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  • Cranking Compression Values

    We were on the subject the other day and I quoted from a 1996 (as I recall) published Merc. Service Manual for 60 Jet through 125 HP 3 and 4 cylinder Mercs of that and later era. I stated that the manual said that one COULD (nice rubber word) expect to have problems if below 120 PSI or over 15% variance between cylinders. One guy who is in the know and posts regularly here said he never saw that. I try to annotate my sources/references (including...opinion, best guess, I think) when that's what they are. I gave away the manual with my last boat but I now have 2 newer manuals, one covering carbureted engines of the (2002 era my interest) and the other covering direct injection engines. So from the two current Service Manuals:

    First is the carbureted manual, Document number...lower left corner of page, dated Nov. 2015, covering 75 Marathon and 75 Sea Pro plus the same engines listed above, from 1994 up :90-8MO110566. Page 1C-19, titled Compression Check:
    Warm up engine
    Hold throttle plate at WOT....(Gotta get some air in the cylinders if you expect to compress it. My addition)
    All plugs out
    Crank engine till peak reading obtained...(didn't mention that this info used to contain: At 200 rpm or better aka fresh battery)
    Record data:
    "All within 15 psi of each other" or "A reading below 120 PSI might indicate a total engine wear problem." (Notice the rubber word MIGHT...my addition again.)

    The Injected V6 Serv. Man. #90-859494 dated May 1999 for 115, 135 and 150 Optimax , Page 1C-5 states: Same basic setup for test as above but did add using a fully charged battery notation.

    "Normal compression for all cylinders should be 110-130 PSI. Cylinders should not vary more than 15 PSI between one another. A variance of more than 15 PSI would indicate the need for a power head inspection/disassembly."

    Still no indication anywhere of what a new engine coming off the assy line and in initial test/setup measures or one just broken in.

    HTH

    PS: While we are on the subject of manual verbiage, they have added paragraphs about where to run your RPMs, effect of trim position when checking, effect of altitude, load, dirty bottom, and temperature on WOT performance and corrective action (prop pitch changes) you are to take if below the min RPMs. Wink! Must have been reading boating forums!
    Last edited by Texasmark; November 10th, 2019, 04:58 PM.
    If you are new to boating or have a new boat, a knowledgeable friend could show you how to operate your boat and save you a lot of grief, maybe some money, and maybe your life.

  • #2
    Since I doubt any of the Carbed and EFI 2 strokes have Compression Ratios more than 8:1, if even that, for a compression ratio, I have trouble believing posts where guys are saying their engines are showing 140 - 150 psi(that was here on the merc Forums. Who? I can't remember, nor am I going to spend time looking for it.) Air pressure at sea level is roughly 14.7 psi. A 2:1 ratio would yield 29.4. 3:1 44.1, 4:1 58.8, etc. To have 150 psi would require more than a 10:1 ratio. Factor in many boaters are not at Sea Level, so the numbers will be lower, in which case "a Specific low psi is still not absolute" when it comers to stating there is a problem.
    Maybe the guy with the 150 PSI was doing the test at Death Valley, it's what, 280 ft below Sea Level? Naw, Atmospheric Pressure there, 15.005 psi, not much greater.

    The newer Higher Performance 4 Strokes might be a different story. I know a lot of Car/Truck engines for the past 25 yrs have 10.5:1, 11.5:1, and higher Mechanical Compression Ratios, that due to valve timing, overlap, and Lobe separation, result in lower more manageable Dynamic Compression Ratios when running, allowing low octane fuels to be the spec
    Last edited by jimmbo; November 10th, 2019, 05:58 PM.

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    • #3
      Throttle does NOT need to be open on that 75 HP 2 stroke.----It just does not need to be open !

      Comment


      • #4
        Originally posted by racerone View Post
        Throttle does NOT need to be open on that 75 HP 2 stroke.----It just does not need to be open !
        For what it's worth, I never did a compression check on any vehicle with the butterflies forced open and have been satisfied that my results represented the condition of that particular engine. Just didn't think about it. But it does make sense.
        If you are new to boating or have a new boat, a knowledgeable friend could show you how to operate your boat and save you a lot of grief, maybe some money, and maybe your life.

        Comment


        • #5
          On a 2 stroke when the piston is at the bottom dead center, the intake and exhaust ports are WIDE OPEN.------The cylinder can only be full of air.-----When piston come up it closes intake ports first.-----Then after exhaust ports close the trapped air is compressed !----No need for throttle plates to be open for compression test on a 2 stroke.

          Comment


          • #6
            Originally posted by racerone View Post
            On a 2 stroke when the piston is at the bottom dead center, the intake and exhaust ports are WIDE OPEN.------The cylinder can only be full of air.-----When piston come up it closes intake ports first.-----Then after exhaust ports close the trapped air is compressed !----No need for throttle plates to be open for compression test on a 2 stroke.
            Good point. Didn't think about that and apparently the reason it has worked for me....and surely others. 👍
            If you are new to boating or have a new boat, a knowledgeable friend could show you how to operate your boat and save you a lot of grief, maybe some money, and maybe your life.

            Comment


            • #7
              Originally posted by jimmbo View Post
              Since I doubt any of the Carbed and EFI 2 strokes have Compression Ratios more than 8:1, if even that, for a compression ratio, I have trouble believing posts where guys are saying their engines are showing 140 - 150 psi(that was here on the merc Forums. Who? I can't remember, nor am I going to spend time looking for it.) Air pressure at sea level is roughly 14.7 psi. A 2:1 ratio would yield 29.4. 3:1 44.1, 4:1 58.8, etc. To have 150 psi would require more than a 10:1 ratio. Factor in many boaters are not at Sea Level, so the numbers will be lower, in which case "a Specific low psi is still not absolute" when it comers to stating there is a problem.
              Maybe the guy with the 150 PSI was doing the test at Death Valley, it's what, 280 ft below Sea Level? Naw, Atmospheric Pressure there, 15.005 psi, not much greater.

              The newer Higher Performance 4 Strokes might be a different story. I know a lot of Car/Truck engines for the past 25 yrs have 10.5:1, 11.5:1, and higher Mechanical Compression Ratios, that due to valve timing, overlap, and Lobe separation, result in lower more manageable Dynamic Compression Ratios when running, allowing low octane fuels to be the spec
              Glad you worked through that. Never took the time to look into compression parameter maxes. I think I read somewhere 2 strokers run around 9:1 compression so that and sea level (if at sea level) atmospheric pressure sets the max one would expect coming off the assembly line. That answers that.

              So, at my 600' for slightly less than a 132 PSIG max. and actuals of 129 PSIG x 3 + 130 PSIG on 115 HP with PTT and a 22P Ballistic, SS (which I'll probably sell) tells me I got a heck of a rig with new radial tires, 3 new batteries, 72# ™, engine through the shop and quality brands of B, M, T, for $5k. Bad news was a few soft spots in the deck which weren't a problem to fix and the titles were lost but I had the time (4 months) for the dealer to work through getting all the paper work in my name and all is well. 😉
              If you are new to boating or have a new boat, a knowledgeable friend could show you how to operate your boat and save you a lot of grief, maybe some money, and maybe your life.

              Comment


              • #8
                Many Johnson / Evinrude motors 2 and early 3 cylinder models show 140 / 150 PSI on my gauge.

                Comment


                • #9
                  I know a lot of OMC engines in the late 50s and thru the 60s were in the 5:1 to 6:1 compression ratio range. The loopers were probably a bit higher, but not by much. Gasoline's octane rating drops when the Oil is mixed with it, and all engines were speced for regular or White Unleaded gas, which was lower in Octane than the leaded gas of yesteryear, so the compression ratio couldn't been too high or Detonation would have occurred.Some compression gauges have been known to read low, so I suspect some may have been reading high.

                  Comment


                  • #10
                    I have 2 compression testers.------They are consistent / accurate !!

                    Comment


                    • #11
                      Lets get to the bottom of this..... so guys, keep up the momentum. As we all know, stuff doesn't just happen, it's caused. I for one would like to know "The Rest of the Story" (Paul Harvey, news caster).
                      If you are new to boating or have a new boat, a knowledgeable friend could show you how to operate your boat and save you a lot of grief, maybe some money, and maybe your life.

                      Comment


                      • #12
                        The T2 I had after boring had 140+psi..A lot of the early inlines with high dome pistons ranged for 130-150psi static. The mid 80's V-6 with hemi head had 125psi.
                        When in doubt,Trim it out!!!
                        Business Owner & Certifed Technician
                        Boats I own & use
                        20ft Javlin w/250 Junkrude
                        17ft Bullet w/200 Merc (SOLD!!)
                        13ft Allison w/150 I-6
                        17ft Hydra-Sport 175GT
                        15ft Allison w/75 Stinger(project)
                        14 x 48 Drifter with 1100SS Merc

                        Comment


                        • #13
                          Originally posted by Faztbullet View Post
                          The T2 I had after boring had 140+psi..A lot of the early inlines with high dome pistons ranged for 130-150psi static. The mid 80's V-6 with hemi head had 125psi.
                          Thanks Faz. So one could assume the ratio was high enough to support those numbers....yes? Do you recall the fuel requirement? WOT timing up over 20 BTDC?
                          If you are new to boating or have a new boat, a knowledgeable friend could show you how to operate your boat and save you a lot of grief, maybe some money, and maybe your life.

                          Comment


                          • #14
                            Originally posted by racerone View Post
                            On a 2 stroke when the piston is at the bottom dead center, the intake and exhaust ports are WIDE OPEN.------The cylinder can only be full of air.-----When piston come up it closes intake ports first.-----Then after exhaust ports close the trapped air is compressed !----No need for throttle plates to be open for compression test on a 2 stroke.
                            At last, someone who agrees with me on this. Almost fed up hearing about folk suggesting the throttle needs to be full open on a 2 stroke for a compression test. Presumably by those that just don’t understand their workings.
                            only exception to the rule, I might mention, would be on some jet ski 2 stroke, when holding the throttle full open holds the fuel off.

                            Comment


                            • #15
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                              Originally posted by Texasmark View Post
                              We were on the subject the other day and I quoted from a 1996 (as I recall) published Merc. Service Manual for 60 Jet through 125 HP 3 and 4 cylinder Mercs of that and later era. I stated that the manual said that one COULD (nice rubber word) expect to have problems if below 120 PSI or over 15% variance between cylinders. One guy who is in the know and posts regularly here said he never saw that. I try to annotate my sources/references (including...opinion, best guess, I think) when that's what they are. I gave away the manual with my last boat but I now have 2 newer manuals, one covering carbureted engines of the (2002 era my interest) and the other covering direct injection engines. So from the two current Service Manuals:

                              First is the carbureted manual, Document number...lower left corner of page, dated Nov. 2015, covering 75 Marathon and 75 Sea Pro plus the same engines listed above, from 1994 up :90-8MO110566. Page 1C-19, titled Compression Check:
                              Warm up engine
                              Hold throttle plate at WOT....(Gotta get some air in the cylinders if you expect to compress it. My addition)
                              All plugs out
                              Crank engine till peak reading obtained...(didn't mention that this info used to contain: At 200 rpm or better aka fresh battery)
                              Record data:
                              "All within 15 psi of each other" or "A reading below 120 PSI might indicate a total engine wear problem." (Notice the rubber word MIGHT...my addition again.)

                              The Injected V6 Serv. Man. #90-859494 dated May 1999 for 115, 135 and 150 Optimax , Page 1C-5 states: Same basic setup for test as above but did add using a fully charged battery notation.

                              "Normal compression for all cylinders should be 110-130 PSI. Cylinders should not vary more than 15 PSI between one another. A variance of more than 15 PSI would indicate the need for a power head inspection/disassembly."

                              Still no indication anywhere of what a new engine coming off the assy line and in initial test/setup measures or one just broken in.

                              HTH

                              PS: While we are on the subject of manual verbiage, they have added paragraphs about where to run your RPMs, effect of trim position when checking, effect of altitude, load, dirty bottom, and temperature on WOT performance and corrective action (prop pitch changes) you are to take if below the min RPMs. Wink! Must have been reading boating forums!
                              Hi texasmark

                              i have a feeling that the reason there is seldom info about new off out the factory compression values, is because they will likely be varied and more crucial, quite low in value, compared to what they would be once the engine has broken in.

                              having had more than a few 2 stroke old motors in bits for various reasons...from proactive maintenance rebuilds to total failure rebuilds. From small outboards to howling 2 stroke Yamaha jet ski engines...I must say that I’ve found the following to be true

                              unless your gauge is calibrated every year, never trust it, other than to give an indication in the ball park and also for indication of being within tolerance of each other cylinder. I’ve known gauges to vary by up to 40 psi to others, within 5 minutes of tests.

                              also I’ve noticed a fair difference in compression with different oils and mixing ratios. Especially when doing a test on a warmed up oil injection engine and a pre mix test straight after off a separate tank (with a heavier mix).

                              battery condition makes a huge difference too.

                              With these particular engines you mention. I’ve only had one in bits. That was a ‘96 merc 90hp. Compression was in my mind healthy at anything over 110 psi on a trusted gauge. Anything below 100 would be thinking about a top end perhaps. Or at least a retest on a different day and different gauge.

                              but for certain, on these motors, it wouldn’t matter much about throttle position. Although it becomes a habit to open the throttle on any compression test, I suppose.

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