Compression Test video

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      I watched the Compression Test video last night from circa 2010. I liked it, but came up with a couple of questions. ‘Thought I would post here to see if anyone had any input/feedback. The part of the video that triggered my question was when Eric did a demonstration of a dry versus wet test and the possibility of different results therein.

      Apart from the larger pressure that was ultimately achieved in the wet test (160 PSI IIRC, versus 150 PSI in the dry test), what I also noticed was that the needle’s jumps along the way to 160 were different too. IIRC in the dry test the first needle stop point was about 90 PSI and in the wet test it was more than that, though I don’t remember exactly what it was now, wanna say 105, but I don’t remember.

      And that got me thinking: are the numbers that are attained at these intermediate “stopping points” (if you will) of the needle while the pressure builds to its ultimate value important? My thinking is that yes, they are. That is, how close the first pressure value (first “stopping point”) is to the last/ultimate pressure value has meaning. So in other words, it isn’t just the final pressure, it is how the cylinder gets there that has importance.

      I mean, if a cylinder has a low first pressure but builds to an acceptable final pressure that’s one thing. But if that cylinder has a higher first pressure before building to an also-acceptable final pressure then that paints a different picture.

      So the question is: is this correct? And if so, how can you put a metric to gauge “how good” a cylinder is based on this data-progression or building of pressure during the test of a single cylinder? Maybe like first number is 50% of the final is ok? 30%?

      And finally, does anyone know of a resource to get the compression values for a whole slew of engines? That is, when you go to do a test on an engine the manual says that each cylinder should produce X-amount-of-pressure (say 180 PSI) with a maximum of 20 PSI difference cylinder-to-cylinder. Is there an online resource/DB that has those numbers for lots and lots of engines or must you really get each metric from each separate manual? My mechanic buddy told me yesterday that Alldata (he calls it “somedata”) doesn’t always have those numbers.

      Anyway, thanks. Appreciate all feedback.


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      EricTheCarGuy 1EricTheCarGuy

        Yes you there are more specific values for the first pass vs the last one. Personally I don’t pay too much attention to that however. The main reason is I look at this test as the engine at a glance. If I see a large discrepancy between cylinders, then I know I’m onto something. But if the numbers are close on all cylinders I know to look elsewhere for the problem. If I see a problem, I remove the schrader valve from the test hose and hook up compressed air to do a leak down test so I can determine where the leak is. Ultimately that’s what you’re looking to find out in the first place. You’re looking to see if you have compression loss, but the next step is to find the cause of that loss. Once I find a problem I know the next step is to either remove the cylinder head or tear down the engine. This is just my method. As I said, you can us other data like the first reading to give you more information. There are also electronic compression testers that give you this information as well as the percentage difference between the first and last readings of the test. Here’s a link.


          Hi Eric,

          Thank you so much for the reply! Funny, but one of the reasons I asked this question is because I have been developing my own digital compression tester. I looked at Innova’s 5612 last night. Interesting design. Has some neat features, but I think their math was a bit off WRT the variance, so I sent them email explaining why. ‘Doubt they’ll reply, but hey, no big thing.

          Anyway, that tester wouldn’t fit the need that I am trying to meet. So I’ll press on with my stuff and perhaps come up with some metric for determining what these numbers mean. Or perhaps I’ll take a page out of Eric’s book and choose to ignore it as a factor in my automated diagnosis code.

          Thanks again!

          EricTheCarGuy 1EricTheCarGuy

            Think of compression testing as a ‘quick check’. The real information is gleaned from a leak down test. Having a compression leak is one thing. Knowing where it’s coming from is what will lead you to the repair.


              Understood, and thank you for the information!

              I asked the question only because I am trying to design my tester with the richest feature set that I can create within the timeframe that I have to work on it. And knowing how to assess individual compression readings within a given cylinder is a part of that.

              Thanks again much!


                Hey Eric,

                I watched your leakdown test video again tonight (saw it for the first time shortly after it came out, dontchaknow). I had a question regarding that tester you used. It seems that all leakdown testers that are on the market come with a regulator for the inlet pressure. I was curious: why is that? Couldn’t you just regulate the inlet pressure at the source, say the compressor outlet? Is the concern the fact that there could be a drop in pressure (albeit somewhat small) between the compressor outlet and the leakdown tester inlet, depending on how long the hose was?

                I saw a couple of videos about how to build one and thought it would make a good Saturday afternoon project, but was wondering about the regulator.


                EricTheCarGuy 1EricTheCarGuy

                  In a way when you set the psi on the compressor you’re setting when the motor shuts off. So whatever you set it at, is what psi is inside the tank when the motor shuts off. In other words, you really can’t set accurate inlet psi that way since you’re doing it after the fact. I hope that makes sense.

                  You want to regulate inlet psi so that one, you know how much psi is going in. This helps you measure how much is being lost when you do the test. Two because you don’t want to add too much psi or the engine will turn over during the test.


                    Ok Eric, thank you for the feedback!

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