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Thread: 2.0L m10 compression ratio..

  1. #1
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    2.0L m10 compression ratio..

    Riddle me this. I've been doin a little math to verify my supposed lower than 8.1:1 compression ratio. I have a 2.0L with flat top pistons and the 1.8L head.

    Ok, preliminary data:
    The 2.0L has flat top pistons, 60cc combustion chambers, and is listed to have 8.1 : 1 CR. The 1.8L has domed pistons, 63cc combustion chamber, and is listed to have 8.8 :1 compression. We all know 1.8 and 2.0 rods are the same and the pistons are not because the compression height is 4.5mm off due to the 80mm stroke vs 71mm. When I assembled my motor, my pistons came to 0 deck height at TDC.

    So, last night I very carefully measured the dome on my old 1.8L pistons, and I calculated an 11 cc dome. For the 1.8L, assuming an 89mm bore, 71mm stroke, 63cc chamber, 11cc dome, .75mm thick head gasket (maybe a little thin, but that's what gave me good numbers), I come very close to the factory 8.8 : 1 CR.

    But for the 2.0, assuming 89mm bore, 80mm stroke, 60cc chamber, 0cc dome, .75mm head gasket, I calculate 8.7 : 1. WTF? Why is it listed as 8.1:1? Is my information wrong?

    For my motor, assuming 89mm bore, 80mm stroke, 63cc chamber, 0cc dome, .75 gasket, I get around 8.36:1 CR. I calculate a 565.36cc cylinder volume at BDC and a 66.67cc cyl volume at TDC (quench volume?). Divide vol @ BDC by vol @ TDC to get CR.

    I found a Compression Calculator, and it gives me the same numbers. The numbers don't seem to add up. Anyone have any ideas? Am I missing some mundane detail?
    Last edited by jrcook320; 10-26-2010 at 10:33 PM.

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  2. #2
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    The carby 2.0 had a lower comp if that helps

  3. #3
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    We didn't have those in the US... The issue is that my calculations match specs for the 1.8L, but not for the 2.0L.

    '81 320i turbo | t25, 931 CIS, 240hp, 13.92@100mph | 2.2L m10 Turbo Build | My E21 Videos |

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    did you take into account the height difference from wrist pin to piston deck?
    i can't remember the exact figure, but it's a pretty significant difference between the 1.8 & 2.0 pistons.

    in otherwords, the difference in piston displacement (from the wrist pin on up) between the 2 pistons is more than just 11cc's.

    plugging in -5 or -6cc's for the 'piston dome' figure gives about the correct CR # for the 2.0 motor. which means w/ the 1.8 head, your motor should be around 7.8:1. sounds like a very nice potential boost happy setup

  5. #5
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    No, the 2.0L is not listed to have 8.1-1. It's 8.3-1. Dont know if that helps...

    waferman

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    Quote Originally Posted by jbob
    did you take into account the height difference from wrist pin to piston deck?
    i can't remember the exact figure, but it's a pretty significant difference between the 1.8 & 2.0 pistons.

    in otherwords, the difference in piston displacement (from the wrist pin on up) between the 2 pistons is more than just 11cc's.

    plugging in -5 or -6cc's for the 'piston dome' figure gives about the correct CR # for the 2.0 motor. which means w/ the 1.8 head, your motor should be around 7.8:1. sounds like a very nice potential boost happy setup
    The compression height? Yes. I thought both the pistons came to 0 deck hieght @ TDC on both motors. If this is true, and the rods are the same, the compression height difference between 2 pistons would have to be 4.5 mm to account for the 9mm increase in stroke. To have a -5 to 6 cc "piston dome", either the 2.0 pistons are dished (and I have 2002 pistons), the deck hieght of the 2.0L block is slightly higher than the 1.8, or the chamber volumes I have aren't right. But, like I said, my pistons come to 0 deck height. There's no -5 to 6 cc's there.

    Quote Originally Posted by waferman
    No, the 2.0L is not listed to have 8.1-1. It's 8.3-1. Dont know if that helps...

    waferman
    If you can show me where you found that info, that'd be great. Jeroen has it listed at 8.1 : 1, for one source. Either way, I calculate 8.7: 1 for the 2.0, so that's no help.

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  7. #7
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    My e21 casting head has 62cc chambers and it has been milled slightly twice so your 63cc is close.

    I'm sure the 1.8L casting head has chambers larger than 60cc as it is quite a bit more open than the e21 casting. I have a 1.8L on my bench now cleaned and about to get ported-once it's assembled I will cc the chambers and know for sure but that won't be for a while.

    My engine calculator shows 89mm bore/80mm stroke/.75mm gasket compressed thickness/63cc chambers/.000" deck height and no piston dome- comes out to 8.5-1. With 65cc (I think that's what 1.8L heads are) chambers that drops to 8.1-1.

    ken

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    Jcrook, I could have sworn I read in the Haynes manual that it is 8.3:1, but online pistons for sale say 8.1:1. Here is another website with some info:
    http://www.bimmers.com/02/

    Also, this is lifted from an old E21 Digest post from one of the guru's back in Y2K:

    Begine Quote**Hence there is NO such thing as a E21 or E12 or E30 HEAD!

    Unfortunately, this is plain wrong. Here is some info:

    Head Name Volume Models
    121 58cc 2002, 1972 and earlier
    E12 61cc 2002, 1973 to 1975
    E21 60cc 2002 & 320i, 76-79
    18i 63cc 320i, 318i, 1980 on

    Each of these heads flows differently, and are used for different purposes in modified cars (volume, compression, valve side, intake vs exhaust flow). They also each have their own weak spots and common failure modes. These are easy to identify because these actual numbers are cast in metal in the side of the head. Go look at your car just above the #4 intake port!

    For a nice set of pictures and descriptions of these different heads, look at the Metric Mechanic online catalog at http://www.metricmechanic.com.

    The Engine info page of the 2002 website at: http://www.bimmers.com/02 has a nice table which shows the various compression ratios available using different OEM and aftermarket pistons with the various heads.

    To quote this page:
    "The 2002 engine was offered with four different cylinder heads during its 8 year production life. When building a high performance 2002, there are certain heads that may lend themselves to higher horsepower applications. Choosing among the various available options is dictated primarily by the intended application. All 2002 cylinder heads are designated by a number (either 121, 121TI, E12, or E21 2.0) which is cast into the head. The location of this number is on the intake side of the cylinder head, above the number four intake
    port. It is easily visible upon opening the hood. "

    Another interesting tidbit:
    "1976 49-state 2002's came with a further revised E21 2.0 head, which also carried over to the 2-liter 320i's. It is basically an open-chamber modification of the old 121/121TI head, but with 46mm intake valves."

    This naming convention is no doubt confusing and annoying, and certainly most times when someone says "E12" or "E21" they are talking about BMW internal model series codes, but in this case, they are talking about head model codes.

    Thanks, Dave**end quote



    waferman

  9. #9
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    great info waferman

  10. #10
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    jr - I am curious how much boost you plan on running wanting a CR lower than 8.1:1? It will kill off-boost response the lower the CR is as you probably know. With such a beefy engine base to build on, I wouldn't want any less than 8.5:1 as long as the ignition is taken care of well. Unless you are tlaking 25-35psi of boost, in which case you should have forged pistons anyway. Not ragging, just curious, as I am working along the same lines, but only planning 10-14psi max.

  11. #11
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    I don't want any lower than 8:1, I was pleased to find that I may have a little higher than that, but I wouldn't want to go over 8.5:1. I just wanted to know for sure what it was to aid in the planning of the setup. I am planning on 8-10 psi for my first setup. I've never done a turbo before so its all new to me, i'm learning as I go.

    Do you guys think .75mm compressed gasket thickness is too thin? Even if the E21 head has larger chambers like Ken measured, it takes a 1.2mm gasket to get 8.1:1

    With a ~1.65mm thick head gasket I get a 8.1 CR for the 2.0 with 60cc chambers, but then I also get 8.1 CR for the 1.8L with 11cc dome and 63cc chamber. It looks like it would take about a 15cc dome for the numbers to jive between the 2.0 and 1.8, but I measured several times, and variance in my measurements yeilded very small variances to the calculated volume of the dome. So, why don't the numbers between the 1.8 and 2.0 jive?? Thats the question.

    I guess whether I have 7.9:1 or 8.3:1, it will be little difference, but I want to know!

    '81 320i turbo | t25, 931 CIS, 240hp, 13.92@100mph | 2.2L m10 Turbo Build | My E21 Videos |

  12. #12
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    Quote Originally Posted by jrcook320
    I don't want any lower than 8:1, I was pleased to find that I may have a little higher than that, but I wouldn't want to go over 8.5:1. I just wanted to know for sure what it was to aid in the planning of the setup. I am planning on 8-10 psi for my first setup. I've never done a turbo before so its all new to me, i'm learning as I go.

    Do you guys think .75mm compressed gasket thickness is too thin? Even if the E21 head has larger chambers like Ken measured, it takes a 1.2mm gasket to get 8.1:1

    With a ~1.65mm thick head gasket I get a 8.1 CR for the 2.0 with 60cc chambers, but then I also get 8.1 CR for the 1.8L with 11cc dome and 63cc chamber. It looks like it would take about a 15cc dome for the numbers to jive between the 2.0 and 1.8, but I measured several times, and variance in my measurements yeilded very small variances to the calculated volume of the dome. So, why don't the numbers between the 1.8 and 2.0 jive?? Thats the question.

    I guess whether I have 7.9:1 or 8.3:1, it will be little difference, but I want to know!

    Hmm. I know the m20 guys boost at both levels, and there are really no problems since both 7.9:1 or *8.3:1 IS considered relatively low compression.
    So the point is - dont worry about it. CR is what Nic and I have talked about. Its seems to be something of an engima.
    There are some people that boost a lot on super eta engines which has around the same, so either one is doable.

    What size turbo and how much boost are you planning on running?
    What engine mgt software? What Hp do you expect?
    Last edited by jjgbmw323; 02-11-2006 at 11:19 AM.
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  13. #13
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    Quote Originally Posted by jjgbmw323
    Hmm. I know the m20 guys boost at both levels, and there are really no problems since both 7.9:1 or *8.3:1 IS considered relatively low compression.
    So the point is - dont worry about it.
    Thanks for making the point. But no crap Holmes, I just said:
    Quote Originally Posted by jrcook320
    I guess whether I have 7.9:1 or 8.3:1, it will be little difference
    This is what I want to know:
    Quote Originally Posted by jrcook320
    So, why don't the numbers between the 1.8 and 2.0 jive??
    Engine management? What's that? I'm going to run K-jet on my first go around and use an MSD BTM box for ignition.
    Last edited by jrcook320; 08-01-2008 at 11:34 AM.

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  14. #14
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    Quote Originally Posted by jrcook320
    Thanks for making the point. But no shit Holmes, I just said:

    This is what I want to know:
    So, why don't the numbers between the 1.8 and 2.0 jive??
    Engine management? What's that? I'm going to run K-jet on my first go around and use an MSD BTM box for ignition.
    Well Sherlock my thoughts are have stroked you engine slightly with the 2.0 crank and pistons. And my quess its this with the head gasket factored in.
    Last edited by jjgbmw323; 02-11-2006 at 02:32 PM.
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  15. #15
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    Here's an idea. Write in to the QandA section of Bimmer Magazine. Mike Miller owns his own shop somewhere, and is very knoledgable on...everything. When he doesnt know something, he tracks down the info from people like Korman, Metric Mechanic, BMW NA, etc. Seriously, give it a shot. He has answered 2 questions for me. Who knows, it might make it into the magazine's printed Q and A.
    -Waferman

  16. #16
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    are you sure the flat-toppers are flush with the deck? If they were lowered a tad it would be like having a dished piston. I only deal with 10+:1 though so I cant help

  17. #17
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    why do i have done top pistons in my e21 M10??

    im carby with the e21 20 head, and when i did my head gasket - i saw nice clean dome tops!!

  18. #18
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    Quote Originally Posted by tlmitf
    why do i have done top pistons in my e21 M10??

    im carby with the e21 20 head, and when i did my head gasket - i saw nice clean dome tops!!
    who knows? take it apart and find out You either have a 1.8L crank or someone shaved the dome off the pistons.

    waferman

  19. #19
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    Well... Euro and other non-U.S. markets I presume got domed pistons that look like this:


    U.S. got flat topped pistons.

    Using headgasket thickness to set compression ratio is the wrong way to get where you want to be-cheaper but wrong... You want your piston to come within about .040" of your cylinder head to promote turbulance. Even the open chamber 1.8L casting has a small quench pad that should be utilized to it's best advantage. Increased deck height (piston top to cylinder head clearance) makes the engine MUCH less efficent and more prone to detonation-not what you want in a boosted application.

    The only way you will know your actual static compression ratio is measure YOUR combustion chambers, YOUR piston deck height, YOUR headgasket compressed thickness (usually the MFG will know this) and YOUR piston dome/dish (if there is one). Using published numbers is not really very accurate and not the best way to plan an engine. Yes, you will have to assemble/disassemble your engine a few times to check things out before final assembly-that's the cost of doing business the right way and knowing EXACTLY what you have and the peace of mind that comes with knowing it was done right and any imperfections/defects were corrected.

    ken

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    very thoughtful

  21. #21
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    Quote Originally Posted by kdanielson
    Well... Euro and other non-U.S. markets I presume got domed pistons that look like this:
    thats is what my pistons look like.

    thanks for that, good to know they are only stock, shouldn't have an issue getting rings to fit then.

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