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Compression Ratio

This is a discussion on Compression Ratio within the Internal Engine forums, part of the LSx Technical Help Section category; If I have a stock LS1 shortblock with heads milled to 62cc's to give me a 11:1 compression ratio. What ...

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    Compression Ratio

    If I have a stock LS1 shortblock with heads milled to 62cc's to give me a 11:1 compression ratio. What happens if I change my shortblock to a 402 does my compression ratio stay the same or does it drop it slightly?

    Lets keep it simple, lets say the new 402 shortblock is using identicle everything same gaskets and not dished or flycut does the wider bore change the compression ratio?

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    Senior Member mrr23's Avatar
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    .1 for each .006" mill.

    all else being completely equal, the bore of the 402 will cause a rise in compression ratio.

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    Quote Originally Posted by mrr23 View Post
    .1 for each .006" mill.

    all else being completely equal, the bore of the 402 will cause a rise in compression ratio.
    I didn't even think about that... so a higher compression ratio because there is more air to be compressed?

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    Quote Originally Posted by Bouvers View Post
    I didn't even think about that... so a higher compression ratio because there is more air to be compressed?
    Yes...think about it this way:

    You have two soda bottles. One is a 20oz and one is a 2 Liter. If you compress each one to a 11th of their size (that's what 11:1 means) then the 2 Liter is obviously going to be bigge, but you still have the same compression ratio. But if you try to compress the 2 Liter to the same size as the 1/11th 20 oz, the pressure inside the 2 liter is going to go up exponentially, because you have more air in the 2 Liter that you are trying to compress into a space that is realistically too small.

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    sweet thanks for the insight

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    to continue along this same point, what would happen if I ran the same cam I'm running now just a baby 228/230 and then used that in conjunction with the same heads but on a 402 stroker...

    would I infact lose power because I have too little air/fuel being supplied and exhaust discharged or would I virtually have the same power or would I see substantial gains... I'm kind of at a loss on this one...

    I would guess that it would stay relatively the same with a slight gain but nothing worth while...

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    Yeah the 402 will have a significant increase in CR.

    (Adding the following so nobody wonders by and figures if the 346 to 402 increase cui increases CR...then boring their engine .030 over will increase CR!)

    Chamber - Ignoring Head and a flat piston (not domed/dished)
    *Obviously not real engine numbers here, but easy math...
    -3.5" diameter piston, 8" stroke in a 9" chamber
    -3.25" diam piston, 8" stroke in a 9" chamber

    Each yields the same comp. rat., 9:1
    -3.5" - 76.97 cui uncompressed, 9.62 cui compressed
    -3.25" - 74.66 cui uncompressed,8.3 cui compressed

    If both cylinders were using the same head...say 3 cui (easy math)
    3.5" - 79.97 cui : 12.62 ... 6.336:1
    3.25" - 77.66 cui : 11.3 .... 6.872:1

    NOTE here!!! -The smaller bore has higher CR, not as big of an effect with real engine numbers, but the same relationship exists...

    If the bore gets larger, piston dish or head volume increases comp. ratio goes down. If stroke is increased, piston dish or head volume decreases comp. ratio goes up.

    The 346 is 3.898" x 3.622" (bore x stroke)
    The 402 is 4" x 4"....the more you stroke to get cui the more CR you run.

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    Bouvers,

    You ain't a big fan of Sunday drives or pump gas are you?

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    Nice Mr. Luos. Much nicer than all the typing I did.


    I like how that one includes piston dish inputs. Now in my favorites.

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    Quote Originally Posted by BowTied View Post
    Bouvers,

    You ain't a big fan of Sunday drives or pump gas are you?
    lol yes I am... just so no one gets the wrong idea if I were to ever go through with this upgrade it would be used on a car that is a weekend warrior and won't see hardly ANY track time but will see considerable highway roll racing...

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    awesome calculator! thanks Luos

    So its looking like with the heads I've got right now with an LS2 402 stroker I would be running 12:1 CR but all I would have to do is dish the pistons -8cc's and I'll be right back to the same CR I will be running with my h/c package.

    So I guess the question is if I did go through with this and bumped up my cam size, would I be able to flycut the pistons to -8cc's or is that too much and therefore have to dish to achieve the lower CR?

    To repost my other question this is a Weekend Warrior car and I'm still very concerned with comfort and drivability thus my present 228/230 cam what is a strong cam that should net me 520rwhp +/-15hp that will still be mild and tame on the street with a 402?
    Last edited by Bouvers; 01-05-2009 at 04:15 PM.

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    BUMP anyone ?

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    I think 6.xx CC's dishing of the piston will do it good enough.

    Kind of a shame though....I wonder if you could potentially swap heads with someone in the market to boost CR????

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    Well these are AFR 205's and I payed high for them brand new so I ain't swapping them... I will end up having them ported for the higher cubes if I were to get a stroker. If I did I could get the cc's milled up to 64cc's or something I'm sure... the thing is if I wanted to run a bigger cam and required flycut pistons could I just flycut the pistons to lower my CR and kill two birds with one stone or is a flycutting to -8cc's way too much? I have no clue...

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    Quote Originally Posted by Bouvers View Post
    Well these are AFR 205's and I payed high for them brand new so I ain't swapping them... I will end up having them ported for the higher cubes if I were to get a stroker. If I did I could get the cc's milled up to 64cc's or something I'm sure... the thing is if I wanted to run a bigger cam and required flycut pistons could I just flycut the pistons to lower my CR and kill two birds with one stone or is a flycutting to -8cc's way too much? I have no clue...
    I've seen some pretty big valve relifes in pistons before....but that was in older small blocks

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    ok so it can be done on SBC engines anyone know if it can be done on these engines running 11:1 CR?

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    11:1 is NOTHING for an LSx series.
    Pump gas is not a problem with 11:1.



    When it comes to LSx compression and being able to run pump gas....it depends on two things. DCR and tune.
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    Quote Originally Posted by Mr. Luos View Post
    11:1 is NOTHING for an LSx series.
    Pump gas is not a problem with 11:1.



    When it comes to LSx compression and being able to run pump gas....it depends on two things. DCR and tune.
    after all stock is 10.1:1.

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    Quote Originally Posted by Bottesini View Post
    after all stock is 10.1:1.
    Yup.


    I run 11:1 with my 402. 91 octane and 31 degrees of timing.
    ZERO knock.

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