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  1. #1
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    Choosing Compression Ratio

    Thanks to everyone that gave input on:

    http://thespeedfreaks.net/showthread.php?12669

    I've decided to go with a milder stroker combination. It's an "interesting" choice :-) .....

    This is a '93 ZJ, 5.2L. I'm keeping as much of the engine as stock considering that the '93 ECU seems too difficult/expensive to reprogram or change out.

    I've decide to use a crank of 3.58. I'm using the 24lb Bosch 4-hole injectors, and 180F thermostat. And everything else is stock.

    It seems the only cheap pistons that will work at 3.94 bore are the Silvolite 1278 ( http://www.summitracing.com/parts/uem-1278-030 ). With Cometic gaskets ( 0.027, 0.030, or 0.036) and the measured deck height of 9.580 (roughly) I can set my 0.040 recommended quench ( http://www.beckracing.com/page05.htm ) . I've measured the head combustion chamber at 64cc.

    I'm going to machine out reliefs from the pistons to increase the combustion chamber volume -- otherwise the compression is over 11:1 -- I'm using my lathe to machine out two flat overlapping craters, thus leaving a quench area at the cusp of the combustion "kidney" shape. I built a jig to hold the pistons offset on my lathe. It works well and I took a shallow test cut already. (See photo.)

    So now I need to know what compression ratio would be good for this car (which will be a daily driver) ????

    I can choose a lower compression by deepening the reliefs. I wanted to go as high as 9.6:1 but not lower that the stock compression of 9.1:1.

    I'd like to run regular gas.

    Can anyone recommend a compression ratio? The stock heads are iron of course.

    lathe-pistons.jpg

    I'll create a project page as soon as the rest of the parts arrive.

    Best

    ]Paul

  2. #2
    RallyJeep GO
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    How much cam are you planning to run, and what fuel are you planning to run it on?
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  3. #3
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    The cam, heads, and valve train are all stock.

    I think I want to run on whatever low-octane gas I find at my local gas station. We are 200ft above sea level here --- 87.

    Well, the gas part is not a strict requirement ---

    Really, I just Don't want to be in a situation where 87 gets into the tank "accidentally" and breaks something.

    The engine needs to be safe from detonation for years and years of daily driving.

  4. #4
    RallyJeep GO
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    With stock cam, etc. you want stock compression if you want to be able to run 87. Nothing more, or you'll be pulling an insane amount of timing to keep it safe. If you're willing to run 91 octane, you could go for somewhere in the 9.5 - 9.8:1 compression range comfortably, probably 10:1 as well. The stock cam is pretty small, so it builds more cylinder pressure at lower rpm than a bigger cam, so less static compression is needed / wanted for a given fuel.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  5. #5
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    Cool, thanks.

    So to put this in context: The stock quench height is more than 0.080 -- i.e. way-too-much, hence a non-existent squish-effect. The stock compression is 9.1:1 and is supposed to run on 87.

    I am going for a 0.040 quench height with a nicely-sized quench area.

    Does my quench count for nothing then???

    In my interwebish-research this past week I understood that the point of good quench is to be able to safely increase the compression ratio.

    I'm missing something here :-(

  6. #6
    RallyJeep GO
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    You could probably bump the compression a little bit with good quench, but not a ton. As-is, with the poor quench, the factory had to pull a lot of timing to run safely on 87, while 91 allowed close to optimal timing. I'd definitely stay to no more than 9.5:1 unless you plan on putting in a bigger cam.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  7. #7
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    So you are saying 9.5:1 with 91 octane is ok for daily driving?
    Last edited by paulsheer; 04-12-2015 at 05:58 PM.

  8. #8
    RallyJeep GO
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    If you're going to run 91, you could comfortably shoot for 9.8:1 - 10:1 without issue.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  9. #9
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    Well if I have to use 91 octane I can live with that.

    I can't change the timing -- re-programming these old ECUs turns out to be really expensive.

    Changing the cam means changing the timing, the springs, or both.

  10. #10
    RallyJeep GO
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    If you're running stock 5.2l timing that was good for 87, you might be ok with 89 depending on how the combo ends up.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  11. #11
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    Uh-oh -- the lathe cut I've made so far puts me under 9.74:1

    So I guess it's too late for the "9.8:1 - 10:1 without issue" you speak of -- LOL

  12. #12
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    Well the cut I made was not as deep as I thought. I'm bang on 9.8. Ok, 9.8:1 and 91 octane I think it will be. But this is all rough -- you can't get any-thickness gaskets, and my volume calculation is murky on the chamfer. Let me finish the mirror cut and then measure the relief with my shiny new buret.

  13. #13
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    There are composite gaskets available at .025, .041 and .049 thickness, IIRC. If you're right around 0 deck, the .041 fel-pro gaskets should get things pretty well sorted. If you're .010" below deck, the Mopar Performance thin composite gaskets should do the trick.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  14. #14
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    I can only find Summit's Cometic gaskets at 0.027 and a few larger sizes. Nearly $100, but I'm fine with that.

    Deck milling is $70 here. Plus I'd have to get the block unmounted off the engine stand and onto my truck --- which I never ever ever want to have to do again. LOL. So no deck milling for me.

    My mirror cut is done. Dunno what the XXXX I was thinking machining pistons as a first lathe project!! Not easy to turn those lathe cranks when you are shaking with terror!!!

    Ok, it came out awesome! Heck you can't even tell they weren't shipped just like that. Buret comes out at 10.4 CC relief. That's 9.65:1 if I can get the quench exactly on 0.040.

    relief-buret.jpg

    piston-cut.jpg

    - - - Updated - - -

    one down and 7 to go...

  15. #15
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    These are the thin Mopar gaskets: https://www.monicattichrysler.com/co...?idProduct=247

    If you get the block to 0 deck, just use the cheap Fel-Pro 1008s, they're .039" compressed, which will put you right on for quench.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  16. #16
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    The thin Mopars seem to be backordered everywhere. Mr. Gasket 1121g is .028 compressed.

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