold Compression Readings

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  • S14 2.5 - "High" Cold Compression Readings

    Compression testing on the bench (cold) of my 2.5 liter S14 engine has produced some "mysterious" values -- has anyone else experienced this?

    Normal values for years were in the 140 psi range when tested hot. I have used the same gauge and hose that screws into the plug hole as before, but in cold testing find the numbers much higher. Tested twice, forgot to block the throttle open the first time so repeated. 1st time came up with 163 psi avg with range of 180-145. 2nd time with throttle blocked open came up with 193 psi avg. with range 197-182 !! Leak down values on Cylinders 1 and 2 are 6-8% and on 3-4 are 12-16%.

    Since the engine had been sitting for some time I was worried about how much oil would be on the cylinder walls so I added a small quantity to each cylinder through the spark plug hole before starting the tests.

    The engine is used in my 1981 320i which has been used for years as a track car running SCCA events at Nelson Ledges and Mid-Ohio mostly with occasional trips to the Glen. It has about 17-18 hrs of track use at the moment. Rebuilt in 2001 after 30 hrs. of track time.

    Any ideas?

  • #2
    Pretty much to be expected. Cold oil does a great job of sealing worn rings.


    • #3
      140 is quite low. Norm for most 2.5's depending on pistons/rings/CR is 185-200+. I suspect you have a bad tester and possibly stuck or as Ironhead suggested worn rings.
      Ron ///Man

      • '91 Gr-A Former CiBiEmme / Ravaglia - Sold
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      • '89 M3 M3T / ITR Club Racer
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      • '09 M3 - Tarmac Terrorist
      • '04 330Xi Sport 6 Speed - Snowmobile
      • '07 530 Xi - Highway Star


      • #4
        What Ron said!

        thats low.

        my stock rebuilt was 200-205 last time I tested a couple years back


        • #5
          Thanks All,

          Appreciate the comments ------ have ordered a new MityVac Digital Compression Tester kit. Will retest later in week and advise results.



          • #6
            Is it possible that the 140 numbers were forgetting to open the throttles, and the 193 avg. this time is the accurate number?
            "It is needless to say that self-propelling vehicles, like other machines, will never do as much for one who does not understand them as for one who does."


            • #7
              Completed the "wet" compression tests (Mityvac term), using the new Mityvac MV5532 Digital Compression Test kit. Avg values are now 237 psi and high to low 243.5 - 231 psi (making sure throttles were full open). Further internet scouring seems to turn up an expected Wet-Dry difference of on the order of 10% higher for Wet. That would give me an "expected " 214 psi for Dry avg. -- more in the typical range for this engine as listed by other members here.
              Based on this data I am now planning to reinstall the engine and then run another "Dry" test for final confirmation. It would appear the engine is in reasonable shape which is what I expected based on the run hours compared to previous rebuilds.
              BTW -- the Mityvac kit is very professional. 1. It includes many adapters and reach lengths including 18 mm, 12 mm and 10 mm as well as the new Ford 16 mm Triton. 2. It stores the cylinder readings so you can run the complete test, then come back and record data. Readings can be taken in PSI, BAR, and kPA. 4. The kit also includes a Pressure Hold attachment which would allow replacement of valve seals without head removal.
              I will post the Dry readings when I get to that stage of work.
              Thanks for the help.


              • #8
                Now you are on a roll, why not do a leakdown test.


                • #9
                  Ah yes, good point. -- I have already completed the leak down tests and listed in my initial post. "Leak down values on Cylinders 1 and 2 are 6-8% and on 3-4 are 12-16%." These values were obtained in two tests and do tend to confirm reading of compression. Cyl 1-2 were 241.5 and 243.5 and the compression values for 3-4 were 231.0 and 232.0 - a bit lower and the leak downs are higher in those two cylinders. I believe the higher leak down and lower compression in 3-4 are"ring" related -- I could hear leakage at the oil fill cap when testing 3-4. We will see how the dry values come out and I will retry the leak down with the engine warm as well.


                  • #10
                    Regarding the leakdown test, do you have an EVO gear installed on the exhaust which retards the timing? Could impact your leakdown values as both cams are not a TDC at the same time.

                    Comments or Opinions?


                    • #11
                      Engine built by Kermit Upton of MASR in Vermont -- using his cams. Don't remember ever hearing any discussion of this feature. Will have to ask.