PSA: Low Compression Fix Adventure

Discussion in 'Mazdaspeed 3/6 Engine' started by HawkeyeGeoff, Oct 17, 2016.

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  1. brainbot1

    brainbot1 Greenie Member

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    I got the car in late April and did notice a power loss after a month. 3rd and 4th gear weren't as punchy as when I first got the car. Did my first compression test in August and everything has been consistent since. I switched over from the free Cobb ots to a conservative tune by dizzy after the first compression test and no noticeable changes since and the compression numbers would agree. Wish I did one before I bought the car. I still want to try meth before replacing the bottom end as there was some improvement though not significant

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    brainbot1, via a mobile device, Nov 17, 2016
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  2. VTMongoose

    VTMongoose John/MD1032 Platinum Member

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    As far as we know he never had good compression to begin with, so, nothing to compare to. Car was bought used <1 year ago. Probably traded in due to low compression like many speeds. I'm sure the motor is down on power with the numbers he has, but still works fine and isn't blowing out the dipstick or anything weird like that, or consuming any oil.

    edit: Also a note on the compression tests, they weren't done at a consistent engine temperature, hence the variability.
     
  3. Redline

    Redline I done fucked up for the last time. BANNED Greenie Member

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    Ahhh, makes sense. Yeah, gotta control variables to track differences.

    I've always been interested on the actual impact of compression on power, quantitatively speaking. For instance, a 175 on all 4 vs a 195 on all 4 comparison. I've heard lots of tuners talk about "new, tight" engines making power easily, but never seen actual figures of comparison.
     
  4. HawkeyeGeoff

    HawkeyeGeoff MSO Chicks Greenie Member

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    I have not really done any logs since I did this....will probably do some this weekend to see where the vehicle ended up.
     
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  5. VTMongoose

    VTMongoose John/MD1032 Platinum Member

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    To some degree, on two identical, healthy engines, I wouldn't think it would be realistic to see a 20 psi difference across the board like that...it seems outside the scope of simple parts tolerance. Maybe @AYOUSTIN can comment.

    FWIW, on my own engine, I haven't seen compression change over time. Of course, I've only put about 70k miles on it since the first time I measured it, too.
     
  6. HawkeyeGeoff

    HawkeyeGeoff MSO Chicks Greenie Member

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    You also have a 2nd gen motor; these just hold up way better. My 2nd gen had 100k of hard hard miles on it....180 across the board. My 6 has 61k miles....not so great.
     
  7. AYOUSTIN

    AYOUSTIN Silver Member

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    There's really too many variables to determine reasons for some engines lasting longer than others.

    Another factor is gauge calibration variances.

    But anyway, the reason why you won't see a huge difference on power vs compression (but there is one) might have to do with the fact our engine is boosted and cylinder pressure will be influenced more from boost than piston compression. Think of it as an exponent, compression is your base number and your boost is your raise to value. There's gains from increasing either but moreso from boost.

    Another thing for your mind to chew on is that engine compression is a ratio and is inversely exponential. You'll have larger gains moving from a 7:1 to an 8:1 than you will from moving from a 10:1 to an 11:1.
     
    AYOUSTIN, via an iPhone, Nov 18, 2016
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  8. Redline

    Redline I done fucked up for the last time. BANNED Greenie Member

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    Right, I hear ya. What I'm trying to gain better understanding on is, say for instance, you have setup A on a car with 175, 175, 175, 175 compression.
    They you have setup B (which is identical in every single way - no other variables in the hard parts, tune, mileage, or anything else), but it has 195, 195, 195, 195 compression. I wonder what, if any, power difference would be seen, and if so, how much?
     
  9. HawkeyeGeoff

    HawkeyeGeoff MSO Chicks Greenie Member

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    Only way to tell is by empirical data my man. No two vehicles are the same so it would be very very hard to get this data; really you'd have to have a pretty decent sample size of very similar vehicles to make this data happen in any reliable way.
     
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  10. Redline

    Redline I done fucked up for the last time. BANNED Greenie Member

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    Indeed. The perfect way to tell is by a laboratory-level standard of complete control of all variables excluding the independent variable being tested, in this case compression. Then we analyze the dependent variable (power production) to see the correlation. Unfortunately, I don't think we'll ever get such a scientifically rigorous and valid test :(

    That's why I'm curious. Anecdotal data is very sketchy, at best. I'd think it'd have an impact, and I've read a ton of threads where the issue came up, even if not the main focus, but in my experience, we really have no substantiation either way on this, especially not on a quantifiable level.
     
    Last edited: Nov 18, 2016
  11. ALPINEST4RS

    ALPINEST4RS Greenie Member

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    If you had more than one low cylinder I wouldn't put my money on this method or a severely low cylinder. Knowing the owner of this MS6 before Geoff, I knew the ringlands weren't going. The car was babied (which can also be bad for a DI).

    Honestly, 165 isn't the end of the world. 135 and below I'd start to get worried. You are still in MIN spec depending on your cranking RPM. I have a graph from Mazda for plotting psi per cranking RPM. Totally understand that everyone prays for 180 when they start cranking though.

    180 psi on a "worn in" engine is pretty good and pretty rare in my opinion. Depending on mileage of course. Typically I see anywhere from 155-175 on a 100k disi untouched. Car runs completely fine and boosts well.

    If you're burning oil excessively or having a lack of performance then yes, it needs to be fixed asap.

    EDIT: I missed the second page of this whole thread. Apologies.
     
  12. Redline

    Redline I done fucked up for the last time. BANNED Greenie Member

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    Indeed. My last hot test (only way to do it, IMHO) was 190, 186, 189, 180. Pre methanol spraying, I was always 180 +/-2 on all 4, and I always test the exact same way (5 or 6 times, so far). Looks like that methanol freed up the rings or something and restored compression. I thought I felt more power, but wasn't sure. Compression rising seems consistent with this. I have 71k on the motor and have been BNR S3'd @ 385/380 since 42k.
     
  13. HawkeyeGeoff

    HawkeyeGeoff MSO Chicks Greenie Member

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    Update on this: almost 2 years later of not checking compression on this engine....

    It now sits at 190, 188, 178, 190
     
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  14. Flyhigh

    Flyhigh Silver Member

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    Compression is still decent and Redline is gone...Nice!
     
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  15. John

    John Full Fledged Member Platinum Member

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    Interestingly that's higher on all cylinders, and a touch more even than when you measured post-soak in Feb 2016. Seems like you definitely brought the thing back from near-death, and like it even may have continued improving after you broke 'em free then re-broke 'em in.

    Just for interest, what oil was run before/after the soak job? I'm wondering if it was something like T6 before, Plat after or something and the supposed additional cleaning pwar of the Plat (which I'm going to assume is true because dexos) helped improve it gradually after the test.
     
  16. Orangatang

    Orangatang Greenie Member

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    If you are interested at all there is bg109 I ran it in my car 7.5k ago with still no issue currently at 207.5xx k miles. (Edit: valve have also never been cleaned.)
    copy and pasted this from another post I did.
    If I am making a new thread about compression test that has already been created then my bad. I tried looking and couldn't find it.
    This was back on 5-18-2014
    "Just did a compression test. Review, stock turbo, engine, clutch & flywheel.
    156k miles.
    compression is(I redid the test on another cylinder after wards to make sure the gauge was correct. Came back the same.)
    180
    175
    170
    182

    Still on stock turbo, motor, trans, clutch and flywheel. Ok so I Did a dry compression test. It was around 110 today and went on a short drive. From passenger side to driver side, 180, 179, 179, 180. Only did a dry test mileage at 200776. Didn't feel like doing a wet test even though I was right there. At around 197k I think I ran a can of bg109 through the car to see if it would do anything. I only ever run synthetic oil (5w-40) and 3-4k oci.
     
  17. HawkeyeGeoff

    HawkeyeGeoff MSO Chicks Greenie Member

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    I was and have been using Pennzoil Plat. Euro 5w-30 or 0w-40. Whatever I found. Car burns zero oil at 3.5k oil changes.

    I think the most logical explanation is the rings basically re-carboned themselves and brought the compression back up in all the cyls because when they were decarbonized the compression went down to normal levels in all the cyls besides #3. Just a guess though.
     
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